A Physiological Primer on Rebreather Technology

By R.W. "Bill" Hamilton

What is a Rebreather?

Unlike most of the names technical divers use for their equipment, the word "rebreather" is both simple and descriptive. A rebreather is a breathing system that enables the user to retain and reuse some or all of her expired gas.

To be suitable for reuse, the expired gas has to be processed to some extent&endash;a function performed by the portable processing unit that comprises part of the rebreather. In contrast, a device that saves and reprocesses the expired gas remotely from the diver is generally called a "reclaim" system. These are designed for commercial bell operations and are not of interest here.

Humans operate optimally at an oxygen partial pressure (PO2) of about 0.2 bars (or atmospheres)1 with some "inert" background gas making up the balance, and without too much CO2. The rebreather reuses expired gas while maintaining these limits.

Even though oxygen is the most vital gas physiologically speaking, the inert gas is also important in this discussion because oxygen must be diluted with some other gas when at an absolute pressure much beyond about 1. 5 bars (in terms of depth, beyond bout 20 fsw or 6 msw)2. Of course, the inert gas is also important to decompression, since it is the source of decompression problems, and technical diving includes a lot of tricks to try to reduce it.

The inert gas compnent is crucial in rebreathers, because its’ conservation is the main purpose of the unit. In addition to conserving inert gas, the rebreather has several other tasks. The first is to provide an appropriate oxygen level. In addition it has to remove carbon dioxide (CO2). Finally, it must provide a space for the diver to breathe in and out of.

Carbon Dioxide Removal

Carbon dioxide is a normal product of body metabolism. It is usually given off at a level of about 0.8 times the amount of oxygen consumed. For practical purposes, a rebreather has to remove about one liter of CO2 for each liter of oxygen consumed in the body. This is not the amount of oxygen breathed, but the amount consumed.

Carbon dioxide is relatively easy to remove, and most rebreathers do this well during low or moderate work levels by passing the expired gas through a canister filled with chemical absorbent such as soda sorb. However, when the canister gets cold, the absorbent becomes far less effective&endash;enough so as to create a serious problem under certain conditions. This on-going problem has been dealt with by redesign of the chemistry and physical structure of the absorbent, or by warming the canister&endash;itself a difficult task.

Most standards call for CO2 to be kept below about 0.5 kPa (they usually say "below 0.5% sea level equivalent," which is the same as a partial pressure of 0.5 kPa).3 Lower CO2 levels than this are no problem, and higher levels to about 3 kPa may be distracting, but are tolerable, although they can lead to serious problems if the diver exercises and the level increases. Excess CO2 causes an increase in the urge to breathe, and above levels of 15 kPa or so can cause severe narcosis, unconsciousness, and convulsions.

Oxygen Control

The task of controlling oxygen is much more complex than regulating CO2 levels. This gas has to be kept between strict upper and lower limits for physiological safety, but there is a strong incentive to run the oxygen level as high as possible to improve decompression.

While the oxygen level can be as low as the familiar 0.21 bars PO2 (21% at sea level), there are advantages to having it higher. First, should the level fall below about 0.10 to 0.12 bars, the diver may suffer symptoms of hypoxia (oxygen starvation). Below this, it can cause unconsciousness, and if the oxygen level gets too low, it can be fatal. Maintaining a PO2 higher than 0.21 bars makes hypoxia less likely.

In the other direction, it is necessary to keep the oxygen below the level that could cause oxygen toxicity. The degree of any oxygen toxicity is a function of both oxygen level and duration of exposure. The main toxicity problem is a neurological one: the risk of a convulsion. This type of nervous system (CNS) toxicity is a relatively short-term effect.

Another manifestation of oxygen toxicity is a general effect on much of the rest of the body besides the central nervous system&endash;particularly the lungs &endash;resulting from longer exposures at somewhat lower levels of PO2 than cause convulsions.

For optimal closed diving, a technique that has been found to be effective is to maintain a PO2 of near to but no greater than 1.4 bars. This is safely below the threshold for CNS toxicity, and this level can be tolerated for the duration of all practical rebreather runs4. It gives near-optimal decompression because the oxygen is about as high as can be tolerated for the entire run, but there is no concern about CNS toxicity as long as the rebreather works properly. (Note that the USN currently specifies a "set point"&endash;the target PO2 level&endash; at 0.7 bar, though they are considering raising this to 1.2-1.3 bar&endash; see Oxygen Tolerance Management by Richard Vann pg. XX&endash;ed.)

The Counterlung

One other function of a rebreather is that it has to provide a "counterlung"&endash;a kind breathing bag for the diver to breathe in and out of. This cannot be a rigid space, and it has to be as large as the largest expected breath. In addition to this counterlung, the rebreather hardware must include absorbent canisters, a means of regulating gas flow, a housing pack of some sort, gas storage, and a mouthpiece or mask.

Regulation of the rebreather’s counterlung function is affected by changes in depth, and this can be the source of some problems. As depth changes, the rebreather unit must adjust to both a change in the gas volume and a change in the oxygen fraction in order to maintain counterlung volume and a constant PO2. Thus, ascents cause a release of bubbles (since gas cannot be put back into the high pressure containers) and descents require addition of gas to maintain system volume. As a result, too many depth changes can deplete the gas supply even though the diver does not actually use gas. Another problem is the placement of the breathing bag relative to the lungs. If it is above the lungs, it is harder to breathe; and if below, the gas is under slight positive pressure. This, of course, may change when the diver shifts position.

Types of Rebreathers

In general there are two main categories of diver-carried rebreathers: fully closed and the semiclosed. The current fully closed rebreathers all have oxygen controllers that sample the gas in the breathing circuit and add oxygen or inert gas as needed by operating a solenoid valve or its equivalent.

The semiclosed unit works by feeding an oxygen-rich mix into the breathing loop at a rate adjusted to match the consumption by the diver. Semiclosed units are of two main types: those that control oxygen input by flow control, such as passing the gas through a calibrated orifice, and those that use the counterlung to adjust the gas by a mechanical ratchet or bellows arrangement.

Systems with oxygen controllers normally use pure oxygen for the oxygen supply, but rather than use pure inert gas they usually rely on a diluent mixture with a small amount of oxygen in it that can sustain life in the event of a failure of the oxygen supply. As a result, the operating range of these units is generally not limited by the gas mixture. However, oxygen-controlled rebreathers are sensitive to exercise rate, and the design must prevent a high oxygen demand from depleting the breathing loop of oxygen.

Semiclosed units usually use only one gas mix, and a given mix is limited to a specific depth range in order to stay within oxygen limits. The mix used is somewhat higher than the desired oxygen level and is bled into the loop at a controlled rate that must be somehow matched to the exercise level and hence the oxygen consumption of the diver. These units are much more sensitive to oxygen demand than units with an oxygen controller.

Still another type of rebreather is the pure oxygen unit. This apparatus needs no oxygen controller nor inert gas, but is limited in depth because of oxygen toxicity. This type of unit seems to have fewer applications for technical divers.

What Are Rebreathers Good For?

The most obvious use of the rebreather is to provide extended diving time independent of gas supply. This in turn allows longer bottom times than can be obtained with carried gas, and makes decompression dives far more feasible than they are with scuba.

Once decompression is a factor, the optimal gas mix with the right tables makes decompression about as efficient as it can get. Having no need for predetermined gas mixes tied to bottom depths makes it easy, at least with fully closed units, to use gases other than nitrogen for the oxygen-diluting gas.

Candidates for alternative diluting gases are helium and, for some, neon. While neon is far too expensive for normal open-circuit scuba diving, the gas conservation of a rebreather makes gas costs just a small part of the overall cost when diving with a rebreather.

The U.S. Navy has developed extensive procedures using nitrogen as the inert gas for shallow rebreather dives. But a case can be made that one would be better off using helium for even the shallowest dives, at least those in which decompression is a factor.

Another factor in rebreather use&endash;at least the fully closed models &endash;is that they do not make many bubbles. This has obvious military implications, but it can also be important in caveand scientific diving and photography.

Rebreathers also have some negative aspects. First is the cost: current rebreather units range from about $15,000 to $40,000. This is likely to change as the market develops. Not far behind are their complexity, need for maintenance, and the extra training required. Complexity brings with it more places for technical failure. This risk may be compounded by the fact that there is no redundancy in gas supply on most current units: no spare octopus for a dive partner, and no spare gas for the wearer. In addition, the units are bulky and heavy.

In some situations the extra endurance provided by a rebreather merely shifts the factors limiting the dive from decompression and gas supply to thermal exposure. For instance, in really cold water there may be no good way to provide adequate thermal protection to take advantage of the diving time allowed by a good untethered rebreather.

To date, the biggest user of rebreathers, both closed and semiclosed, has been the military. Their use generally has involved swimming relatively long distances at relatively shallow depths. In this circumstance, there is no real need for redundancy or even a bailout; the surface is a safe haven in most cases. Another important application is ordnance disposal and mine clearance. Here, greater depth may be needed, and there is a need for silence, no bubbles, decompression, and non-magnetic construction.

So far, the commercial diving world has not embraced rebreather technology. There are a number of reasons for this, including the matters of cost, complexity, training required, and perceived reliability. But probably the most important is that rebreathers just don't fit into the established patterns for typical commercial diving work. Recently, however, a serious interest in closed circuit technology has developed for lockout from saturation at depths beyond 300 msw. In the future, closed circuit "bail out" systems are likely to become a standard in deep saturation work. In that environment all the gas a diver could carry would be good for only a couple of minutes of open-circuit breathing.

Rebreathers make a lot more sense for scientific diving. It may not be possible to replace a particular archaeologist or biologist on a specific project, and many tasks require long bottom times. In addition, the absence of bubbles may be important in some instances.

Technical Diving Applications

While rebreathers may offer real potential for technical divers, they are not generally available to very many people right now. Of the two types of systems, closed circuit shows the greatest promise for technical applications. Semiclosed rebreathers are simpler and less expensive and might meet the needs of some, but they are sensitive to exercise level and depth, and do not allow the optimal PO2 for decompression purposes.

To date, practically all of the operational experience has been with military rebreathers. This has made it possible for these units to be developed and used extensively without a bailout capability built into the system, since most of their use has been in shallow water where a bailout bottle is not normally needed.

In contrast, most technical applications involve diving to depths that makes a carefully planned bailout system essential. This means having enough gas of the right composition to get to the surface or another gas source under all conditions of operation. This must be factored into rebreather operations.

Clearly, considerable work remains before rebreathers can be readily embraced by the sport diving community. But in time, they will undoubtedly become a important tool for the few who can afford the cost and training necessary. In the meantime, closed circuit will remain on the diving frontier: unavailable to most, and demanding to those who do gain access to them .

A contributing editor to aquaCorps Journal, Dr. RW Bill Hamilton is a diving physiologist and principal of Hamilton Research Ltd. with over twenty years of decompression management experience in then hyperbaric and aerospace industries. He can be contacted at: 80 Grove St., Tarrytown, NY, 10591 USA, f: 914.631.6134.

This article was originally titled, "Technologically Inspired: The Closed Circuit Rebreather" and first appeared in aquaCorps Journal, N2, "SOLO" JUN90.

Footnotes:

1. The pressure exerted by each gas within a mixture of gases is called its partial pressure and depends on the fraction of that gas present in the total mixture. Mathematically, the partial pressure of a given gas equals the total pressure of the gas mixture times the fraction of that gas present (Pp = total pressure x fraction).

At sea level, the fraction of oxygen in air is 0.21, or 21%. The total atmospheric pressure at sea level is defined as 1 atmosphere (atm), so the partial pressure of oxygen (called PO2) at sea level is 0.21 atm or 0.21 bars (1 atm = 1.013 bars, so for physiological purposes they are more or less interchangeable).

Obviously, as depth increases and total pressure grows, so does the partial pressure of gases in the breathing mix. This is crucial, since the physiological effects of oxygen and carbon dioxide are very much a function or partial pressure, as is the narcotic effect of nitrogen.

2. Feet of sea water (fsw) and meters of sea water (msw) are actually pressure units used to measure depth. Common definitions are:

1 fsw = 1/33 standard atmosphere

1 msw = 1/10 bar

1 msw = 3.2586 fsw

3. the kPa, or kilopascal, is a metric pressure unit with great utility. A pKa is 1/100 of a bar - very close to 1/100 atm - and this makes it handy to use. In time, most physiological pressure will use kPa.

Gas costs for open-circuit diving may be high sometimes, but they rarely approach the other operational costs of a job. In most diving situations, it makes more sense to simply use more than one diver than to put up with the longer decompression times and higher risks associated with extending the time of one diver as a rebreather would allow.

4. For a method of calculating allowable exposures at different levels of oxygen, see R.W. Hamilton, Tolerating Exposure to High Oxygen Levels: Repex and Other Methods, Marine Technological Society Journal, Vol. 23, No. 4, 1989; or see the U.S. Navy Diving Manual, 1981, Figure 9-20 and Section 15.2.1, which has been reprinted in the NOAA Diving Manual and in many other places.