Re: Thanx for the URL

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Posted by Steve on July 09, 2001 at 17:09:44:

In Reply to: Re: Thanx for the URL posted by MHK on July 09, 2001 at 16:10:39:

"What you need to do is convert your SAC rate to your Respitory Volume per Minute while at depth."

That's not the way I do it. I know my SAC rate and then I convert it to DAC, my depth air consumption to plan my air needs.

RMV = The RMV is the total volume of gas used at body temperature and atmospheric pressure. I wouldn't convert SAC to RMV. I take my SAC of .46 and multiply it by ATA's to figure out my expected usage. You do it any way you want.

My average RMV while scuba diving is equal to my SAC surface air consumption. They are the same thing. But my RMV while watching TV has nothing to do with diving.

Why do we keep butting heads? I hoped this wouldn't happen again this week. SAC is specific to diving RMV isn't necessarily diving related.

This is what I found doing some research.

On open circuit SCUBA, each lungful of gas is exhaled and lost into the water. On a closed circuit rebreather, the only oxygen used is that which the body metabolises. So, when comparing gas consumption, we need to consider Respiratory Minute Volume (RMV) for open circuit, against metabolic oxygen consumption for closed circuit.
The RMV is the total volume of gas used at body temperature and atmospheric pressure. Oxygen consumption is the amount of oxygen that the body metabolises and is measured at standard temperature and pressure.
Some example figures taken from the US Navy Diving Manual are:

Oxygen consumption RMV
(litres/min) (litres/min)
Standing still 0.4 9
Walking 2 mph 0.7 16
Swimming, slow, 0.5 knot 0.8 18
Walking 4 mph 1.2 27
Swimming, average speed, 0.85 knot 1.4 30
Running 8 mph 2.0 50
Swimming, 1.2 knots 2.5 60

An RMV of 18 l/min (slow swimming) will be compared against an oxygen consumption of 1.5 l/min (moderate swimming).
Open circuit gas consumption calculations are based on the gas laws [3], and are summarised below.
Pressure = depth/10 + 1
so at 30m, pressure = 4 bar
Gas needed = RMV * pressure * dive duration
so for a 1.5 hour dive at 30m, gas needed = 18 * 4 * 90 = 6480 litres
Gas available = tank volume * pressure
so with a 10 litre tank, pumped to 232 bar, gas available = 10 * 232 = 2320 litres
Number of tanks needed to spend 1.5 hours at 30m if breathing at 18 litres/min open circuit = 6480/2320 = 3.
Oxygen consumption is not depth dependent, so for this example remains at 1.5 l/min whatever the depth. For a 90 minute dive, oxygen consumption is 1.5 * 90 = 135 litres.
The oxygen cylinder in the UT 240 is a 3 litre water capacity cylinder, pumped to 232 bar. It therefore contains 3 * 232 = 696 litres of oxygen. The same dive would use less than a fifth of the cylinder.
Some other depth comparisons for a 1.5 hour dive are given in the table below.

Open circuit Closed circuit
Depth Pressure Gas used Tanks used Oxygen used Tanks used
m bar litres litres
10 2 3240 2 135 0.19
20 3 4860 3 135 0.19
30 4 6480 3 135 0.19
40 5 8100 4 135 0.19
50 6 9720 5 135 0.19
60 7 11340 5 135 0.19
70 8 12960 6 135 0.19

At constant depth, almost no diluent is used up at all - it just keeps on going around the loop from lungs to breathing bags. (Some small amount will be absorbed into the tissues of the body). On descent, some diluent is added to the loop to maintain a constant volume. On ascent, as the gas in the loop expands, so some of the mix is vented off. So, for a dive profile where you descend to the desired depth and stay there, diluent use is very low. If the dive involves going up and down many times, more diluent will be used. If gas is allowed out of the closed circuit loop by exhaling through the nose, for example to clear your mask, then again gas consumption will increase.
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