Freef

Custom Mix Calculating Tables With Known vO2


Steve Sprague adapted a series of TDI tables for use when the diver wants to quickly calculate their loop fO2 from a known flow rate, cylinder fO2 and vO2 when access to a computer running a spreadsheet isn’t available. His original reworked table appears below. Note how the range of vO2 covers values of 0.40 to 2.50, and how dangerous the wrong gas choice can be, dropping the loop fO2 well below 0.21 [21%]. The range of flow rates for the chosen gas is also illustrated at the top of the table, as is the orifice fO2 designation and the ideal flow rate. A useful feature of the tables is that the coloured band that is fitted to the jets is used as the band at the top of the table. It was from this original table design the author calculated ones more specifically suited to his own vO2 levels, with smaller steps between the vO2 levels.








When compiling tables such as these, vO2 readings outside the normal vO2 range of the diver should also be used. By doing this the diver can quickly check for any out of range problems a given cylinder fO2 could give.

Calculating desired flow rates for variable supply valves.


Moving away from the standard Drager design, some manufacturers SCRs are equipped with a valve that the user can set themselves to determine the flow rate of nitrox and hence the loop fO2. CCR units such as the KISS work on a similar principle of adding a continual flow of pure O2 [rather than nitrox] to the loop to maintain a near constant pO2 with the diver adding O2 as needed.

The following formula is used to determine the flow rate when the vO2, supply fO2 and desired loop fO2 are known:



Which, when calculated out will give a flow rate of 5.29 litres/min.

Again, it is best practice to work out a contingency flow rate for higher and lower vO2 levels. Varying either of the desired loop fO2, cylinder fO2 or vO2 readings while leaving the other values constant can do this.

Dive planning when using CMF SCR


When planning a decompression dive on a CMF type SCR unit, the diver needs to plan for all of the factors that would affect a normal nitrox dive in terms of oxygen exposure and decompression while calculating the loop fO2. The Author always assumes that the dive will be slightly harder work than the previous one, and the loop fO2 is taken as 2% lower than calculated.

For example a dive using 50% through the 60% jet at a vO2 of 0.95 L/min would result in a loop fO2 of 40%, so the plan would be as if the dive were being conducted on 38%-the same as if the vO2 had gone up to 1.1 L/min. The diver must also make a note of the highest pO2 reading they get at the deepest part of the dive to calculate oxygen exposure. It is also worth planning the dive as it was being conducted on air as a back up in case the vO2 rises with the consequence of the loop vO2 dropping below 38%.


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 may be performed by the user.

However, consideration needs to be given to the wisdom of relying completely on these methods of decompression management. 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. Putting complete trust in a dive computer without the knowledge of how long you can spend underwater for a given decompression profile, or no decompression at all, increases the risk to the diver.






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 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 divers vO2 for safety]. 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. A second bail out slate should be carried assuming that the loop fO2 drops below the planned percentage.

Decompression on a CMF SCR.

Many manufacturers prohibit the use of their SCR models for decompression diving. They also want only the correct gasses to be married up with the flow rates they are designed for. It should also be noted that most dive computers carry warnings that they are not to be used for decompression diving, so it appears that the restrictions may be more for a liability function than any shortcomings of the equipment.

Decompression using the loop of a SCR is certainly possible, but the possibility of a problem with the loop should always be considered. If a loop problem occurs the diver has two options: bail out immediately to the OC gas, or stay on the loop in OC mode before switching over. Depending on the nature of the problem it may not be possible to stay on the loop, so sufficient bailout needs to be carried for the dive. It is also an idea to calculate the bailout gas required at a higher RMV than usual to take account of the raised stress levels in the event of a problem. For example if at 35m, with a stop of 2 min at 9m and 8 min at 6m, a diver has to bail out, and assuming an RMV of 18, the diver will need at least 572 litres of gas-191 bar out of a 3L cylinder. When it is considered that gas will be added to either the wing or drysuit on descent it becomes apparent that for decompression of any length a larger source of decompression gas is required.

In the UK a popular way of achieving this is to use a 10L cylinder with an H valve. This will provide enough gas for bailout and decompression obligations. In the example used above, the diver will require just over 57 bar of gas from the 10L to meet their decompression needs. The only disadvantage of a 10L from the redundancy point of view is that a catastrophic failure of the cylinder neck O ring or an O ring in the valve itself will deplete the gas the diver was depending on for decompression. Fortunately this is an extremely rare occurrence and one that is not likely to be encountered by the user.

A completely redundant source of decompression gas, such as a sidemounted stage is another way of diving beyond no decompression limits. A 232 bar, 7L stage will provide over 1600 litres of gas to the diver. While it is possible to plumb this in and provide drysuit inflation, it can be kept totally separate and for decompression only. In the dive above, using a 7L will cause a drop in tank pressure of 82 bar, so it is perfectly feasible to dive the tank for two dives and still have a healthy reserve.

Another point to consider is the step up to the decompression gas fO2 from the loop fO2. If a 10L is being used for both loop and decompression, the gas required to achieve the desired loop fO2 may not provide much of a decompression advantage over staying on the loop.

For a dive to 36m for 35 min using a cylinder fO2 of 40% and a flow rate of 5.8 L/min, a diver with a vO2 of 1L/min would have the following dive profiles, depending on which gas was used for decompression.

Note that in these calculations the safety factor of planning for a loop fO2 2% below the calculated loop fO2 has NOT been used, and the calculations are based on a loop fO2 of 28%.



So it can be seen that using the option of a higher fO2 supply can reduce the dive time in this example by up to 16 minutes, especially important when diving in cold water. Of course the down side of this is the pO2 of the deco gas is not useable as bailout at depth. OC divers use this method for decompressing, and their bailout is that their twinset [two tanks manifolded together] features valves that can isolate a failed regulator, preserving as much gas as possible for an ascent where a switch to a deco mix can take place. The rebreather diver does not have this option in the event of a catastrophic loop failure, which although rare must be planned for. However it is preferable to take a breath from a too rich mix gas at depth and have the possibility of an O2 problem than it is to drown for certain.



Abbreviations used in this article.

CCR Closed Circuit Rebreather. Mechanically [mCCR] or electronically [eCCR] controls a pure oxygen supply into the loop. Two cylinders are required, one for a dilutent [also called diluent] gas, which will be air or trimix, and the other 100% O2.

CMF Constant Mass Flow. Most semi closed rebreathers are of this type, where there is a continual metered flow of oxygen rich gas into the loop.

fO2 Fraction of oxygen. The measure of how much oxygen is in a gas, can be writted as a percentage [ie 21%] or a decimal [ie 0.21]. IfO2 is used for inspired fraction of oxygen elsewhere, which this article refers to as loop fO2.

OC Open Circuit. Conventional SCUBA equipment.

OPV Over Pressure Valve. Usually found in the exhale counterlung, and vents the excess gas from the loop.

pN2 Partial Pressure of Nitrogen. May also be written as ppN2, the pressure of nitrogen within a gas in Bar.

pO2 Partial Pressure of Oxygen. May also be written as ppO2, the pressure of oxygen within a gas in Bar. Normal diving is limited to 1.4 bar as the ‘working’ pO2 and 1.6 bar for decompression.

RMV Respiratory Minute Volume. The amount of gas consumed by a diver while on open circuit, in litres per minute at the surface. Also known as SAC [Surface Air Consumption].

SCR Semi Closed Circuit Rebreather. One where a flow of nitrox is supplied into the loop, usually from a single cylinder. Some recent SCRs have two tanks that can have different mixes, usually one for the bottom mix and one for decompression.

vO2 The measure of how much oxygen the diver consumes, in litres per minute [L/min].

__________________
David.

Currently owner of two differently sized ankles.