SCR Gas Extension Calculations

Constant Mass Flow [CMF] Type Semi Closed Rebreather Gas Extension Calculations
By David Morris


Introduction

The following is based on my training and experience with the Drager Dolphin rebreather. The formulae featured can be used on other CMF SCR rebreathers to fine tune gasses against flow rate to optimise the gas duration or the loop mix for the dive that you are doing. Any SCR diving using different gasses from those advised by the manufacturers increases the risk of diving the unit, and you must use a pO2 gauge and suitable bail out for this type of diving.


Background

The CMF SCR system is a very simple rebreather that controls the mix that the diver will breathe by continually supplying a known quantity of high oxygen gas to the breathing loop. As the gas is continually supplied, there will be venting of the excess gas through either an overpressure valve [OPV] or the diver exhaling off loop, usually through the nose.

As the quantity of oxygen that is supplied is fixed, the diver will have a quantity of oxygen in the loop that will vary with workload and other factors such as water temperature. The consumption of oxygen by the diver will vary. For example if workload increases this will drop the percentage of oxygen within the loop. In extreme cases this may lead to hypoxia.

Another consideration is the decompression obligation. As each dive has an element of decompression built in, but the diver must not to let the fO2 [fraction of oxygen] in the loop drop below 0.21 [21%] unless they are diving mixed gasses beyond the range of the diving being discussed here. An fO2 of 0.21 will give a partial pressure of oxygen [pO2] of 0.42 at 10m, 0.63 at 20m etc. Reduction in the loop fO2 will give a corresponding rise in the loop fN2 [fraction of nitrogen], which will increase the uptake of nitrogen by the body.


Calculation of vO2 [Section three of spreadsheet S1]

“vO2” is the term used to describe the amount of oxygen in litres per minute a diver metabolises during diving. At rest, on the surface the average person will use exhale up to 17% oxygen, which is why mouth to mouth resuscitation will keep a non breathing persons blood oxygen at a life supporting level. Under exercise the vO2 will rise, and a very fit person may reach a vO2 of up to three litres/min.

To calculate the vO2 of a diver four things need to be known: the percentage of oxygen supplied to the breathing loop [cylinder fO2], the flow rate of the gas [in litres per minute], the divers depth and the pO2 reading at that depth. The fO2 the diver is breathing needs to be determined first, using the ‘pressure T’, and from this the vO2 can be found using the following formula:


Using the following figures the vO2 of a diver can be determined:

• Cylinder fO2 = 60%
• Flow rate = 5.8 litres/min
• Depth = 12m
• pO2 = 1.12 bar

From the depth and pO2 it can be determined that the loop fO2 is 50.9%

Placing the figures into the formula gives:


Which gives a vO2 of 1.07 litres per minute of oxygen consumption.


Methods for Determining vO2

While the single example above gives a vO2 of 1.07, this figure alone must not be used. Relying on a single figure will not give an accurate picture of the actual vO2 of the diver, so multiple readings must be taken. By taking at least five readings per dive over a number if dives prior to adjusting the cylinder and flow rate combinations the SCR diver will be able to determine the optimum mixes for a dive without endangering themselves.

It is also important to consider a heavy workload dive. To simulate this a hard fin for at least two minutes against an immovable object at a fixed depth will raise the vO2 so that the higher vO2 can be taken into consideration when dive planning. While it not seem like a very long time to exercise, if you really go for it you will be slowing down at the end of two minutes. My typical normal and heavy workload diving will yield a vO2 of 0.90 and 1.20.


Determining the Loop fO2 from a Known vO2 and Cylinder fO2. [Section two of spreadsheet S1]

Once the vO2 is determined over a series of dives, the fO2 of the loop can be derived from the cylinder fO2 and the vO2. It is important to remember the effect of raised vO2 on the fO2 of the loop, and I always plan the dives, and set my computer with an assumed vO2 of 2-4% less than I have calculated. The following formula is used to determine the loop fO2:


If we assume a cylinder fO2 of 50%, a flow rate of 7.3 litres per minute and a vO2 of 1.1, use of this formula will give:


Which works out at a loop O2 of 41% [fO2 of 0.41]

For this dive I would plan for a loop fO2 of 38% and set my computer accordingly. As I dive with a Vytec I have a choice of three gas mixes, so I would set 21% as a back up in case of a high vO2. If my vO2 had dropped, the fO2 in the loop, and consequently the pO2 would rise, which I would be alerted to by monitoring the pO2 meter. I also make a note of the expected pO2 meter readings at various depths. For a loop fO2 of 0.41, and diving on a 38% plan I would expect the following:

• Depth 5m = pO2 of 0.57
• 10m = 0.76
• 15m = 1.02
• 20m = 1.23

Finding a Cylinder Mix from a Desired Loop Mix and Known vO2. [Section one of spreadsheet S1]

This is probably the most useful of the mix and match formulae. With a known vO2 and a desired loop mix for a given depth, it is possible to determine the desired cylinder fO2.

If we assume a dive to 25m and a loop pO2 of 1.2 is desired, this will require a loop fO2 of 0.34 [34%]. It will be better to fill the cylinder to the mix calculated from these figures than assuming the 4% lower loop mix used when planning the dive than fill to a higher cylinder mix. The following formula can then be used to calculate the cylinder mix:


Assuming a flow rate of 5.8 litres/min and a vO2 of 1.10, the cylinder should be filled to:


Considerations when using these formulae

There are circumstances when the flow rates and cylinder mixes could present a hypoxic situation to the diver. The most serious situation is using a too lean cylinder mix through a low flow rate jet. From the example given in 6.3, if the divers vO2 were to reach 1.9 then the loop fO2 would be 0.20. Even at a vO2 of 1.35 the fO2 would be 0.30, which would be the 30% that the dive had been planned to. Even a loop flush at depth may cause problems with high pO2 levels, so this must be avoided as well.
As these situations need to be avoided, each diver must calculate the window of tank fO2 they can use for a given flow rate and desired loop fO2. My personal minimum acceptable cylinder fO2s for the four jets in the Dolphin [flow rates of 5.8 to 15.6 litres/min] are as follows:

• 5.8 litres/min [60% jet] cylinder fO2 of 40%
• 7.3 litres/min [50% jet] cylinder fO2 of 37%
• 10.4 litres/min [40% jet] cylinder fO2 of 35%
• 15.6 litres/min [32% jet] cylinder fO2 of 30%

Section four on spreadsheet S1 has a means of calculating the range of tank fO2s from a known depth an pO2, and shows both of the worst case scenarios [high vO2 and minimum loop fO2 and low vO2 maximum depth allowed].

Of course there are benefits to using the formulae given. The main advantage is to extend the time that a cylinder can be used for, and to tune the mixes for the optimum fO2 for a given dive. The divers vO2 must be determined before any such cylinder fO2’s adjustments are undertaken. Diving a 50% mix through the 60% jet is unlikely to cause problems, but there is always the possibility for error.


An Experiment by Paul Leyland [Odin on YD/Rebreather World]

Paul emailed me the following information with regard to an experiment he carried out on his Dolphin.

I usually run on a 40% mix through a 60% jet. I did manage to get it down to 20% when dragging a trainee through the surf, and out into clearer water! Working like stink I did look at the gauge at one point and saw 20%. I did an experiment on how long it takes for the mix to drop when the cylinder switched off.

• 40% mix 6m water.
• Start reading 37%
• Gentle finning
• After 4 mins still reading 24%
• 6 mins alarm went off for 19%

I was by this time in 2m water. So surfaced and switched to earth air open circuit 21%...
I was interested to see how long I had if the cylinder did empty on me (and I do always carry bailout…)

Paul


David Morris


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