Semi Closed Rebreather diving gas calculations.

4ster

I am a relatively new Dolphin diver so am probably missing the obvious.

In Dolphin class my instuctor went over how to calculate vO2 as described by Freef in this thread (it was not explained well in the TDI textbook). I have been recording pO2 and depth on every dive to establish my baseline O2 metabolic rate. I get wildly variable numbers on each dive. Basically, at the begining of the dive, vO2 makes sense with a personal vO2 of between 0.8 to 1.0 lpm and a high work load vO2 of about 1.4 lpm. However, towards the end of my dives my calculated vO2 goes to zero, obviously bogus measurements.

At first I thought this might be caused by a combination of an old O2 sensor and/or increased condensation in the loop towards the end of the dive. I replaced O2 sensor in my Oxygauge, and there is only a little bit of water in the exhale bag at the end of my dives. I don't record depth and pO2 unless I have been at a steady state for a few minutes.

It just occured to me that the problem might be related to flow rate and depth. Since I am usually decending or at planned depth at the begining of the dive, the amount of gas venting from the OPV is minimal. But towards the end of the dive I am shallower (duh) and the amount of gas exiting the OPV is a lot greater.

If I am at 10 meters running a jet/mix that is optimized for 20 meters, a lot of the gas entering the loop is being vented without being used. Doesn't this raise the loop fiO2, all other variables being equal? That would make the vO2 appear to approach zero.

If gas is exiting the OPV, doesn't this screw up vO2 calculations on a SCR?

Steve

Freef

As the O2 monitoring is on the inhale side of the loop, you are monitoring the mix you are breathing in for these calculations. The O2 in the part of the loop you aren't breathing in from [exhale hose, CL, scrubber] doesn't matter.

I have had some odd vO2 readings on occasion, and at the end of the dive they sometimes get less, but they will reduce slightly [on a stop near the surface you are likely to be more relaxed than finning at the bottom].

__________________
David.

Currently owner of two differently sized ankles.

dozer

hi all, curious by this thread and i would like to put my two pence worth in!! i play with the flow and mix on my drager as previously mentioned, i just took the formula supplied with my manual and transposed it to to find my vo2 then worked the forlmula again with my calculated vo2 to find a better flow and mix for deeper diving, i wont put figures here, then i cant be accused of missleading the inexperienced but you would be surprised by the suitable mixes for acceptable loop fo2's.

also to comment on the Vo2 being at 2.5l/m as used in your examples (and i do appreciate they quote these figures in the drager manual) .. but in my experience (i consider myself a fit guy 30 yrs age and 12 years in military) ive done some testing to find my Vo2 and 2.5l/min is a bit of an unrealistic figure, and to actually maintain it for 4 minutes..good luck to you!!


think we shoud have a poll of vo2s achieved and how long you could mantain it (with some test criteria obviously)

si

4ster

Rereading my post it was unclear. I was trying to make a point about moisture level in the loop, not where the O2 partial pressure is measured. What I meant to say was that after a couple of dives there is a little moisture in the loop due to what I think is condensation. My inhale bag is not dry, but it doesn't drip any water from the drain when I take the pieces apart for cleaning. There is usually a dew like accumulation on the p-port mounted O2 sensor as well. I think this may be a source of error - a damp O2 sensor.

But this thread is about calculations for SCRs NOT O2 sensor problems.

Again, I can't figure out how calculations can be valid when a diver is significantly shallower than the SCRs ideal depth, since it is blowing off so much gas. Sure, the OPV is on the exhale bag but in shallow water a significant portion of that gas only went to the diver's lungs once before exiting.

Steve

Freef

Hi Steve,

If you are diving the Rebreather on the 32% or 40% jets the high flow rates will cause a lot of venting compared to the 50% or 60% jets. The only way to prevent this is to use the gas calcs to tune your mix, BUT this increases the risk of using the unit.

Try a couple of shallow dives using 60% through the 60 jet, and if the vO2 is acceptable then gradually lean off the mix while still using the 60 jet, all the time taking your pO2 readings to determine vO2.

The O2 cell is placed on the top side of the lung [when in the diving position] and has a guard to prevent the lung surface touching it. I haven't had any readings of zero L/min O2 use, but I haven't dives on the 32 or 40 jets. If you are fit and don't use much O2, then while doing the stops while shallow on a high flow rate jet could make your unit work almost as OC, which may explain your readings.

__________________
David.

Currently owner of two differently sized ankles.

4ster

The Dolphin vents least at the max depth for the jet (and tuning the mix makes it vent even less compared to Drager's recommendations). I have been mainly diving the 50% jet in the neighborhood of 60 to 70 feet (20+m). As expected I don't vent a lot there and the vO2 calcs "make sense" at those depths.

I'll try and arrange to dive a shallow dive on the 50% jet taking a bunch of readings at a steady state depth to see what I find. If at the begining of the dive, when condensation is not an issue, I have a stupidly low vO2 then that would seem to point to gas venting being a source for error don't you think?

I know I can rule out being fit as a reason for low vO2.

I am questioning how one knows he is collecting valid data to calculate vO2. I have not got it clear in my mind that a venting loop does not affect the validity of the data. Can anyone give me a logical explanation why it doesn't?

As an SCR diver I think it is important to have a good understanding of your personal vO2 in various conditions, no matter if you are diving your Rebreather on or off label. In hindsight my training course (TDI) was woefully inadequate in this area. I'm just trying to fill in the gaps and appreciate this forum for helping me do that.

Steve

Freef

I find that the Dolph vents fairly constantly, but it's not something I have noticed if it changes. There is a continual flow of gas into the loop, apart from the O2 that you convert to CO2 and is absorbed, the rest must escape via the dump valve [apart from on ascent, obviously]. I'll have to have a look on my next dive to see if there is a difference.

On the descent you may be activating the bypass valve which will cause a rise in loop O2 as you are adding more than the constant flow rate. You will notice the same effect if you do a loop flush.

Not super fit, [I'm not] just less of a couch potato. If you are relaxed and in warm water it also helps, my vO2 is slightly higher in cool conditions [4-10C] than in warm ones [10C+]

You must remember that it is a portion of the volume of the gas that is being dumped, not a specific component. The proportion of O2 on the exhale side remains the same as when you exhaled it, which is why the numbers are still valid. If you had a couple of 25 litre CL's you would get the same results as if you had a couple of 5L CL's or the Drager ones.

With the proportion of O2 in the exhale side remaining constant even if the dump operates the calculations are still valid.

__________________
David.

Currently owner of two differently sized ankles.

4ster

(slaps forehead) That makes sense.

I'm still going to do that shallow dive on a 50% jet just to see what happens.

Steve

Freef

It does after I had edited it, you should have seen it first time around!

Post the answers, it should be interesting.

__________________
David.

Currently owner of two differently sized ankles.

Freef

[Revised Nov 2006, unable to edit original post]

1. Introduction

1.1 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.

2. Background

2.1 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.

2.2 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.

2.3 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.

3. Calculation of vO2 [section three of spreadsheet S1]

3.1 “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.

3.2 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:



3.3 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.

4. Methods for determining vO2.

4.1 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.

4.2 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.

5. Determining the loop fO2 from a known vO2 and cylinder fO2. [Section two of spreadsheet S1]

5.1 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:



5.2 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

6. Finding a cylinder mix from a desired loop mix and known vO2. [Section one of spreadsheet S1]

6.1 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.

6.2 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:



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



7. Considerations when using these formulae

7.1 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.

7.2 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].

7.3 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.

8. An experiment by Paul Leyland [Odin on YD]

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

8.2 “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 2 m 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”
[Revised Nov 2006]

__________________
David.

Currently owner of two differently sized ankles.