Methods of measuring basic gas consumption for recreational divers (2) SCR/RMV

[Rick Murchison]

Methods of measuring basic gas consumption for recreational divers (2) SCR/RMV

Note: In addition to the assumptions in the previous thread on SAC rate, (diving in the (average) ocean, and not in the eye of a hurricane or under the world’s tallest high pressure system, so when I say “the Surface” I’m assuming “sea level” on an average day, where 1 atmosphere is about 33 feet of sea water or 10 meters of sea water or 1 bar or 14.7 psi or 760mm Hg or 29.92” Hg), for the purposes of this thread, I am assuming that the reader already knows how to calculate SAC rate. Furthermore, the term SCR is in CFM, while the term RMV is in LPM – hopefully this will avoid the picking of some (but I’m sure not all) nits.

In the previous thread I discussed SAC rate, which was a useful measure of gas consumption for those divers who use a single tank size for their diving. But… different size tanks have different capacities and working pressures, so a SAC rate of, say 30 psi/2.0 Bar per minute on an AL 80 would not be 30 psi/2.0 Bar per minute if the same diver were to use a LP 120 or an E7 80.
So…
If we’re going to dive a variety of tanks, we need to be able to convert SAC rate into something that’ll give an accurate measure of gas consumption in any tank. That something is a measure of gas volume consumed per minute rather than pressure at the surface.
To make the conversion we need to know the relationship between pressure and volume for the tank involved. In the metric world this is ridiculously easy… an eleven liter tank holds 11 liters per bar, so if a diver has a SAC rate or 2.0 bar/min on an 11 liter tank, that is a Respiratory Minute Volume (RMV) of 2 X 11 or 22LPM, and if the same diver were to strap on a 15L tank, we’d see that diver’s SPG reflect a 22/15 or 1.6 Bar/min SAC rate for the larger tank.
- All you metric bubbas may commence gloating now… J -
For those of us who use the imperial measurements… things are a little more convoluted. For example, for a standard AL 80, we just have to know that the tank actually holds 77.4 standard cubic feet of gas at its rated pressure of 3000 psi. To convert a SAC rate from psi/min to CFM, we have to either divide it by the number of psi per CF for the tank (in this case 38.76), or multiply my CF per psi (in this case .0258). In either case, a SAC rate of 30 psi/min on an AL 80 comes out to be 0.77 CFM
Now, to see how much gas per minute we’d use at that rate from, say, a low pressure 104, we need to know that a 104 holds 104 CF at 2640 psi – that’s 25.38 psi/CF, so at a SCR of 0.77CFM, we’d use about 20 psi/minute.
- All you metric bubbas stop gloating now! –
Rick

 

[Tek Adventure Diver]

SAC is your rate and does not reflect your cylinder. It depeneds on the cylinder you use as to the pressure drops but you use the same amount of air. A LP drops slower then a HP but it carries the same amount of air. So it doesnt matter which tank you use, and to plan a dive you should not plan the dive to your cylinder . You should plan your dive then pick the attiquite cylinder. Interesting formulas but there is no added advantages to this. Just remember all tanks drop pressure differently.

 

[Rick Murchison]

... you should not plan the dive to your cylinder . You should plan your dive then pick the attiquite cylinder...

That's a fine recommendation for me, as I have 30 cylinders ranging from 30 to 130CF loaded with air and nitrox and trimix and deco gas and rigged single, double and stage sitting in my garage... but this is the "Basic" Scuba forum, and in the real world most "Basic" divers are going to dive what's available. Even hardened old tekkies have to take what they can get on the road sometimes... I certainly do
Rick

 

[MikeFerrara]

SAC is your rate and does not reflect your cylinder. It depeneds on the cylinder you use as to the pressure drops but you use the same amount of air. A LP drops slower then a HP but it carries the same amount of air. So it doesnt matter which tank you use, and to plan a dive you should not plan the dive to your cylinder . You should plan your dive then pick the attiquite cylinder. Interesting formulas but there is no added advantages to this. Just remember all tanks drop pressure differently.

I disagree.

I own and dive al80's or double LP 104's. If a dive requires too much gas for an 80 or I need redundancy I use the doubles. If the dive requires more than the 104's carry I add stages as needed and the stages are al80's.

My wife on the other hand dives a single 80 or double HP 100's. She dives the HP 100's because the size of the tank and weight distribution fits her body size better than the 104's.

But, we dive cold water and wear dry suits for most dives. If we needed doubles in warm water we'd probably double up some AL80's.

Tanks size is usually not only chosen based on gas capacity but also physical size, weight and balance.