The Dangers of Gas Matching on Constant Mass Flow Semi-Closed Rebreathers.

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Introduction.

Previous articles have discussed the calculations used in determining the gas mixtures other than recommended by the manufacturer that can be used with a CMF SCR rebreather. Those articles have been based on the author’s personal experience with the Drager Dolphin CMF SCR, although if the flow rates for other models are known, these too can be used with the calculations to fine-tune the fO2 in the breathing loop. However, there are dangers associated with this practice, and the risks that the diver exposes themselves to is the subject of the following article.

The CMF SCR Design.

The design of the CMF SCR is remarkably simple. With the continual supply of oxygen rich gas to the breathing loop electronic or manual O2 addition is not required, and the unit can be dived without a means of monitoring the loop fO2 [often indirectly by the means of a pO2 monitoring device] to the manufacturers standard mixes, although this is not recommended. Extra gas is introduced into the loop during descent by an automatic valve similar to the second stage of open circuit scuba equipment, or by manual addition through a valve similar to a drysuit inflation valve that is usually found on one of the counterlungs.

As there is a continual flow of gas into the breathing loop, the excess must be vented. On Closed Circuit Rebreathers [CCR] this is achieved by exhaling “off the loop”, usually through the nose, or by loosening the seal between the mouthpiece and lips. When diving on a CCR the only time that off loop exhalations will usually occur is on ascent as the gas in the loop expands. Most SCRs have an overpressure valve [OPV] built into the exhale counterlung which allows the diver to stay on loop through all phases of the dive.

Having a continual flow of gas into the loop presents a problem for the diver, in that the fO2 of oxygen they inhale will vary with workload, temperature and stress levels, as each of these has an effect on the respiration rate, as does a loop flush [exhaling off loop to completely refresh the gas in the loop]. It is because of these factors that care needs to be taken in choosing the gas for a given dive.

Calculating Mix Choice With a Known vO2.

The reasons for wishing to calculate a non standard match between cylinder fO2 and flow rate are varied, from wishing to dive deeper, using the unit as a gas extension tool or only being able to obtain a gas mix that is not specified in the manufacturers literature.

vO2 is the term used to describe how much oxygen a diver consumes in litres per minute. Before attempting any variations on the standard flow rates and gas choices the divers range of vO2 MUST be determined. Just determining the normal relaxed fining pace vO2 is not enough, and the higher vO2 experienced during exercise must also be taken into account [see appendix 1].

Once the vO2 levels of the diver are known then it is possible to then choose an ideal gas for a given depth, depending on how the diver wishes to use the gas, from lowering the fO2 within the loop to go deeper than the mix for that flow rate is designed for to raising the fO2 for reducing the decompression penalties.

Longer Duration Diving.

By using a slower flow rate than standard a longer duration dive can be undertaken. On the Drager Dolphin, with a 5L cylinder at 200 bar, and with surfacing with 50 bar in the cylinder, the gas duration is between 48 and 129 minutes, depending on the jet selected.

If a dive is to take place at 15m, the fO2 in the loop can be anything from to 21% to 56%. Although theory allows a lower loop fO2 than 21% to be used at 15m this is outside normal SCR diving practice. At a loop fO2 of 21%, with a vO2 of 1 the cylinder fO2 can be found for each of the four flow rates found on the Dolphin from the following formula:



It can be seen that an increase in dive time of over 2½ times can be gained from changing from the 32% jet to the 60% jet and increasing the cylinder fO2 by 9%.

The problem of gas matching now appears, in that if the divers work load increased and their vO2 went up to 1.75, the cylinder mixes that were giving a loop fO2 of 21% would now give the following:



The figures for the 5.8 L/min flow rate [the 60% jet] are particularly noteworthy. At a depth of 15m the loop would supply the diver with a partial pressure of oxygen of 0.195. While this would sustain life, the partial pressure of nitrogen [pN2] would be 2.305, which is the nitrogen pressure that would be found at 20m, rather than the 15m that the dive is being conducted at. This obviously has an effect on the decompression obligations of the dive. Worse still, on ascent the loop pO2 would drop rapidly to 0.078 bar on the surface, too low to enable the diver to ascend safely.

So, for simple gas extension at shallower depths the cylinder fO2 is chosen based on the upper vO2 that will still leave a loop fO2 of 21%. From the examples above, and using a peak vO2 of 1.75 the cylinder contents would be as follows:



At the fairly shallow depth of 15m, all the above mixes are safe to breathe on open circuit, so if a loop flush was required while using the 60% jet the peak pO2 would only be 1.12 bar, well below the maximum of 1.4 bar that is the usual ceiling for the bottom part of a recreational dive. If there is a risk of a hyperoxic situation occurring then the safest option is to use a higher flow rate with a lower cylinder fO2.

Deeper Diving Using Slower Flow Rate Jets.

The maximum recommended depth for diving the Dolphin is 40m using a cylinder mix of 32% through the 32% jet. This combination will give a loop fO2 of 27.3% for a vO2 of 1 L/min and 23.4% for 1.75 L/min. While this combination is the safest way of diving the Dolphin, there is only 48 min gas duration in the cylinder. It should also be noted that the maximum operating depth [MOD] of a 32% mix is 33m, so if a loop flush is performed at 40m the pO2 could in theory reach 1.6 bar, although this is unlikely as there is always a diluting effect with the gas that is already in the exhale side of the loop before the bypass valve operates.

When deeper diving with a lower cylinder fO2 it may be possible to get a hyperoxic mix in the loop as well as a hypoxic one. For example if a diver plans on a dive to 30m, using a tank fO2 of 38% and a flow rate of 10.4 L/min, [the 40% jet on a Dolphin] and a vO2 of 1.25 was used to plan the dive.

At 30m the loop fO2 should be 29.5%, and the pO2, 1.18 bar. If the diver is relaxed and their vO2 drops to 1 the loop fO2 will increase to 31.5% and the pO2 to 1.32 bar. The following table illustrates what a further drop in vO2 would do to the loop fO2:



It can be seen that the loop is not likely to go into a hyperoxic state during this dive with a more relaxed vO2 than was planned for.

If the vO2 were to rise then the following loop fO2 and pO2 would be expected.



Again, it can be seen that with this particular combination of cylinder fO2 and flow rate that the mix supplied to the diver is unlikely to drop to a hypoxic level

For a typical gas choice that can be used 5m deeper, a cylinder fO2 of 43% and a gas flow rate 5.8 L/min, and the effect of various values of vO2.



From these figures the dangers of using non-standard cylinder and flow rate combinations can be seen. The loop can become hypoxic at high workloads and hyperoxic at low values of vO2. Bear in mind that a loop flush at 35m with the cylinder providing 43% will give a loop pO2 of up to 1.93 bar.

This said, with careful selection of gasses and knowledge of their personal vO2 allows divers to extend the usefulness of their SCR. The diver needs to know the oxygen window for the choice of cylinder fO2 for a given flow rate.

Using the Oxygen Window to Determine Choice of Cylinder fO2

If the diver takes their time and uses a pO2 meter to determine their vO2 then a range of cylinder fO2’s can be found to match any given flow rate. As an example the author has determined his vO2 to vary from 0.80 L/min in normal diving conditions to 1.25 L/min when exercising hard. The dive details are entered into a spreadsheet of the authors devising, with these two vO2 values to determine the safe cylinder fO2:



Once the desired pO2, depth flow rate and high and low vO2’s have been entered the spreadsheet calculates the loop fO2 that will give the pO2 at the dive depth, and then the cylinder fO2’s that will give the loop fO2 required. Note that as well as the range of cylinder fO2s there are also loop fO2s that give the worst-case scenarios.

The first gives the loop fO2 that will occur if the lowest cylinder fO2 is used [which assumes that the vO2 will be the lowest value] but the diver is working at their maximum vO2. In the example above it can be seen that the loop is still above 21%.

The second reverses the vO2 and cylinder fO2 to give an indication of the loop fO2 if the diver assumes that the dive will be harder work than it really is. Assuming a high vO2 and working at a lower vO2, the loop fO2 will increase, and in this case it can be seen that the loop may contain 34.9% O2, which would give a pO2 of 1.57 bar at the depth the dive is planned to [35m] and the MOD [for a maximum pO2 of 1.40 bar] is also given as 30.1m. The following table gives the readings for the same dive on different flow rates.



It can be seen that range of choices of gas narrows with the increase in gas supply flow rate, from 5.6% on the 5.8L/min flow rate to 2.1% at 15.6 L/min. This is because the diver has the oxygen in the loop replaced at a faster rate on the higher flow rate jets. Once the range of flow rates has been determined the diver can make a choice of the gas they wish to employ for the dive.

For the vO2 ranges and depth given above the it is possible to choose a cylinder fO2 of 40% which is fed through the 60% to get best gas optimisation. This will also result in a worst-case pair of loop fO2s of 24% and 30%, with a MOD that is deeper than the planned depth. The choice of 40% is also close to the middle of the permissible fO2 limits.

The table below illustrates that as a diver goes deeper on a fixed jet the cylinder fO2 range widens. vO2 values of 0.80 and 1.30 are used to provide the maximum and minimum rates of consumption, with a supply fO2 of 40%,a flow rate of 5.8 L/min, and a maximum depth pO2 of 1.3 bar.



At 36 and 39m the minimum cylinder fO2 must not be used as this would lead to a loop fO2 of less than 21%. Even at 33m the figure of 22.5% is below the minimum that the author finds acceptable [25%] for a buffer against the loop fO2 getting too low. So, realistically the oxygen window starts to close at 30m, with a minimum cylinder fO2 of 41.8%. Using this figure the 33-39m section of the table above can be revised thus, and extended to 42m:



…where it can be seen that the oxygen window has closed. Any deeper and the minimum required fO2 will exceed the maximum required fO2. The diver must also be working at their maximum planned vO2 [1.30 in this example] to prevent the risk of the loop fO2 rising. If the diver in this example had a vO2 drop to 1, the loop fO2 would rise to 29.6% giving a pO2 of 1.54 bar. This shows how critical it can be to choose gasses with care, and to understand how easy it can be to get a hyperoxic mix at depth, or if the diver tries a fix of assuming incorrect vO2 levels how quickly the loop fO2 can move outside the planned parameters.

It must be remembered that a loop flush at these depths will lead to a spike in the loop fO2. At 42m with a cylinder fO2 of 41.8 a flush would cause the loop pO2 to exceed 2 bar, which is an extremely dangerous situation.

Deeper diving with gas extension is a higher risk form of diving the CMF SCR. On the Dolphin the maximum depth is realistically 40m whichever gas mix you choose. Anyone choosing to dive in this way must be fully aware of, and accept the risks in doing so.

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1 Methods for determining vO2

When diving a CMF SCR in the way described above the vO2 of the diver must be determined. It is also important to keep tracking vO2 as experience grows as the value is likely to drop as the diver becomes more familiar and relaxed with the unit.

Simply by noting depth and pO2, and knowing the cylinder fO2 and flow rate the vO2 can be determined. It is worth taking many vO2 readings at various depths to begin with, tapering off to five or so on each dive as the user gains more experience.

Using this formula will give the vO2 for the readings taken. Again, the author uses a spreadsheet to perform the calculations.



The following readings were taken by the author on a dive in October 2006, with a cylinder fO2 of 40% and a flow rate of 5.8 L/min.



It is worth considering these readings and the vO2 that are calculated to understand what causes readings that appear to be incorrect. During the descent phase of the dive the bypass valve operates adding more O2 to the loop than the metered flow alone. This extra oxygen gives a higher pO2 reading, which leads to an apparently lower vO2.

The spike in vO2 at a depth of 15.5m is again easily explained as this was when an exercise test was being performed. A stationary object, in this case a submerged double decker bus, was used to push against for two minutes of hard fining. At 11.2m the author was stationary while watching fish, and relaxed allowing for a lower pO2 to occur compared to fining. Finally at 6.6m the diver was just returning to the loop after a switch to OC to inflate a delayed SMB to mark their ascent as is required by the dive site. This allowed O2 to be supplied to the loop without being consumed by the diver.

These readings were taken as an illustration of the factors that can lead to spurious vO2 readings, and when a diver is taking notes to determine vO2 then a period of settling at a depth is advised to take into account the extra O2 injected into the system during descent.


2 Dive computer use with CMF SCR

There is increase in popularity of dive computers that have the ability to read the loop fO2 from a cell and provide real time decompression information to the diver in the same way that the fixed O2 levels in open-circuit can be set on a normal nitrox or trimix computer. It is also possible to use multi mix nitrox computers where gas switching can be performed by the user.

However, consideration needs to be given to the wisdom of relying completely on these methods of dive planning. Dive computers are battery powered electronic devices that are submerged in water. They are also operated by a human who is not perfect. The author has twice mis-set a dive computer, but the error was realised due to having a knowledge of what the readings should have been, and has personally seen three computers fail while in use by others.

With pre planning and carrying a run time slate for any planned decompression diving dive computers can be used to assist the diver in optimising their dive. With a multi mix computer it is possible to plan for a loop fO2 above 21%, a fall back of 21% if the vO2 rises, and if a third mix can be programmed in then a decompression gas can also be entered.

For example, if a dive to 36m for 30 min is planned with a 40% cylinder fO2 and a flow rate of 5.8 L/min and a vO2 of 1 L/min, it is safe to assume a loop fO2 of 26% [2% lower than calculated for the vO2 for safety]. With a deco mix of 50% a three mix dive computer could be programmed with 21%, 26% and 50% as the three options.

The diver would then make a note of the minimum pO2 they should allow before switching to the fallback of 21%. At 36m with a loop fO2 of 26% this is 1.19 bar. It is worth noting the expected pO2 at other depths as well to ensure maximum safety. If the pO2 drops it is simple to switch to 21% to continue to dive safely. Due to the spiking of loop fO2 on descent it may be advisable to keep the computer switched to 21% until the correct level of loop fO2 has been verified during the dive.