Freef

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 selected 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 [lowest lines].

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%-although a larger buffer is preferred to prevent problems with lowered oxygen and raised nitrogen levels.

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 35.3% O2, which would give a pO2 of 1.59 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 29.7m. 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, 39, and 42m 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:



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 42m dive was undertaken, and the diver in this example had their vO2 drop to 1, the loop fO2 would rise to 29.6% giving a pO2 of 1.54 bar. This shows how critical it is 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.

The table is repeated below, with the same parameters except with a higher flow rate of 10.4 L/min which is the rate of the 40% jet on the Dolphin.



Again, it can be seen that at 36m or deeper the minimum fO2 is below 25%, so the figures can be recalculated to give:



Again, the effect of the oxygen window closing can be clearly seen.

If the two tables above are compared, for each of the depths given, the flow through the 40% jet gives a far better margin of safety compared to the 60% jet. A loop flush at the maximum depth of 42m would result in a peak pO2 of 1.79 compared to 2.17 when using the 60% jet. The MOD for the richest mixes is also deeper, with a 3.6m shallower MOD for a depth of 42m on the 40% jet compared to the 8.9m difference when using the 60% jet. It can be seen that the benefit of using the 60% jet is purely in terms of gas duration, and it must be remembered that the slower the flow rate, the greater the risk of the diver having an incorrect loop fO2.

The chart below shows the oxygen window more clearly with the minimum and maximum vO2s for the two flow rates compared. The red and blue lines are for a flow rate of 5.8 L/min [the 60% jet on the Dolphin], and the green and yellow for the 10.4 L/min [40%] jet. It must be remembered that just because the lines are further apart it does not make the choice of gas mixes safer. In fact, the risks increase with depth, especially when the low vO2 calculated cylinder fO2 bottoms out to prevent a loop mix of less than 25%. The only way to really minimise the risks of CMF rebreather diving is to dive the manufacturers recommended mix and jet size combination.


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. Bailout must be carried, and it must be of a size that will get the diver to the surface including any decompression stops. The standard 3L/232 bar cylinder in use in the UK will just about get a diver to the surface from 30+m when full. If the diver is on their second or third dive and they have a loop failure of any sort then the 3L may not be enough to get the diver to the surface safely. It must be remembered that the bailout is usually used to supply gas to the divers buoyancy control, be it a wing or drysuit as well, so there will be less gas available to the diver than when they descended.

It is possible to use a nitrox mix as bailout, but as this must be a safe gas to breathe at the deepest planned part of the dive, it is unlikely to be of benefit for decompression. Conversely, if a decompression mix is carried, this may be unsafe to breathe at the maximum depth of the dive, which is where bailout use must be planned from.

Further calculations regarding the size of bailout can be found in the section below entitled ‘Decompression on a CMF SCR’.

__________________
David.

Currently owner of two differently sized ankles.