Extending The Envelope: A Primer On Self-Contained Diving Technology

© 1991 Michael Menduno. All rights reserved.

GAS SUPPLY

Supplying divers with an adequate volume of compressed gas represents the fundamental constraint in diving. Without adequate thermal protection a diver may survive for many hours. Given an improper breathing mix this margin shrinks to a few or many minutes but in an "out of gas" emergency, survival is a matter of seconds. No where is this problem more acute than in self-contained diving.

Since the development of the "Gagnan-Cousteau" demand valve, popularly known as the "AquaLung," in the 1942, open circuit scuba has become almost synonymous with self-contained diving. Indeed, as a result of its low cost, high reliability and minimal training requirements, open circuit technology opened the way for the development and growth of sport and scientific diving and has has greatly extended our diving range.

Today, with standardized training, bigger tanks and improved breathing equipment, recreational divers enjoy access to a wide range of diving environments while explorers are pushing back boundaries that would have been thought inconceivable fifty years ago. Nevertheless, in spite of the advances that have been made, the limit of these pushes, whether it involves extended cave penetrations or deep diving, is clearly in sight in what closed circuit pioneer and inventor, Dr. Bill Stone, refers to as the "Open Circuit Barrier."

With the advent of closed circuit (C2) breathing systems just around the corner, many observers believe that the days of open circuit scuba are numbered. For good reason. Offering a virtually unlimited gas supply and near optimal decompression in a 50 lbs. pack, the next generation of closed circuit systems promise to revolutionize self-contained diving and extend our envelope even further. But don't get rid of those steel 104s just yet.

Though C2 technology represents the diving system of the future, it will likely be some years before its price, currently projected in the $7500-$15,000 per system range, reliability and maintenance requirements make it a competitive alternative to open circuit for most applications. In the meantime, new methods and thinking brought about through the growth of technical diving promise to broaden the reliable range of open circuit technology still further and improve diving safety.

Fig. 5.1 Gas Constraint Diagram

OPEN CIRCUIT GAS MANAGEMENT

Insuring that divers have adequate gas supplies is at the heart of the problem of self-contained diving and represents it's major risk factor as demonstrated by accident analysis (see Chapter 7, " Safety First). Fatalities are rarely caused by running over a table or getting separated from the boat. Rather the accidents that take divers lives are usually due to "running out of gas" whether caused as a result of poor planning, equipment failure, entanglement or getting lost in a wreck or cave.

Unlike surface supplied diving where gas is continuously fed to the diver under pressure through a surface umbilical, which also serves as a navigation device to guide the diver home, the self-contained diver must carry and stage all of the gas he or she plans to consume on the dive. As a result, the self-contained diver is at constant risk from equipment failure and in the event there is insufficient gas to complete the dive. What's more is that in comparison to their commercial counterparts, self-contained divers have "precious little time" to correct any problems that may occur, for example, getting lost in a wreck or cave or dealing with entanglement, due to limited gas supplies. This is particularly true as depth increases, increasing consumption dramatically. These are some of the reasons that many commercial operators have "banned" the use of self-contained breathing equipment from the workplace, regarding it as "unsafe," and why the US Navy recommends that self-contained decompression dives be conducted only under "exceptional" operational circumstances. Clearly proper gas management is critical to self-contained diving operations, particularly for technical diving applications where divers may be many minutes to hours from the surface and problems must be solved at depth. Accordingly the objectives of gas management are two fold:

1) To insure that sufficient volumes of the appropriate gases needed to conduct the dive are available at the proper time during the operation.

2) To minimize the probability of supply disruption through the use of proper equipment, and to insure there are sufficient reserves to return the dive team to safety in the event of an emergency.

In technical diving these objectives are accomplished through planning, utilizing redundant breathing equipment and carefully managing gas supplies throughout the dive.

Though there have been few real changes in open circuit hardware since the development of the single hosed regulator in the early 1960s, the need to reliably extend the range of technical diving operations has driven the evolution of gas management strategies and the types of hardware configurations used. Though planning and gas management have long been a recognized part of recreational diving, recreational methods and equipment are inadequate for most technical exposures. The differences in approach can best be summarized by comparing operational requirements.

Unlike recreational diving, most technical operations are mission or task specific and require a detailed "pro-active" approach to gas planning. Where recreational divers typically are content to jump off the boat and "swim around" until some predetermined constraint such as a "no-stop" decompression limit or specified gas level&endash; whichever comes first&endash; is reached and divers surface, this is rarely the case for the technical diver seeking to accomplish a specific objective. Rather than asking what can be accomplished with the gas on hand, the question asked by technical divers is usually, "how much gas is required to accomplish the dive reliably." In this case, requirements include gas for both planned consumption and reserves for dealing with emergencies.

What's more is that these dives typically involve extended bottom times and significant decompression. As a result the single tank of gas carried by most recreational divers is simply inadequate for the exposure. Most technical dives require that multiple cylinders be used. Double cylinders containing the diver's bottom mix are typically carried in a back-mounted "set" while one or more diver carried "stage bottles" are used to extend range and carry the intermediate mixes and oxygen used during decompression. Surface supplied oxygen is also sometimes used to supplement limited diver-carried supplies. In addition, for some applications, "bail out" bottles such as a 13-40 cf pony are mounted to the diver's set in the event of an out-of-gas emergency. Because of the number of cylinders that must be carried by the diver, diver propulsion vehicles (DPVs) have become a standard tool for extended and or deep dives.

FIG. 5.2 How Much Gas Is Needed To Accomplish A Dive?

(A Gas Consumption Comparison)

Though a redundant second stage regulator and increasingly an "alternative air source" are typical of recreational diving applications, redundancy has been elevated to a fine art in technical diving. A standard set includes both

a full primary and secondary regulator rated for the planned working depth along with independent shut-off valves on the doubles manifold. In addition one of the regulators is fitted with a "long hose," a five to seven foot whip to facilitate gas sharing in the event of an emergency. Similar to recreational diving, the "buddy" still represents an important source of gas redundancy though not the primary one. Self-sufficiency is key. What's more is that in technical diving the loose concept of the buddy has been replaced with that of a "team member" who has a pre-planned and usually rehearsed set of responsibilities, though a "team of one" is sometimes appropriate in certain circumstances.

Staging is also an important consideration in most technical diving applications. Though in some environments such as underwater caves, cylinders can be reliably "staged" along the planned route taken by divers or at a decompression stop, this is typically not the case in open water diving. Though in the past it was not atypical for divers to leave their cylinder of decompression gas clipped off to the anchor line, today this practice is viewed as unreliable in most circumstances. In the event the diver is unable to return to the line he or she may find themselves with inadequate gas to complete the dive. Instead the thinking today is that dive team should carry all the gas that they will need to to complete the dive if possible. However this does not preclude staging cylinders of decompression gas along the pre-planned route, for example at the exit point of a wreck penetration environmental conditions permitting. Strategically placed pre-staged "gas depots" are also sometimes utilized for additional safety on cave exploration pushes or on a wreck where multiple teams are in the water or dives are planned over an extended period.

A final difference with recreational diving is the way in which divers manage their gas supply. The traditional "surface with 500 psi" rule that is still taught in recreational classes provides an insufficient reserve for most technical exposures. Instead technical divers have adopted more sophisticated "gas management rules" that better reflect the requirements of the overhead environment. Originally developed by the cave diving community to insure that a dive team has adequate gas to exit from a cave in the event of an out-of-gas emergency, the basic "Rule of Thirds" is one example. The strategy here is to "turn the dive" when the first member of the team consumes one third of their gas supply. In the event that one of the divers has a catastrophic gas failure at the point of maximum penetration&endash; Murphy loves cave divers!&endash; the team still has sufficient gas to exit the cave. This basic rule can be modified downward, for example to fourths or fifths for greater conservatism depending on environmental and operational considerations such as having to exit against a flow or when diving scooters. By doing this the integrity of the teams gas supplies can be assured.

HOW MUCH GAS

Most technical operations involve a detailed plan of the amount and types of gases to be consumed during the dive and the gas reserves that are needed in order to accomplish the specific mission at hand safely, taking into account contingencies in the event that something goes wrong. These calculations determine a diver's set configuration, the number of cylinders that will be required, and the manner in which the operation will be conducted. Though "realtime" gas management rules such as the thirds rule are used in calculating needed reserves and for conducting the actual dive, they are not a substitute for prior consumption planning.

Surface Consumption Rate

Calculating planned gas consumption is easy and begins with determining a diver's expected "surface consumption rate " or SCR, the amount of gas consumed per minute if the dive were conducted at the surface (1 atm). Conversely, the SCR can be thought of as the amount of gas to be consumed during various phases of the dive adjusting out the effects of pressure i.e. the surface equivalent consumption rate in cubic feet or liters per minute per atmosphere. For the purpose of this exposition we will treat the "per atmosphere" term as a given.

Extensive US Navy studies have shown that a diver's SCR, also referred to as respiratory minute volume (RMV) i.e. the amount of gas actually inhaled and exhaled per minute, is about 26 times the amount of oxygen metabolically consumed by the body. Metabolic oxygen consumption is a function of exercise rate and typically ranges from about 0.5 liters/minute for a diver at rest to about as 1.0-2.0 liters/min. under moderate to heavy workloads. Converting liters to cubic feet (1 liter is equal to about .036 cubic feet) and multiplying out terms, surface consumption rates typically range from about 0.5- 2.0 cf/min (i.e. 0.5-2.0 liters/min. x .036 cf/liter x 26 = 0.47-1.9 cf/min.).

During the working portion of a dive when exercise rates are generally high, SCRs are typically about 1.0 cf/min. or less but may be as high as 1.5-2.0 cf/min. if the diver is swimming hard for example in a heavy current. Conversely during decompression when a diver is usually at rest, SCRs generally fall in the 0.3-0.8 cf/min. range. Note that SCRs generally vary with water temperature, experience and familiarity with the dive site. Surface consumption rate values typically used for planning gas consumption requirements are shown in Table 5.1 below.

Table 5.1: Surface Consumption Rate Planning Values

Dive Phase	Gas Mix	Planned SCR (cf/min.)
Descent	Travel/Bottom Mix	1.0-1.5(light to mod. work)
Working	Bottom Mix	1.0-1.5(light to mod work)
Decompression:
Intermediate Stops	Intermediate Mix	0.6-0.8
Oxygen Stops	Oxygen	0.3-0.6

Sidebox: Calculating Individual SCRs

An individual's Surface Consumption Rate (SCR) can be readily calculated during a test dive and later used for planning purposes. The procedures are as follows;

1. Begin the dive and descend to the depth (D) at which the test will be conducted.

2. Record starting time and starting tank pressure on a dive slate upon commencing the test. Generally a 5-10 minute test period is sufficient.

3. Conduct the dive breathing normally while maintaining a single fixed depth (D) for the duration of the test..

4. Record ending time and ending tank pressure.

5. Using the test data, calculate your SCR as follows:

SCR= Gas Consumed (cu. ft.)/ Depth (atm)/Test duration (minutes).

where Gas Consumed (cu. ft.)= (psi consumed/rated tank psi) x rated tank volume (cu. ft.) and Depth (atm) = (Test depth (fsw)/33) +1.

This exercise can be carried out during the working phase of a dive as well as during intermediate gas and oxygen decompression stops to derive a range of SCRs.

Calculating Gas Consumption Requirements

Once an appropriate SCR is chosen, planned gas consumption can be readily calculated by applying the basic gas laws1 to the planned depth/time profile according to the following formula:

Consumption At Depth (cf) = SCR (cf/min) x Depth (atm)

x Duration (min.)

For example, if the planned SCR is 1.0 cf/min. and the dive is to be conducted at 100 fsw for 15 minutes on air, making it a "no-stop" dive, then planned gas consumption is given by:

1.0 cf/min. x ((100 fsw + 33)/33) atm x 15 minutes= 60 cubic feet.

Note however that this simple calculation does not take into account the gas consumed during the descent or ascent portion of the dive which can be an important factor particularly on deep dives. Fortunately this is easy to include. By calculating the duration of the descent, based on the planned descent rate, and observing that algebraically, the gas consumed during descent is equal to the descent duration times the average depth in atm1 , consumption can be calculated as follows:

Consumption During Descent (cf) =

SCR (cf/min) x Average Depth (atm) x Descent Duration (min.)

From the example above assuming an average descent rate of 60 fsw/min. to 100 fsw, total descent time should be 100 fsw/ 60 fsw/min= 1.66 minutes. Using the SCR above of 1 cf/min, gas consumption during descent is given by:

1 cf/min. x (50 fsw/33 fsw +1) x 1.66 minutes = 4.2 cf.

where 50 fsw is the average depth during the descent. Note however that since bottomtime is calculated from the surface, the 1.66 minutes of descent time must be subtracted from the bottomtime. Recalculating gas consumed on the bottom yields:

1 cf/min x (100 fsw/33 +1) x (15-1.66 min.)= 53.7 cf

Similar to the descent, the same procedure can be used to calculate the gas consumed during the ascent phase of the dive. Assuming an average ascent rate of 30 fsw/min. to the surface, gas consumed during the ascent is given by:

1 cf/min. x (100 fsw/30 fsw/min.) x (50 fsw fsw/33 + 1) = 8.3 cf.

If in addition, a three minute safety stop at 15 fsw is included for conservatism, this can be readily calculated by adding the three minutes of consumption at 15 fsw using the consumption at depth formula above. For simplicity we assume that the diver's SCR is 1 cf/min for both the working and decompression (safety stop) phases of the dive. Note that the safety stop consumption can simply be added to the gas consumed during the ascent

Safety Stop Consumption = 1 cf/min x 3 minutes x (15 fsw/33 fsw +1)

= 4.35 cf.

Hence total planned consumption for the dive is given by:

Dive Phase	Consumption (cf)
Descent	4.2 cf
Bottomtime	53.7 cf
Safety Stop	4.35 cf
Ascent	8.3 cf
Total Planned Consumption:	70.6 cf

Calculating Gas Reserves

Based on the analysis above, the diver should consume approximately 71 cf of gas (air) for this 15 minute dive to 100 fsw, however these figures do not include gas reserves needed for dealing with an emergency such as an equipment failure or an out-of-gas situation. In the event of an out-of-gas emergency, it does little good to "share air" with a dive partner, if the partner has insufficient gas for both divers to safely exit from the surface.

In many cases, the "500 psi rule," i.e. "surface with 500 psi remaining in your tank," is probably sufficient for "no-stop" recreational dives in the absence of a "decompression ceiling," though this may not always be the case. In the example of the 100 fsw "no-stop"dive above, the analysis shows that 8.3 cf is needed to ascend from 100 fsw at an ascent rate of 30 fsw/minute not considering the safety stop. Assuming that a pair of divers were using aluminum 80 cf cylinders pumped to 3000 psi and one of them had a catastrophic failure upon completing the planned 15 min working portion of the dive, each team member would require 8.3 cf or about 311 psi for a total of 16.6 cf (622 psi) to surface assuming SCRs of 1 cf/min. at a 30 fsw/minute ascent rate. If instead we assume that their SCRs increased to 1.5-2.0 cf/min due to the stress of the situation, 25-33 cf or about 940-1240 psi would be needed to effect the exit. This implies that if the diver with air had only reserved about 900 psi ( 500 surfacing psi + 8.3 cf=311 psi = 811 psi total) at the end of the working phase of the dive&endash; enough for one diver to surface with 500 psi remaining in his or her tanks&endash; the team could potentially sustain an injury. This analysis suggests the "500 psi rule" should be regarded at most as a "minimum" for recreational dives and probably is insufficient&endash; even on recreational exposures&endash; to cover a team emergency when diving deep2 . It also points out the need to pre-plan gas reserves for contingencies specific to the dive being conducted.

If for the purpose of this analysis, the "rule of thirds" is applied to the dive above i.e the divers would plan to surface with one-third of their gas remaining, "Total Gas Requirements" i.e. the total planned consumption in addition to required reserves would be:

Total Planned Consumption: 71 cf

One Third Gas Reserve: 36 cf

Total Gas Requirements: 108 cf

Note that the one thirds reserve would allow ample gas for the team to exit in the event of a catastrophic gas failure as discussed above.

Pull Quote:

Total Gas Requirements = Planned Consumption + Calculated Gas Reserves

This analysis shows that at a planned SCR of 1 cf./min. each diver would require slightly more than a 100 cf tank to conduct the dive and should consume approximately 71 cf. Note that if steel 120 cf cylinders (rated to 3500 psi) was used, the divers would plan to surface with about 1430 psi

(i.e. (49 cf remaining/120 cf) x 3500 psi= 1429 psi). If planned reserves were reduced to 29 cf, which would be sufficient to conduct an emergency exit based on the analysis above (since the dive was a no-stop dive in open water), and the dive was conducted using single 100 cf tanks (@ 3500 psi), the divers would plan to surface with 1015 psi (about 29 cf.).

Calculating Total Gas Requirements On Technical Dives

Calculating planned gas consumption for a 'technical level' exposure involving staged decompression and multiple gas mixtures is slightly more complicated then the example above but is based on the same logic. Beginning with a planned decompression schedule which includes the type of gases to be used, gas consumption calculations are iterated until a workable operational plan is arrived at (in terms of the amount of gas/ number of cylinders that will be carried) based on the environment and operational parameters of the dive. The logic for these calculations is shown in flowchart form in Fig. 5.3 below. These calculations are usually carried out with the help of a gas planning worksheet as shown in Fig. 5.4 or a computer program designed for this purpose. Eventually most special application tables will incorporate gas consumption requirements in their calculations.

Fig. 5.3: Gas Planning Flowchart (****Discuss)

Fig. 5.4: Gas Planning Worksheet-(blank & filled out)

During the actual operation divers typically log their actual gas consumption. This information can then be compared to planned consumption values during the post dive analysis and planning values can be modified to better reflect 'actuals' on subsequent dives.

Gas Planning & Analysis

The best way to appreciate the operational information that can be derived from gas planning analysis and the manner in which gas requirements change with depth is to apply this analysis to a variety of dives. The analysis below considers gas requirements for a series of "20 minute" open water wreck dives conducted in Key West, Florida. These could just as well be deep water reef exploration dives in the absence of penetration. Maximum depths range from 90-425 fsw. Each of these dives are planned based on appropriate gas mix and decompression schedules as discussed in Chapters 3 and 4 and are summarized in Table 5.3 below. Gas requirements were calculated using the relevant decompression schedules.

Table 5.3: Gas Planning Analysis

Depth (fsw)	90	180	250	310	425
Bottom Time	20	20	20	20	20
Bottom Mix:	EAN 36	Air	Trimix 17/50	Trimix 13/50	Trimix 10/60
Intermediate Mixes:
#1	EAN 36	Air	EAN 36	EAN 36	Air
#2				EAN 50	EAN 30
#3					EAN 46
Oxygen:	n/a	20 fsw	20 fsw	20 fsw	30 fsw
Wrkg PO2 (atm)	1.34	1.4	1.46	1.4	1.4
END (fsw)	66	180	85	127	141

Generally for the purpose of planning gas consumption, dives are treated as square wave profiles and the maximum working depth is used in the calculation which tends to add a healthy conservative factor. In this analysis an SCR of 1.0 cf/min. is used in calculating gas consumption for the working portion of the dive and ascent to the first decompression stop while SCRs of 0.7 cf/min. and 0.5 cf/min. are applied to the intermediate and oxygen decompression stops respectively. With experience working SCRs generally fall slightly below 1.0 cf/min (i.e. 0.7-0.8 cf/min.) on light to moderate swimming dives and when using diver propulsion vehicles (DPVs). Note also we have assumed that the ten foot oxygen stops are pulled at 20 fsw unless noted, a common practice in open water diving, yielding a slightly more conservative oxygen consumption requirement. In practice these calculations are sometimes based on the actual planned (multilevel) dive profile or average working depth if this information is available and the team's known historical SCRs. Bottom times are calculated from descent at the surface. Descent rates are assumed to be 60 fsw/min unless noted. Ascents are conducted at 60 fsw/min. to the first decompression stop and 30 fsw/min. there after.

Gas management rules are used both for calculating needed gas reserves as well as conducting the actual operation. The objective during planning is to calculate the amount of gas likely to be consumed in accomplishing the mission and adding an adequate safety reserve for contingencies, specifically to allow the dive team to survive a catastrophic gas failure at any point in the dive. In the field, this becomes a matter of balancing the need for total self-sufficiency with operational practicality and depends on the environment and specific operational parameters of the dive.

Required bottom mix reserves are generally more flexible for "open water dives", even in the event a wreck penetration is planned, as divers are typically able to "turn the dive" in the event of gas shortages or an equipment failure and ascend to their first decompression stop in short order. This is generally not the case in cave diving where there is a physical overhead and significant return transit time is usually required to reach safety. In this case, bottom mix reserves must be strictly calculated and adhered to. Typically for open water exploration dives and when diving a wreck for the first time the rule of thirds is used in calculating reserves. The objective is for divers to reach the first planned decompression stop with a third of his or her bottom mix remaining. With experience at the dive site, reserve factors can be gradually adjusted to better reflect the actual consumption/profile.

Insuring adequate decompression gas reserves is generally a more complicated issue and further serves to highlight the differences between open water and overhead environment diving operations. During the working portion of the dive, the thirds or other appropriate reserve rules serve as a sufficient buffer to assure the integrity of the team's gas supply. In the event of gas shortages or equipment failure, bail out procedures are initiated and the team begins their exit. However this is not the case during decompression where divers have a mandatory schedule of stops that must be completed prior to surfacing which may involve many minutes or hours. In the event of a decompression gas emergency such as a regulator failure, missing stage bottle or over extended stay at the bottom the dive team cannot simply exit to the surface. In order to be "fully self-sufficient" the team must either "carry" or "stage" a fully redundant supply of decompression gas (at least twice the required decompression gas) to handle emergencies. Unfortunately for deep extended dives carrying a fully redundant supply is not always feasible.

As shown in the examples below, oxygen used for decompression is rarely the limiting factor for most deep open water dives though it can become significant for long (more than two hour) shallow dives. Rather the problem is one of carrying sufficient intermediate gas to insure self-sufficiency in the event of a gas emergency3 . Based on field experience and analysis, the self-sufficiency "breakeven point" for extended working dives seems to be to about 250-300 fsw for a two person team depending on the length of the dive. Though intermediate gas requirements are rarely an issue for short (less than 10 min.) "bounce" dives, intermediate gas requirements increase exponentially with bottomtime. As a result, carrying a fully self-sufficient supply of intermediate gas for extended dives beyond about 250-300 fsw becomes impractical due to the number of cylinders involved.

In cave diving these supplies can be conveniently staged along along the diver team's route, in this case the dive team's mainline. In addition, for some operations, strategically located gas depots can be "pre-staged" in the cave during prior set-up dives. Unfortunately the situation is not that simple for open water boat operations where decompression is usually conducted in the water column. In this case additional back-up cylinders must either be staged on the ascent line or ferried to divers by a surface support team which means that dive team self-sufficiency is lost. The result is that "surface support" i.e. a direct link between the dive team and diving platform/surface personnel is generally a requirement for extended open water diving operations beyond about 250-300 fsw. What's more is that DPVs are generally a requirement because of the amount of tankage involved. These requirement imposes stiff restrictions on diving operations and what can be "reliably" accomplished by the team as discussed in in Chapter 6, Planning & Operations and in the examples below.

Today, the thinking is that intermediate gas requirements should include a minimum one half or 50% reserve for contingencies (1.5 times planned consumption) and that stage bottles be rigged with a redundant regulator. In addition it is recommended that the team's stage bottles be rigged with either DIN or post connectors but not both. In the event of a failure, regulators can be swapped between cylinders underwater if necessary . Though insufficient in most cases to insure that the team can pull the entire planned decompression in the event one member has a catastrophic intermediate gas failure, these procedures and other conservative factors built into the plan provides the team with sufficient time to correct any problems that arise at depth and/or to signal the surface support team for help, for example by the pre-planned use of lift bags as signaling devices. This is one area where wireless "diver-to-surface" communications systems offers significant safety advantages and why many people believe that communications represents the coming standard technical diving. Of course if all else fails the team's remaining bottom mix can be utilized as an emergency back up though this would likely result in decompression problems.

Though divers typically carry all of the oxygen required for the dive, an increasing number of open water operations utilize a surface supplied oxygen system to supply the divers during decompression. In this case the diver-carried oxygen stage bottle is kept in reserve as a back-up. The fact that divers end up carrying additional oxygen serves as a safety factor in the event of becoming separated from the support vessel or an extended unplanned decompression, for example if divers are forced to overstay their planned bottom time and do not have sufficient intermediate gas for their decompression. One emergency field procedure that is used is to breathe down intermediate gas supplies to reach the 20-30 fsw stop and then to breathe oxygen until the entire supply is exhausted and or surface support can be reached. In many situations this is far better approach than surfacing prematurely and risking decompression illness.

The results of calculating gas requirements for this series of dives is presented and discussed below.

Table 5.4

Dive Profile #1: Cayman Salvage Master

Depth: 90 fsw

Bottom Time: 20 minutes

Total decompression: 3 min. Safety Stop

First decom stop: 15 fsw

Tables: NOAA Nitrox II

Gas Mix	Planned Cons.	Gas Reserve	Total Gas Reqmts.
Bottom Mix
EAN 36	72 cf	36 cf	108 cf
Intermediate Mix:
EAN 36	3 cf	0 cf	3 cf
Total	75 cf		111 cf

Though this dive could be conducted on air without incurring a mandatory decompression obligation, a single EAN 36 mix is used to provide an additional safety margin to the diver. What's more is that a 3 minute safety stop is included as a standard operational procedure. Based on the gas analysis the diver would theoretically require a single tank in excess of 100 cf., for example a single "120." In this case the diver should surface with approximately 1300 psi assuming the 120 cf tank was pumped to 3500 psi (120-75 = 45 cf remaining/120 cf available=0.375 x 3500 psi).

In practice since the dive is conducted in open water and there is no mandatory decompression, the team would likely dive the wreck with a single "100" or "95" and use the thirds rule to "turn the dive" when the team had exhausted a third of their available supply. Since the decompression requirement of 3 cf. is insignificant and would not have to be considered separately. In the case of a lengthly decompression the team would subtract out their planned decompression requirements before calculating thirds.

Note that the planned dive could be conducted on a single 100 if experience proved that SCRs were actually about 0.9 cf/min. or less versus the 1.0 cf/min planned. In this case the divers would surface with roughly one third of their gas supply less the 3 cf. used for the safety stop. Assuming the tank was initially pressurized to 3500 psi, the dive team would surface with about 1130 psi remaining. To conduct the dive using an aluminum 80 cf tank, actual SCRs would have to be about 0.7 cf/min.

Table 5.5

Dive Profile #2: The Curb

Max. Depth: 180 fsw

Bottom Time: 20 minutes

Total decompression: 26 min.

First decom stop: 50 fsw

Tables: Submariner Research Ltd. Air w/O2

Gas Mix	Planned Cons.	Gas Reserve	Total Reqmts.
Bottom Mix:
Air	130 cf	65 cf	195 cf
Intermediate Mix:
Air	29 cf	15 cf	34 cf
Oxygen	11 cf	6 cf	17 cf
Total	161 cf		246 cf

Based on the planned requirements of 229 cf of air, which is used as both a bottom and intermediate mix, this dive could be conducted with double 120s with a single oxygen pony or stage bottle. The divers would surface with approximately 61 cf in their doubles or about 1000 psi assuming the cylinders were pressurized to 3500 psi. Note also that in case of a decompression gas emergency, for example a manifold failure upon commencing their ascent, a dive team of two would have ample reserves to conduct the decompression independently from the boat, assuming both divers carried their own oxygen or had a double regulator oxygen stage.

The Curb is a low lying wreck with a deck at 160 fsw and the bottom at 180 fsw and consequently there is minimum gas padding in the calculations due to differences in the average versus maximum working depth. However, based on experience, if actual SCRs were found to be about .85 cf/min. or less instead of the 1.0 cf/min. used for planning purposes, total air requirements including reserves would be reduced to 200 cf making the dive feasible with double 100s along with an oxygen stage. If double 80s were used instead and the thirds rule adhered to the divers would be able to conduct about a 15 minute dive versus the planned 20 minutes.

Table 5.6

Dive Profile #3: USS Wilkes Barre

Max.Depth: 250 fsw

Bottom Time: 20 minutes

Total decompression: 70 min.

First decom stop: 110 fsw

Tables: Hamilton Research Ltd.

Gas Mix	Planned Cons.	Gas Reserve	Total Gas Reqmts.
Bottom Mix:
Trimix 17/50	172 cf	86 cf	258 cf
Intermediate Mix:
EAN 36	55 cf	28cf	83 cf
Oxygen	26 cf	13 cf	39 cf
Total	253 cf		380 cf

Special mix dives on the Wilkes Barre are usually conducted with three different gas mixes. Typically these are carried in doubles and two stage bottles. Based on the analysis, the diver would require slightly more than double 120s to complete the working portion of the dive assuming an SCR of 1.0 cf/minute. Note however that this assumes the entire dive is conducted at the maximum working depth of 250 fsw which adds a healthy margin in gas calculations. The depth of the Wilkes Barre ranges from about 150 fsw at the smokestack to a maximum depth of 250 fsw to the sand. For analysis sake if we assume that the average working depth were 210 fsw instead of 250 fsw total bottom mix requirements would be reduced by about 38 cf (25 cf. planned cons. with a 13 cf. reserve) reducing total requirements to 220 cf. though the same decompression schedule was followed for conservatism's sake. With practice and site familiarity actual consumption rates are usually 0.9 cf/min. or less and the dive can be carried out using double 100s.

A 20 minute dive to 250 fsw seems to be close to the "breakeven point" for dive team self-sufficiency in open water. Though oxygen requirements for this dive are minimal and could be carried out with a 40 fsw pony, planned intermediate gas consumption is estimated at about 55 cf. In order to insure complete self-sufficiency, each diver in a two person team would have to carry about 110 cf of EAN 36, for example in a suitably pressurized stage bottle with a Y-valve and dual regulators which is feasible. Note that team self-sufficiency becomes an easier proposition with a three person team. In practice the decompression for this dive is usually be conducted with two single 80 cf EAN stages using a continuous mooring line system from the wreck to the decompression line (see Chapter 6: Planning & Operations), safety divers and a surface supplied oxygen system. Additional redundancy is provided by the surface support team.

Table 5.7

Dive Profile #1: USS Kendrick

Max.Depth: 310 fsw

Bottom Time: 20 minutes

Total decompression: 119 min.

First decom stop: 140 fsw

Tables: Hamilton Research Ltd.

Gas Mix	Planned Cons.	Gas Reserve	Total Gas Reqmts.
Bottom Mix:
Trimix 13/60	238 cf	119 cf	357 cf
Intermediate Mix:
EAN 36	23 cf	12 cf	35 cf
EAN 50	73 cf	37 cf	100 cf
Oxygen	41 cf	22 cf	63 cf
Total	375 cf		555 cf

Extended diving beyond 300 fsw in open water is limited by how many cylinders can be practically carried by the dive team. In this case, each diver would need to carry doubles, for example 104's pressurized to about 3400 psi (about 290 cf) or the new 150s, along with a bottom mix stage bottle to complete the working portion of the dive. Operationally the team might breathe down their stage on descent and into the dive until a third was exhausted and then switch to their doubles. The stage would provide some additional redundancy in the event of a doubles manifold failure. Note that the descent and ascent alone not including decompression requirements would require about 80 cf. In addition, team members would each carry two intermediate gas stages and an oxygen cylinder for a total of five cylinders each. Diver propulsion vehicles would be a major asset on a dive such as this due to the tankage involved.

In order to be completely autonomous from the boat, each member in a two person team would need to carry an addition EAN 50 stage (a total of six cylinders) and both the EAN 36 and oxygen stage would need to be equipped with a Y-valve and dual regulators. DPVs would likely be required. A three person dive team would add additional flexibility.Though operationally "doable" this example approaches the limits of open circuit "self-sufficiency." In the absence of a redundant EAN 50 stage or surface support, the EAN 36 stage could be used as a back up in the event of a problem with the second intermediate mix until the divers were able to reach 20-30 fsw and breathe down their oxygen bottle. Note however because of the possible problems of oxygen toxicity the EAN 50 mix could not be reliably used for stops deeper than 70 fsw.

Table 5.8

Dive Profile #4: USS Saufley

Max. Depth: 425 fsw

Bottom Time: 20 minutes

Total decompression: 173 min.

First decom stop: 210 fsw

Tables: Hamilton Research Ltd.

Gas Mix	Planned Cons.	Gas Reserve	Total Gas Reqmts
Bottom Mix:
Trimix 10/60	295 cf	148 cf	443 cf
Intermediate Mix:
Air1	57cf	28 cf	85 cf
EAN 30	53 cf	27 cf	80 cf
EAN 462	99 cf	50 cf	149 cf
Oxygen	69 cf	35 cf	104 cf
		Total	861 cf

1. For this dive, air is used as a "pad" gas during decompression on the 200-160 fsw stops before switching to EAN 30 at 150 fsw. Oxygen breathing was initiated at 30 fsw to facilitate off-loading. Note that this is not a recommended procedure for technical dives in the absence of a full face mask.

The Saufley dive is close to the limits of open water, open circuit scuba, the so called "Open Circuit Barrier." Clearly significant surface support including safety divers and an adequate guideline system are required for the operation as the dive team would be unable to carry all the tankage required for autonomy.

Operationally this dive might be conducted using large volume doubles (greater than 300 cf) in addition to a bottom mix stage. Tank overpressurization would likely be required to achieve the necessary volumes. In addition divers would carry or have available an air, EAN 30, EAN 46 stage and oxygen bottle for a total of seven cylinders making DPVs an essential tool for this operation. "Quads", discussed in the equipment configuration section below, would be a viable way to configure this tankage. In addition, safety divers would be used to supply divers with the addition gas required for the dive and to provide needed redundancy. Operationally this might be carried out as follows.

The dive team could be met by safety divers at their first decompression stop of 210 fsw who would swap out the used bottom mix stages with air cylinders used as a pad gas during the decompression. Shadowed by the safety divers, the team would then be carrying sufficient gas to ascend through about their 60 fsw stop at which point the safety divers would ferry additional intermediate mix, in this case, EAN 46, to the divers if it were needed. An alternative would be to supply the divers with EAN 46 (for the 80 fsw through 30 fsw stops) through a surface supply system. This would have the advantage of reducing the number of cylinders carried by the divers however it raises the issue of self-sufficiency in the event the dive team failed to reach the ascent line. Note that if this dive were conducted in a cave, the intermediate gases and oxygen needed for the dive would likely be pre-staged in the cave prior to the exploration run reducing the number of cylinders to be carried by the dive team.

Gas Planning Summary

Table 5.9 and the accompanying Fig. 5.5 summarize how gas requirements and hence operational complexity increase with depth for these fixed duration dives. As shown in both the table and graph, though bottom mix requirements increase nearly linearly with depth (a fourfold increase in depth produces about fourfold increase in bottom mix requirements), decompression gas requirements including both intermediate mixes and oxygen increase exponentially by about a factor of 100. Note that these represent planning values and not actual consumption. As the depth of the dive increases meeting decompression gas requirements generally becomes the limiting factor. Note also that as discussed above, though oxygen is certainly a critical component of these dives carrying sufficient supplies isn't an issue even when reserves are considered. For that reason oxygen ponys are usually sufficient for most open water operations though an increasing number of divers are carrying full stage bottles. You can never have too much oxygen on hand during an operation.

Note that a similar analysis could be performed by fixing the depth of the dive, say at 70-90 fsw, and analyzing changing gas requirements (in this case an enriched air mix with oxygen for decompression) as the duration varies, for example from 20 to 240 minutes. This would give the reader a good feel for gas requirements for long "shallow" dives.

Table 5.9

Gas Planning Summary

"20 Minute" Bottomtime

Depth(fsw)	90 fsw	180 fsw	250 fsw	310 fsw	425 fsw
Gas Requirements:
Bottom Mix:	108 cf	195 cf	258 cf	357 cf	443 cf
Intermediate Mix:	3 cf	34 cf	83 cf	135 cf	310 cf
Oxygen:	n/a	17 cf	39 cf	63 cf	104 cf
Total Reqmts:	111 cf	246 cf	380 cf	555 cf	857 cf
# Mixes	1	2	3	4	5
# Cylinders	1 cyl.	3 cyl.	4 cyl.	6 cyl.	7 cyl.

Fig. 5.9-5.10: Gas Planning Analysis

Gas Management In Caves

Though the same method is used to calculate gas requirements for both open water and cave diving, gas management in caves is much more exacting due to the nature of the physical overhead environment. In the event of gas shortages or an equipment failure in a cave, divers are rarely able to ascend to their decompression gas or gas cache and significant return transit time is typically required. As a result the cave community has had to rethink the problem of how much gas to carry and how to best plan its use. The philosophy that has evolved is that each dive partner must carry sufficient gas and equipment for two divers to successfully exit the cave in the event one diver has a catastrophic gas failure at the point of the team's furthest penetration. Strict adherence to this rule along with well rehearsed bailout procedures are critical to the team's survival.

Originally developed and promulgated by the cave community to reflect these unique requirements, the rule of thirds is generally the starting part for gas management in caves though it is often modified downward based on environmental and operational considerations. For example, when diving the downstream or "syphon" side of a cave system, i.e. "going with the flow," the thirds rule would leave insufficient gas for a reliable bailout if a catastrophic gas failure occlude at the point of maximum penetration since the exit would be conducted against the flow. What's more is that the stress of the situation would likely increase consumption rates. Similarly when conducting an operation with DPVs, the thirds rule would likely prove in adequate in the event of a combined gas and DPV failure since the diver team might have to swim their way out. In these cases, gas rules would likely be modified downward to "turn the dive" at one fourth or even one sixth depending on the operation to insure an adequate team reserve. Note that though these rules have been developed around a two person dive team, a team of three provides greater safety and flexibility from a gas management perspective as more reserve gas is available.

Decompression gas reserves must also be considered by the dive team. In the event a single mix, for example an enriched air mix, is planned to be utilized for both the working and decompression phases of the dive, decompression gas requirements including a reserve must be subtracted from the total gas supplies carried by each team member before applying the appropriate gas rule. Better yet, the National Speleological Society-Cave Diving Section (NSS-CDS) recommends the placement of a decompression stage bottle with at least 1.5 times the anticipated gas requirements one stop deeper than the team's first anticipated stop. Note also that because the decompression phase of the dive can typically be carried out while resting against the floor or a surface of the cave versus hanging in a potentially surging open water column, divers generally have greater control over their breathing rates and can more readily stretch out these reserves in the event of an emergency. Similarly when conducting a multi-mix operation, oxygen and other intermediate decompression gases can be staged at the appropriate location in the cave. This practice of staging decompression gas highlights one of the unique aspects of cave diving.

In spite of the rigorous gas management challenge imposed by the cave environment, the ability to reliably stage cylinders along the dive team's planned route, in this case along the "continuous guideline" to the surface, offers significant advantages not available to open water divers.

Freed from the requirement of carrying the team's decompression gas, the standard practice during open water operations, diver-carried stage bottles are typically used by cavers to extend distance and duration. Operationally, the team breathes down their stage bottles according to a predetermined plan and "drops" the cylinder leaving it securely placed or clipped along the team's exit route after switching to their main gas supply. This procedure can be repeated numerous times and dives involving two to four stage bottles are not uncommon. What's more is that gas depots containing additional stage bottles and even redundant DPVs can easily be "pre-staged" in the system on prior set-up dives to maximize the team's safety. As a result, in comparison to open water operations, much longer duration dives are generally feasible in the cave environment and require less immediate surface support.

Using the same planning parameters discussed in the dive series above, several cave diving planning examples illustrate theses differences.

Table 5.10

Dive profile: Devil's Eye, Ginnie Springs

Max. depth: 105 fsw

Bottom Time: 80 minutes

Total decompression: 37 min.

First decom stop: 30 fsw

Tables: Submariner Research Ltd. EAN 32 w/O2

Gas Mix	Planned Cons.	Gas Reserve	Total Reqmts.
Bottom Mix:
EAN 32	333 cf	167 cf	500 cf
Intermediate Mix:
EAN 32	7 cf	4 cf	11 cf
Oxygen	28 cf	14 cf	42 cf
Total	368 cf		553 cf

Assuming that each member of a two person dive team carried 300 cf of EAN 32 in their doubles, the dive could be conducted using dual EAN 32 stages to extend their range within the thirds rule, each containing about 105 cf of gas. Oxygen cylinders would be staged near the mouth of the system for use during the 20 and 10 fsw stops. Operationally the team would initiate the dive on their stage bottles and switch to their doubles after the second stage had been breathed down to a third and clipped off to the line. Note that Devil's Eye is an upstream system i.e. the team's exit is made in the direction of the flow, hence the thirds rule is generally more than adequate for swimming divers.

In practice, though the maximum depth of Devil's Eye is 105 fsw the average working depth is closer to 80 fsw. Recalculating bottom mix requirements for an 80 fsw working depth yields a planned consumption of about 272 cf plus a 136 cf reserve based on the thirds rule, for a total requirement of 408 cf versus the more conservative calculation of 500 cf. Adding the 10-15 cf needed for decompression, the dive could easily be accomplished using large volume doubles and two 80 cf stage bottles. In either event, stage bottles would be dropped when the first member of the team reached their third, and the dive turned when thirds had been reached by either member of the team.

If the dive was conducted on scooters, a common practice at Devil's Eye, a one fourth or one fifth rule would likely be used for conducting the dive i.e. stage bottles would be breathed to the this planned level and the dive would be turned when divers hit either fourths of fifths on their doubles. If instead the plan was to reach a specific location in the cave instead of simply turning the dive when the appropriate gas levels were reached, the reserve calculations above would need to reflect the gas rule used. If planned bottom mix consumption was 333 cf and a fourth rule was to be used, bottom mix reserves would be 333 cf for a total gas requirement of 666 cf i.e total round trip gas consumption represents 1/4 x 2= 1/2 of the gas carried . In order to have this type of volume available the team might consider staging a depot.

Table 5.11

Dive profile: The Sullivan Connection

Max. depth: fsw

Bottom Time: minutes

Total decompression: min.

First decom stop: fsw

Tables: Hamilton Research Ltd.

Gas Mix	Planned Cons.	Gas Reserve	Total Reqmts.
Bottom Mix:
Intermediate Mix:
Oxygen
Total

OPEN CIRCUIT EQUIPMENT & CONFIGURATION

Because of the gas volumes involved, doubles and stage bottles are generally a requirement for most technical diving operations though diving "sets" vary depending on the specific application (operation) and the diving environment. The factors that must be considered in determining a proper set configuration for a particular operation are these:

1) Required gas volumes (determined by pre-dive planning)

2) Required level of redundancy

3) Bailout requirements

4) Environmental Constraints ex. physical overhead, restrictions, mobility/drag etc.

Though set configurations can vary widely among communities, much of the recent development and thinking behind hardware systems has been driven by the cave diving community which has served as a model for technical diving. Due to the unforgiving nature of the overhead diving environment, the cave community has been forced to deal with the issue of systems reliability and survivability and has evolved a basic approach to hardware configuration that is gradually being adopted by other communities and modified accordingly to suit their specific needs. At the heart of this approach is the principle of redundancy as the means for dealing with potential breathing equipment failures.

Redundancy

Breathing system redundancy is the key to self-contained diving safety. The idea is that the individual should be as self-sufficient as possible in dealing with an equipment failure. This is a non-trivial matter. The problem can best be summarized by applying a corollary of "Murphy's Law" to diving,"If a piece of life support equipment can fail, it will, at the worst possible moment." Redundancy is a means to protect against this inevitability based on the simplifying observation that most equipment failures are "independent events."

Sidebox: Survival Probabilities

If the probability of a second stage regulator failing on a dive is "X" and a diver carries two separate second stages, the probability that the alternate second stage will fail on the same dive is "X x X=X2." Assuming that the initial failure probability is small, for example a "1 in 100" or 1% chance, the probability of a double failure leaving the diver without a functioning second stage would be .01% or "1 in 10,000." Hence by carrying two second stages the diver can greatly reduced the odds of a complete second stage failure. This type of thinking is the motivation behind the "safe second" of recreational diving. Of course carrying a back-up second stage will do little good if the common first stage suffers a complete failure. In order to insure against a first stage failure, the diver would have to carry two completely independent regulators, for example mounted on a Y-valve with a single tank or in the case of doubles, a double's manifold. However, neither of these configurations would protect the diver in the event of a valve or manifold failure. And on it goes.

Table 5.13: Levels of Redundancy

Equipment redundancy can be thought of in levels as shown in Table 5.13 below.

Level Of Redundancy	Failure Coverage	Problems Not Covered	Typical Application
Second Stage	Second Stage Failure	First Stage Blown O-Ring Valve Failure	Recreational Diving
Regulator			Overhead Environments
Y-Valve Dual Manifold (DBLS)	Second stage First Stage Blown O-Ring	Burst disk Manifold Failure
Dual Manifold w/ Iso-lator		Manifold Failure
System
Single w/ Pony	All of the above and manifold/valve failure	Double reg failure	Deep recreational diving
Independent Doubles	" "		Overhead Environment
Doubles w/ Bailout Bottle	" "		Overhead Environment

To cover all contingencies, the diver would have to carry a completely redundant breathing system. However this is generally not feasible or practical for many technical dives and a balance is needed. Fortunately in the event of a failure, all that is needed is sufficient gas and functioning equipment to make a successful exit which simplifies the problem considerably. By applying the concept of redundancy to the diver's life support system and pre-planning for the worst, the individual diver can cover most contingencies with the dive team serving as an essential back-up. In this way the team can reduce the "odds game" to one of an acceptable degree of risk. The discussion below summarizes some of the more common methods of set configuration as practiced today in the technical community.

Single Tank Configurations

Though most technical dives require a minimum of double cylinders, a single tank set is sometimes used for convenience on short exploratory dives, set-up dives, for example, tying into a wreck or staging cylinders, and by safety divers supporting a technical diving team. Typically these dives involve minimum if any decompression. With the large volume tanks available today, for example 120's or 150's, a diver can easily conduct a 10-15 minute open water dive to 200 fsw with experience and can extend bottomtimes much longer at shallower depths.

DIN connectors are generally recognized as the standard among the technical diving community because of their greater reliability. In overhead environments such as a cave or wreck, a Y-valve is used in order to run two independent regulators providing first stage redundancy. Typically one of the regulators is equipped with a 5-7 foot long hose to facilitate gas sharing. In the absence of a Y-value, many divers elect to carry a bailout bottle, either a pony (13-40 cf) or a full stage bottle depending on the planned depth and time of the exposure. Since set-up and safety divers typically dive alone, a bailout bottle provides an important alternate gas source to the diver in the event of a catastrophic failure and can also be used to aid an "out-of-gas" team member. Generally the thinking today is that single tank divers should carry a bailout bottle when diving below about 100 fsw.

Though BCD jackets are still common when diving a single tank set, the trend today is to use a doubles harness and backplate system (with back mounted inflation) adapted for a single tank mount. This allows the diver to use the same harness system which is typically rigged to suit individual preferences and needs, independent of the tank choice. In addition, the harness system provides greater flexibility in clipping on and off stage bottles which is important to a safety or support diver and can be fitted with pockets for carrying diver accessories.

Figure 5.5 Diver w/Single Tank Set

Figures 5.5 shows a support diver outfitted with a single tank set configured for an open water dive. Note that gauges and the backup regulator are fastened down using clips and rubber tubing to provide for instant access and to minimize drag and the potential for entanglements. These are referred to as "danglies" by the cave community and a good deal of thought and effort is applied to eliminate them. Similarly, the diver's light system battery pack is mounted to the backplate in order to minimize protrusions. In this configuration, the diver breathes off his long hose which is wrapped under the battery pack and over the shoulder and carries a regulator/inflator for a backup. In addition a lift bag signaling device and reel are clipped off to D-rings on the tank. A 40 cf pony ("mini-stage") is clipped onto the harness to provide an alternate gas source.

Sidebox: Regulator Performance

Doubles

In contrast to the typical recreational diver's single "aluminum eighty," large capacity doubles have become the "basic set" used in technical diving. Though double 80's are still sometimes seen on wreck diving boats and in beginning cave courses, most technical divers have opted for steel tanks with a capacity of at least 100 cf relegating aluminum 80s for use as stage bottles. For most operations, a diver's bottom mix is carried in doubles and stage bottles are used to extend range or time and for carrying decompression gas though this is not always the case.

Though set configuration and placement have been strongly influenced by the cave diving community, rigging is largely a matter of individual preference and every community seems to have its own nuances which tend to be environment specific. What's more is that individual's rigging preferences seem to 'evolve" and change with time. Some of the more common configurations are discussed below.

Basic Double Set

As shown in Fig. 5.6, a basic double set begins with a DIN doubles manifold with independent valve shut-offs, dual high performance regulators (with long and short hoses) and a single submersible pressure gauge (SPG) mounted to the primary regulator. Notice that this set includes a "manifold cage" to protect the manifold and valves from collision.

Typically the diver's secondary regulator (with long hose) is worn against the chest on a necklace made of surgical tubing for immediate access. The long hose is usually wrapped neatly across the manifold or up and down along one of the tanks using elastic bands so that it can be rapidly deployed. Though some individuals choose to mount an SPG on each regulator this is generally considered "overkill." In the event of a primary regulator failure the dive will be turned and an immediate exit initiated independent of gas pressure readings. In this regard, the extra SPG only serves to add an additional gauge and hose to worry about. On the other hand, having a redundant buoyancy system, typically a drysuit and back mounted inflation system, is critical.

Figure 5.6 Diver w/ basic doubles set

As mentioned above, the emerging standard today is the use of a harness and back plate along with a back-mounted inflation bag referred to as a set of "wings." These have the advantage of minimal frontal bulk and drag and offer a system of pre-rigged D-rings for clipping on stage bottles and accessories. In this configuration, inflation hoses are rigged so that the drysuit and inflation bag are run off separate regulators. In this way, the diver maintains buoyancy capability even in the event that one regulator must be shut off. For some deep diving applications, divers use dual inflation bags (run off separate regulators) for additional redundancy when diving a drysuit, or in the event a wetsuit is worn. Independent suit inflation bottles are also growing in use, particularly for special mix applications to avoid thermal problems when using helium mixes (See Chap. 3, MIX, for a discussion on thermal considerations when using helium.). Suit inflation bottles are typically mounted to the back plate or to the doubles though this can create potential drag and entanglement problems.

Most technical divers opt for 35-75 watt plus "cave style" lighting systems which feature a hand or head mounted "light head" and an independent battery pack. The battery canister can be mounted to the back plate, belt or underneath the diver's tanks depending on the type of cannister and preference of the diver. Likewise, a system of D-rings and rubber tank bands are usually mounted to the tanks for carrying accessories such as a decompression or hand reel and liftbags.

Basic Doubles Set With Pony

Bail out or "pony" bottles (13-40 cf) containing bottom mix or oxygen for decompression are sometimes used in open water applications such as wreck diving and are typically mounted between the diver's doubles to minimize drag and potential entanglement. A typical set-up is shown in Figure 5.7 below. For some specialized applications, "dual ponies" have been used to provide self-sufficiency from the surface for deep mix bounce dives. These can essentially be considered "mini-quads," as discussed below. Figure 5.8 diagrams one such rig designed for deep reef collection work to depths of 300 fsw plus.

Figure 5.7: Double set w/pony

Figure 5.8: Deep Reef Set

Independent Back Mounted Doubles

Though double cylinders are usually manifolded together into a single unit, there are applications where independent tanks are preferable. Having two separate tank, valve and regulator systems increases redundancy by providing two completely independent gas sources (protecting against a doubles manifold failure which could result in a complete gas loss), and when rigged in "sidemount" configuration, presents a much smaller profile for negotiating tight restrictions found in both wrecks and many cave systems. Unfortunately these redundancy and profile advantages come at the expense of diver "task loading." The diver must continually "switch" back and forth between regulators during the dive in order to balance out gas volumes in each cylinder and maintain an adequate reserve. Figure 5.9 shows a wreck diver utilizing a back mounted independent doubles set up.

Figure 5.9: Diver w/ independent back mounted doubles

Sidemount

Originally developed for the tight low visibility sump diving that is common in Europe, sidemounts allowed spelunkers to more easily transport single cylinders through a dry cave to the dive site. In the U.S., the use of sidemount equipment has allowed exploration into small silty areas that were once thought impassable and has opened up entire new cave systems that were simply inaccessible with back mounted doubles. Sidemounts reduce the strain of carrying heavy doubles up steep inclines, lowering cylinders down into a hole, and making long walks through the woods to the dive site. In addition they make it possible to minimize silting in tight silty environments. Though to date sidemount systems have been limited to cave diving the potential benefits for wreck penetration applications has not gone unnoticed.

In a sidemount configuration, the diver wears the cylinders on the hips instead of the back.The cylinders are fastened in the middle with a snap to a harness at the waist. The necks are clipped off at the armpit using bunjii material ( a bicycle inner tube is preferred) so that the cylinders are forced to lay parallel to the divers' body . Adjustments are usually needed at first to insure a snug comfortable fit. By adding additional D-rings and bunjii tubing stage bottles can be easily accommodated.

Similar to back mounted independents, when diving with sidemounts, gas supplies must be balanced for adequate reserves throughout the dive. The regulator and SPG hoses no longer lay across the back and instead are clipped across the chest area. The management of these is critical for proper monitoring of gas supplies and switching regulators during the dive. Back-up and emergency equipment must be streamlined and tucked away to achieve the desired profile&endash; no thicker than the the two cylinders that lay along the divers' hips. Figure 5.10 shows a typical sidemount configuration.

Figure 5.10: Sidemount

Quads

For long deep exposures, particularly those associated with expedition-level pushes, carrying sufficient gas volumes to do the job becomes a major operational consideration. Fortunately, most of these dives are conducted in cavernous passageways or open water where restricted space is not the issue. According quads ("tank packs") are often used in conjunction with stage bottles in order to carry sufficient bottom gas, and that required for decompression, where self-sufficiency is the key. DPVs are generally a requirement when using a quad configuration to overcome hydrodynamic drag.

Figure 5.11: Quads set-up

A typical quad set-up, shown in Figure 5.11, consists of doubles, often 104's with a crossover manifold containing bottom mix, mounted to a pair of side tanks containing decompression gas, for example an enriched air nitrox and a bottle of oxygen, each with an independent regulator. Depending on the dive, the diver may also carry a bail-out bottle of bottom mix, and a second cylinder of intermediate mix for decompression&endash;six cylinders in all &endash; making the diver relatively self-sufficient. High performance regulators are the standard, as well as double buoyancy compensator bags.

Stage Bottles

With the advent of extended and special mix diving and the use of hyperoxic mixtures for decompression, stage bottles have become a standard component of the technical diver's basic rig and are used with each the configurations above for extending gas supplies and carrying decompression gases. Typically stage bottles are carried by the diver using clips and D-rings mounted on the harness and or can be "staged" along a diver's planned route. Figure 5.12 shows a single aluminum 80 configured for stage bottle use which includes dual second stages and the necessary clips. DIN connectors are standard and Y-valves are often utilized to provide additional first stage redundancy.

Pull Quote:

A Pony And A Prayer

Taking a cue from the underground, crossover manifolds and stage bottles are gradually upstaging the pony as the bailout system of choice among safety-conscious deep wreck divers. For good reason. As summed up by Mike Hanna, general manager of Ginnie Springs Dive Center, " A pony will give you enough time at 250 to say the Lord's Prayer ." Amen.

Figure 5.12: Stage bottle

BAIL OUT PROCEDURES

Whether diving in open water or the overhead environment of a wreck or cave a well thought out and rehearsed bailout procedure is essential for the survival of the team in the event of an out-of-gas emergency. One of the key differences between technical and recreational diving is that technical divers "expect" equipment failures and other emergencies and incorporate procedures to deal with them as a part of their dive plan. The considerations that must be addressed include having;

1) adequate redundant breathing equipment,

2) ample gas reserves in terms of volume and type to exit, and

3) the proper support equipment to insure that a safe exit can be conducted, for example, contingency tables, emergency lines, signaling devices etc.

Implicit in these procedures is the fact that potential out-of-gas emergencies can and will occur. Accordingly bailout procedures should be planned, discussed and regularly practiced prior to conducting the operation. Dealing with an incapacitated diver adds another level of complexity and is discussed in Chapter 7, Diving Safety.

As previously discussed, the thinking today is that divers should be "self-sufficient" to every extent possible and be able to deal with an out-of-gas emergency without assistance. Minimally this means that each member of the team carries a redundant breathing equipment, ample reserves for dealing with contingencies and the necessary support equipment. As described above, redundancy can be accomplished in several ways depending on the operation. This may involve using a Y-valve or carrying a pony bottle when diving singles, a dual outlet manifold with dual regulators when diving doubles, or dual independent singles in either a back or side mounted configuration. Each of these configurations offers varying levels of protection against failure. Redundancy must also be considered with regards to the diver's intermediate and oxygen supplies. Note also that self-sufficiency assumes that each diver adheres to their gas plan. Running gas supplies past the limit or tricks like relying on a pony bottle to make an exit from a wreck not only invite disaster for the diver in question but jeopardizes the safety of dive team as a whole.

Bailout procedures begin with an in-water "equipment check" at the surface&endash; prevention is the key to safety. The team checks each other for "bubbles" indicating potentially faulty breathing equipment, makes sure both the primary and back-up regulators are working and everything is in order. Any problems or a malfunction should be corrected at the surface before commencing the dive.

Properly equipped each member of the team should be able to deal with minor emergencies such as a first stage failure, a run away regulator or blown O-ring. In the event that such a failure occurs during the working phase of the operation, the diver with the failure would shut down the appropriate valves and or switch regulators or cylinders, the dive would be turned and each member of the team would exit on their own equipment. Typically the team would be carrying contingency decompression tables with a shortened schedule for handling such a situation. In addition, depending on the operation, the team might signal the boat and or safety divers to notify them that the dive plan had changed and or request assistance. There is one area where wireless communications would greatly improve dive safety. If an equipment failure occurs during decompression, the diver would again correct the problem and continue the schedule. Again the surface support team would probably be notified.

Though diver self-sufficiency is usually the first line of defense in an out-of-gas emergency, self-rescue is not always feasible.. The primary example is in the event of a catastrophic failure such as a manifold failure or a blown burst disk which resulted in a total loss of a diver's primary gas supply.

In open water diving, a catastrophic gas failure is usually easier to deal with then in the overhead confines of a cave, depending on the operation. In most cases the diver(s) usually has only to ascend to the depth where his or her intermediate gas, typically a nitrox mix, can be safely breathed, assuming the divers are carrying decompression gas. Note however that in special mix diving, prior planning and care must be given to the bailout gas to be used in order to avoid oxygen toxicity problems. Though there is typically 4-5 minute latency period before the onset of symptoms, bailing out on an enriched air or oxygen stage at 250 fsw is hazardous and should only be attempted in the absence of other alternatives. Even though open water diving generally offers fewer restrictions compared to the overhead confines of a cave, an immediate ascent is not always feasible. In the event that the team's decompression gases are staged, for example near the anchor line on the wreck, the team would likely have to transit back to their stage bottles before beginning their ascent or be stuck without adequate gas for decompression.

These scenarios, i.e. a catastrophic failure, are the reason that many wreck divers carry a pony "bailout bottle." The problem is that a pony bottle offers minimal bailout volumes (time) at depth particularly if a failure ocurred during the penetration phase of the run. In most cases, the dive team becomes the critical resource in the event of an emergency. The team provides the essential back-up that is needed to effect a successful bailout. This is certainly the case in cave diving where the diver in distress cannot immediately ascend to the safety of a decompression stop and lengthly transits are usually required. Under these circumstances, team gas management becomes critical.

Fig. 5.13: Bailout Volumes

Most gas management rules were designed to insure the survivability of a two person team. In the event one of the members of the team suffers a catastrophic gas failure, the remaining partner should have sufficient reserves for both divers to make an immediate effective exit. This is accomplished through gas sharing. The community standard today is for each diver to have at least one regulator equipped with a five to seven foot hose in order to facilitate this procedure. Using the long hose the team can exit the confines of a wreck or cave in single file or swimming side by side in open water. The extra length of the hose greatly reduces the stress of having to "hold on" to one's diving partner during the exit as required with recreational dive equipment. What's more is that the long hose makes it possible to share gas while scootering. In this case a tow line is usually also utilized to insure the DPVs don't separate during the exit run leaving the distressed diver without gas.

A three person team adds additional flexibility and many people believe it represents the safest dive team configuration. In this case the out-of-gas diver usually alternates sharing between the two partners in order to balance the remaining reserves thus protecting the team's options against the possibility of further incidents during the exit.

For some operations a "one person team" is not only desireable but appropriate, however "solo diving" places special restrictions on divers. The common practice is for solo divers is to carry an independent "fully redundant" gas supply sometimes referred to as a "buddy bottle" (an independent stage bottle containing bottom mix) or a pony, though these are limited in volume as discussed above. Contingency planning is obviously essential here to insure that the diver has adequate equipment and gas supplies to recover in the event of an out-of-gas emergency. Solo diving is generally limited by the ability to meet this contingency. In addition, for many solo operations good topside support is essential.

The Future of Open Circuit Systems

The major challenge today for the technical community is to do a much better job of planning and managing critical gas supplies. This means pre-planning gas requirements prior to conducting the dive and giving careful thought and analysis to reserves, equipment and bailout procedures. To not do so is hazardous and is likely to result in injury or death. Clearly recreational diving thinking and methods are inadequate to handle technical-level exposures and in some cases provide marginal safety even on extended recreational dives, for example in the 100 fsw plus range. The recreational diving community would do well to follow the lead of the technical community in this regard and could like improve diving safety by adopting more rigorous planning and operational procedures4 .

Due to the economics and training requirements, it will likely be some years before closed circuit (C2) technology achieves broad acceptance in technical self-contained diving. In the meantime, many people believe there will be one final evolution in the development of open circuit systems: the incorporation of full face masks (FFM) and blocks common in professional and commercial diving applications. In fact the conversion to FFM technology is already well underway in the technical diving community.

Full face masks and the use of blocks for gas switches offer significant safety and performance advantages to the self-contained diver in particular protection against immediate drowning in the event a diver loses consciousness. What's more is that they will enable self-contained divers to effectively incorporate wireless communications systems into their operations. The advantages and use of FFM technology and communications is discussed in Chapter 8, Emerging Technologies.