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.