Closed Circuit Hardware Review

by John L Zumrick

Closed circuit systems made their first foray during the second world war where they were used by underwater swimmers to support clandestine "bubble-free" operations. These first operational units were oxygen rebreathers. Because the divers breathed oxygen, their maximum operating depth was limited to less than 50 fsw (15 msw) and for longer dives to less than 20 fsw (6 msw) to prevent oxygen convulsions.

The general construction of a closed circuit oxygen rebreather is shown in Figure F1. The system contains a breathing circuit consisting of a breathing bag (7) or counterlung, a carbon dioxide absorbant canister (6), and inhalation (3) and exhalation hoses (4) connected to a mouthpiece. The diver inspires oxygen from a breathing bag to the inhalation hose through a one way valve into the mouthpiece. The divers exhalation is channeled through the exhalation hose where it passes through the carbon dioxide absorbant cannister, and is returned to the breathing bag. The carbon dioxide produced by the diver is absorbed in the carbon dioxide absorbant cannister via a chemical reaction with materials such as calcium, or lithium hydroxide which manufactured under various trade names.

Because a portion of the oxygen inspired by the diver is consumed and converted to carbon dioxide and absorbed by cannister materials, the volume of gas in the breathing bag decreases with time. When the breathing bag reaches a small enough volume on inhalation, a trip valve (8) similar to a scuba regulator is opened and oxygen is added to the breathing bag. This serves two functions, it restores oxygen consumed by the diver, and it also maintains adequate breathing bag volume which is reduced by depth changes or gas leaks. An overpressure exhaust valve (9) prevents overexpansion of the breathing bag.

When using an oxygen rebreather it is important to purge the system with oxygen exchanging all air in the apparatus, and in the divers lungs, with oxygen. Since the nitrogen in air is not consumed by the diver to an appreciable degree, this nitrogen can accumulate in the system preventing the breathing bag from collapsing and adding oxygen to the circuit. In this case, the diver would continue to breath a increasingly hypoxic mixture with little symptomatic warning of impending unconsciousness. As a result this system can only be used with oxygen as the supply gas.

Oxygen rebreathers have evolved since WWII and are still in use primarily by combat swimmers because of their small size, simplicity, and bubble free operation. These systems were used extensively in the exploration of the shallow cave sumps of Great Britain until the late 1950s when they were replaced by side mounted scuba cylinders. Some scientific divers are also using this apparatus to observe shallow water sea life which are more easily approached due to the absence of bubbles. Oxygen rebreathers are also beginning to be used by the law enforcement diving community.

Semiclosed circuit rebreathers were developed somewhat later. Unlike the oxygen rebreathers these apparatus can use gas mixtures such as nitrox or heliox. The breathing circuit, Figure F2, is similar to that of the oxygen rebreather and contains a breathing bag (8), carbon dioxide absorbant canister (5), exhalation (4) and inhalation (3) hoses, and a demand valve gas addition system (9). However, in this design the demand valve gas addition system is used only to make up breathing gas volume lost from leaks or depth changes. To replace the oxygen consumed by the diver, a gas mixture is added to the breathing circuit at a constant mass flow rate independent of depth (23). The rate is calibrated to adequately replace the oxygen consumed by the diver. A similar volume of gas is then exhausted from the breathing bag through a specially designed exhaust valve (10).

Since oxygen is added at a constant rate but the diver's oxygen consumption varies with exertion level, the oxygen concentration in the rig varies, being higher when the diver is at rest and lower during heavy exertion. The concentration of oxygen in the mixture and the rate of gas addition must be carefully matched to insure that toxic or hypoxic concentrations of oxygen will not be encountered during the dive. Since the gas inflow rate cannot be changed during the dive, the gas mixture selected can only be used over a limited depth range. Thus, while the physical construction of a semiclosed breathing apparatus is simple, dive planning and rig set up can be complex particularly when the goal is to maximize gas efficiency. A thorough knowledge of their design and operating characteristics are essential to their safe use.

Several years ago at a NAUI IQ , a vendor demonstrated a simple semi-closed system designed for sport divers using air as the supply gas. They clearly understood the design of the apparatus but failed to appreciate how to set one up and calculate the necessary gas flow rate. In this particular unit, they chose a gas flow which resulted in a hypoxic gas mixture when the diver was shallow. This was dramatically demonstrated by a potential customer who became unconscious while using the rig at a demonstration in the hotel pool. Needless to say this rig never made it to market. Because of these types of limitations, the use of semiclosed circuit apparatus has almost been entirely replaced with mixed gas closed circuit breathing systems.

The breathing circuit of a fully closed circuit mixed gas scuba is almost identical to that of a semi-closed circuit rebreather. The major difference is that oxygen is added by an electronically controlled solenoid valve rather than a pneumatically controlled mixed gas constant flow orifice. When the oxygen partial pressure within the breathing loop drops below a preset level (called the set point), an electrically controlled solenoid is activated and oxygen is added to restore the mixture in the breathing loop to its preset level. Because oxygen is the only gas added, there is no excess gas exhaust from the rig, and thus no gas is lost to the sea.

The oxygen sensors and electronics control the partial pressure of oxygen in the rig at a preset level regardless of depth. Reviewing Dalton's law of partial pressure, this means that as depth increases the percentage of oxygen in the rig drops. Table T1 shows the effect of depth on a rig set to control the oxygen partial pressure at 0.7 atm. At the surface the rig will continue adding oxygen to the breathing bag until the oxygen concentration is 70%. At 297 feet the rig will not add oxygen until the concentration of oxygen falls to 7%.

Controlling oxygen to a constant partial pressure has two major implications in dive planning. First since the percentage of oxygen varies decompression must be calculated differently. Suppose we were using the rig with air as our "make up" or diluent gas source. Referring to the table you will notice that at depths less than 66 feet the percentage of oxygen is greater than air and hence the amount of nitrogen proportionately less resulting in a lessened decompression penalty. However, deeper than 66 fsw decompression penalty is increased. The second consideration relates to nitrogen narcosis. When using air or other nitrox mixture as the make up gas, diving deeper than 66 fsw results in a greater narcosis than air at the same depth. For example, the narcotic effect of using such a rebreather at 198 fsw is equivalent to breathing air at 230 fsw.

Under normal operation no gas should be lost from the rebreather to the water. This high gas efficiency should allow a rig of relative small size, to achieve a long duration that is unaffected by depth. In actual practice this is only partly true. The duration of a closed circuit rebreather is limited by the capacity of the battery to supply power to the electronics, the quantity of make up gas and oxygen carried, and by the duration of the carbon dioxide absorbant canister. Due to advances in electronics, battery power which was often a limiting factor in early designs is seldom a problem today.

The amount of oxygen that must be carried to support diving operations is relatively small. A diver swimming at 2 knots consumes about 1.5 liters of oxygen each minute. This rate of oxygen consumption is unaffected by depth. Thus, an exercising diver will consume about 3.2 cubic feet of oxygen per hour. As a consequence only a small oxygen cylinder is needed while still allowing durations of six hours or greater provided precautions are taken to avoid unanticipated gas loss.

In routine use, diluent or make up gas volume can prove to be a significant limiting factor in rig duration. Once the make up gas is depleted there is no way to maintain the breathing bag volume to compensate for gas lost through leaks, or as a result of depth change. As a diver descends make up gas is added to the breathing bag to maintain its volume. When the diver ascends the excess breathing bag volume is vented to sea. Thus a sea-saw type profile can result in the depletion of diluent. In addition, gas is lost in mask clearing and from small gas leaks around the mouthpiece. The volume of these losses is greater with depth. As a result, careful monitoring of make up gas supply and care used to minimize gas lost in mask clearing and leaks around the mouthpiece are essential when using a closed circuit system. Moreover, if gas is lost under conditions where the oxygen concentration in the breathing bag is less than the concentration of oxygen in the make up gas mixture, additional oxygen will be added. Thus, gas leaks can cause a decrease in rig duration both due to loss of makeup gas and oxygen.

The most difficult performance parameter to characterize in a rebreather is the duration of the carbon dioxide absorbent canister. Canister duration is dependent on multiple factors such as the rate of carbon dioxide production, water temperature, depth, and the type of absorbant used. Absorbants are available in various porosities and water content, all of which may effect the performance of a canister.

Cold temperatures markedly decreases the chemical activity of an absorber and the duration that it will remove carbon dioxide. Increased gas density as a result of depth and the resulting cooling of the canister will also decrease cannister duration. Tests studying the effects of all these factors and canister duration are limited. The work that has been done suggests that canister duration is not a linear function of carbon dioxide production. Thus, doubling exercise rate may reduce the canister duration by more than one half. Unfortunately, most manufacturers have limited resources in which to test canister duration in a rigorous manner. Consequently careful dive planning is essential particularly during cold, deep, high exertion dives, since these are most likely to tax the canisters capacity.

The main hazard in using mixed gas rebreathers are hypoxia, cerebral oxygen toxicity, and chemical burns. If the oxygen addition system fails hypoxia and unconsciousness are distinct possibilities. Hypoxia produces minimal symptoms and may not be recognized by the diver prior to unconsciousness. High oxygen levels may occur during rapid descent with an improper diluent gas mixture, going deeper than is safe for a make up gas mixture, or failure of the oxygen solenoid in the open position. This can result in an oxygen toxicity seizure. As a result it is essential that these rigs have a primary and backup oxygen level displays.

If a significant amount of water leaks into the canister the rig may become unusable. Smaller amounts of water may result in blockage of gas flow through the canister and as a result high breathing resistance. A large leakage may cause a "caustic cocktail" which if inhaled will result in chemical burns though this is rarely a problem in most current models. Since a leak is always a possibility, planning to deal with such problems is necessary.

Today, the cost of closed circuit system is high, but if production expands the price can be expected to decrease. However, in addition to the purchase price, the cost of operation, support equipment, and maintenance must be considered. Consumables alone, including absorbent, oxygen and diluent can run $30-50 per dive, not to mention regular maintenance.

Dr. John Zumrick is an active cave diver and practicing anesthesiologist with the US Navy. Prior to serving his residency at Bethesda Naval Hospital, he served as a medical officer at the Navy Experimental Diving Unit at Panama City, Florida. He can be contacted at: 1588 Chain Ferry Way, Orange park, Florida 32073.

T1:

The Results of Maintaining A Constant Oxygen Partial Pressure (Set point of 0.7 atm) Versus A Constant Percentage.

Depth % Oxygen Equivalent

Air Depth

Surface 70 ---

33 35 21

66 23 63

99 17.5 105

132 14 147

165 11.6 188

198 10 230

297 7 355