Unique failure modes of semiclosed circuit constant mass flow (CMF) rebreathers.

by Rod Nairne

CMF semi closed circuit rebreathers are simple and reliable devices, and may be second only to oxygen rebreathers in terms of common usage. The whole system is dependent on the correct and accurate functioning of the CMF orifice, which meters the flow of the mixed gas into the breathing loop.

As the whole purpose of a rebreather is greater gas efficiency, the CMF is usually set with a small margin of safety, most commonly to give a fraction of oxygen in the loop equal to 19-21% (to avoid hypoxia on the surface) assuming a maximum oxygen uptake (VO2) of 3 liters per minute. The F02 in the loop can be estimated by the formula:

F02= ((flow rate *F02 in supply gas) - Max Vo2)/(flow rate -V02)

For example a nitrox mix with 60% oxygen assuming a flow rate of 6 liters per minute and a diver with a V02 of 3 liters per minute will give us a F02 of 20%.
The upper VO2 limit represents a reasonable upper limit of oxygen metabolism that the average diver will not be able to exceed for prolonged periods. Actual VO2 uptakes have been recorded twice this level however, which would result in hypoxia if repeated for a reasonable period underwater. Instantaneous hypoxia does not occur due to the loop acting as a oxygen reservoir and the increased pressures whilst diving giving an increased PO2 for a low fraction of oxygen.

Whilst this formula can be manipulated to calculate the flow rate required for any gas mix, in reality the flow rate will be limited to one of 3 as determined by the CMF orifices available in the rebreather. Each of the 3 orifice sizes flows a different mass per unit time of gas, and is designed to be used with a supply gas with a specific fraction of oxygen and a specific inert gas diluent, in all but rare cases this is nitrogen. The most common mixes are nitrox 60, 40 and 32 for diving to 16, 30 and 40 msw respectively. This depth limit s calculated by the maximum safe PO2 of 1.6ata of oxygen. (note: military divers may use a max safe depth calculated on a P02 of 2.0 ata, or 23, 40 and 52msw respectively. )

The CMF orifice will operate according to the following formula, provided the supply pressure is 2 times the ambient pressure, and the supply gas is composed of only nitrogen and oxygen.

flow rate in cubic feet per minute = 11(supply pressure)(diameter of orifice in inches)squared.

e.g. for a supply pressure of 450psi and a required flow fate of 6 liters per minute as in the example above, the approximate orifice size is =0.0065 inches or 0.185mm.

This formula leads us to a failure mode unique to CMF rebreathers: particle impingement (PI). PI of an orifice as small as 0.188mm is possible with a wide variety of substances commonly found in diving equipment: corrosion particles can either block the orifice directly of indirectly, as can sand particles, and even (and most commonly) salt water that can bypass the usual block for particles (brass sintered filters) in soluble form to form sodium chloride crystals either between the filter and orifice, in the orifice or after the orifice upon evaporation of the solvent.

To demonstrate the effect of particle impingement, assuming a moderate blockage of 10% of the orifice diameter and using the above formula we have a reduction of flow from 6 to 4.85 liters per minute. Substituting this into our original formula gives a negative value for F02, this means the gas mix will be insufficient to sustain life at a V02 of 3 liters per minute.

Unfortunately should blockage by salt crystals result in a fatality, it would be difficult to establish this as a cause. Firstly, if salt crystals were responsible, they would almost certainly redissolve in the time intervening the accident and the recovery of unit as it would have flooded. Secondly, if salt crystals are found, it is most likely they formed once the unit was recovered and sufficient time had elapsed for the water solution to have evaporated leaving the crystal solute.

The insidious nature of such a flow reduction, is that it is not reliably detectable by the diver once in the water (there is no reliable warning). Unfortunately pre dive checks will not prevent either subsequent formation of crystals or other particle impingement's which may occur during the dive. This is in stark contrast to open circuit equipment where a failure of the life support apparatus is instantly apparent by inability to take a breath. The rebreather diver may be able to continue to breath normally, unaware of any problem, until oxygen reserves in the loop are depleted and hypoxia occurs. This outcome is more likely in the deeper range of diving depths due to the oxygen fraction making up less of the loop volume.

Particle impingement is only one example of hypoxic outcomes possible in semiclosed circuit rebreathers. Should the supply cylinder run out un-noticed during a dive (an unfortunately common event in scuba diving) again the diver may not be aware of the flow stopping, as the only indication would be a cessation of a small amount of bubbling from the unit which reduces with depth. This could easily go un-noticed, especially if the diver is pre occupied with another task, is wearing a hood in cold water, or diving with comparatively noisy open circuit divers. Again the comparison with open circuit: if the cylinder runs out the diver can not breath so is instantly alerted.

Pre-dive flow rate checks may be compromised if a supply mix with a fraction of oxygen less than that required for the orifice size is used for a dive. The flow rate check in this case will pass the rebreather for use whereas in fact the rebreather will give a mix containing insufficient oxygen. A possible mistake might be if the 32 mix is connected to the 60 orifice, of if the nitrox mix is substantially oxygen deficient, for example the 60 mix contains only 50%. (note should this occur, and hypoxia result it may be difficult to test for the nitrox mix in the tank after it has been totally discharged).

The responsible and properly trained semiclosed rebreather diver will be aware of these risks, an be vigilant in pre dive checks. The diver will also be aware, (despite pre dive checks) of the risk of reduced flow during the dive, and always flush the loop with fresh gas before ascents, where hypoxia is most likely due to the reduction of pressure and the resultant drop in PO2 to hypoxic levels should a flow impingement have occurred.

Ask the A.A.R.G.