This math stuff hurts my head

... so forgive me the rudimentary question.

Today I've decided to obsess about RMV and SAC. These are numbers and, sadly, numbers and I haven't gotten along since a falling out over some money they owed me a few years ago.

Regardless...

So I've found some threads explaining how to figure your SAC/RMV after having spent so much time at this much depth.

The best I've found so far (let me know if someone has something better {or, worse, if I'm wrong}) is:



rated tank pressure/cubic volume = some number I'm not sure of what it is, but you need it in a second. It looks like it's psi/volume, but that doesn't make a hell of alot of sense to me, as the pressure is pretty even in that tank, isn't it?

filled pressure - used pressure = your little test number.

little test number / freaky nonsence number = cubic volume used

cubic volume used / (ATA * test time) = RMV in cubic volume.



Now, you have all the nice little numbers and all the nice little formulas right in front of you. What I can't find is, how do you reverse it.

For instance. Say I know my RMV is .54. I figured this using a 3000PSI AL80, so now I know all the times I need using that bottle. But now I want to dive using a 2640 Steel 98cf; how can I find the times I need without an expensive scientific calculator and years of psychological counseling?

If you know that your SAC=.54ft^3/min and you did this with an AL80.

Then:

AL80 holds 77.4ft^3 @ 3000psi

77.4ft^3/ .54ft^3/min = 143.33min of air at the surface (if you drain it).

LP Steel 98 holds 98ft^3 @ 2640psi

98ft^3/ .54ft^3/min = 181.48min of air at the surface (if you drain it).

This time will change at depth and amount of exertion of course.

Fun....
Let's play with these numbers a bit.
Suppose you want to see how much gas you need for a 50 minute dive to 60 feet, using your 98.
Your RMV/SAC/SCR is .54.
Your depth in atmospheres at 60' is (60+33)/33 = 2.82
Your gas consumption at depth is 2.82 X .54 = 1.52 CF/minute
In 50 minutes you'll burn 1.52 X 50 = 76CF, leaving 22 CF in your 98 CF tank, or (2640/98) X 22 = about 600 psi.
If you then make a standard ascent with a 3 minute safety stop, you'll use another
.54CFM X 3.82/2 (that's your average depth during your ascent in atmospheres) X 2 (minutes for the ascent) = 2 CF for the ascent
+
.54 X (15+33)/33 X 3 = 2.35 CF for the safety stop, leaving about 17.6 CF or about 450 psi in your 98 at the boat.
Piece of cake.
E.

is the tank conversion factor. Knowing that number is what allows you to adapt your personal RMV to any sized tank. For an AL80 it is approxiamately 78cuft/3000psi=0.026.

If you have a NAUI shop near you they can sell you a SAC calculator that is very easy to use and quite inexpensive. The booklet that comes with it tells you how to work the equations forwards and backwards.

Forget about tank conversion factors. Just do everything in cubic feet of gas. That way no matter what tank you use, you will always be referring to the same thing, how much volume of gas you will consume.

Your SAC (surface air consumption rate) of 0.54 ft3/min that is the key. Remember, that goes up with cold, stress and exertion. If you dove my 18L (water volume) High Pressure Faber, you will figure out that it holds 138.5 ft3 of gas at its maximum fill pressure of 220 bar (about 3203 psi).

If it hits the water at 3000 psi then I know I have 130 ft3 (3000/3203). Remember, the fill is maybe hot in the shop. When it hits the water, you have to figure the pressure will decline again. My rule of thumb is a loss of 10% when hitting the freezing lake water from a summer day at 30C.

If you go on the dive with me this Sunday, at 102 ffw (fresh water), you will use 0.54 X 4.0 Atm = 2.16 ft3/min. But let's make it 0.7 ft3/min for the added cold (38F) and excitement of diving in Ontario. That is 2.8 ft3/min. Ignore the lesser consumption during the descent, ascent and during the safety stop, for conservatism's sake.

So the tank hits the water and is showing 3000 psi, and will last you 46 minutes at 102 feet depth. As you will want to budget a reserve of one third for your return and ascent, and one third for your buddy in case of emergency out of air situation, I suggest the dive plan is to turn around the dive after you have consumed 42-43 ft3, or at 15 minutes (2.8x15=42) and this will be about 1000 psi off your guage or 2000 psi. 1000/3000x130=43.3 ft3. Don't forget to add a few ft3 for the dry suit.