Calculating air consumption SAC / SCR / RMV

[ckharlan66]

Walter,

Were you my statistics teacher in college.

That explanation brings back painful memories of classroom lectures.

Thank your for explaining it though that should be a useful calculation.

Chad

 

[roakey]

Same thing Walter did, just a little different way to explain it.

Let’s say you want to compute your surface air consumption (SAC) for swimming. SAC is expressed in cubic feet per minute, which frees you from making any cylinder volume assumptions.

Get to a certain depth, note your pressure and swim at that depth for a number of minutes. Again note your pressure.

Let me pause here, this is probably best done by example, so let’s say you dove a standard AL80 and at 66' you swam for 10 minutes and ended up going from 3000 psi to 2400 psi, so you used:

3000 – 2400 = 600 psi.

The number of cubic feet you used would be psi used / working pressure psi * cylinder volume at working pressure or: 600 / 3000 * 80 = 16cf over 10 minutes.

Divide by 10 minutes and you have how many cubic feet of gas per minute you use at that depth. So that's 16 / 10 = 1.6 cf/min at 66 feet.

Now you have to figure out your rate at the surface. At 66 feet you're under 66 feet / 33 feet per atmosphere = 2 atmospheres + 1 atmosphere for the air on top of the water = 3 atmospheres absolute (ATA).

So the final calculation is 1.6 cf/min at depth / 3 ATA = call it 0.54 cf/min at the surface, which is your SAC.

Once more without all the verbiage:

1) (Psi used / working pressure of cylinder) * cubic feet of cylinder = cubic feet used at depth.

2) Cubic feet used at depth / minutes of test = cubic feet per minute at depth.

3) (depth / 33) + 1 = ATA at depth

4 )Cubic feet per minute at depth / ATA at depth = SAC or consumption in cubic feet per minute at the surface.

Basically four simple steps.

So how do you use this? Armed with the knowledge that your SAC is .54 cf/min, how long can you dive on a wreck at 99 feet before you start your ascent with 1000 psi left in your AL80 cylinder?

How many cf of gas do you have to use? Well you’re planning on using 2000 psi for the dive, so that’s 2000/3000*80 = 53 cf of gas to use for the dive before you turn.

At 99 feet, you’re exposed to (99 / 33) + 1 (for the atmosphere) = 4 ATA, so your .54 cf per min consumption rate quadruples to .54 * 4 = call it 2.2cf/min.

How long will that 53 cf last at a consumption rate of 2.2 cf/min?

53 cf / 2.2 cf per min = 24 minutes.

So all conditions being perfect, you can dive that 99-foot wreck for 24 minutes before you turn, and you should have 1000 psi in your cylinder when you do.

This is by no means a substitute for an SPG

Roak

 

[Warhammer]

The main problem with a PSI figure is that if you change tank types or sizes, then that figure is useless to you, since your new tank's volume and pressure will be likely be different. Hence 30PSI per minute from an AL80 is quiet different from 30PSI a minute from a LP steel. Whereas if you know your cubic feet per minute, it's univerisal and can be apllied to any tank regardless of the tanks characteristics.

 

[Rick Murchison]

Roakey gives a nice easily followed explanation... with only one minor problem - An AL 80 doesn't hold 80CF at 3000 psi! It holds 80 CF at 3100 psi, and only 77.4 CF at 3000 psi. So in the example given, the proper SCR is ((600/10)/3100)*80/3, or .52CFM vice .54.
Rick

 

[Fishkiller]

a person who I know sujested I rent a tank and put my head in my fish aquarium till the the scuba tank was empty and do the math that way.
If it takes me x time to empty a 40cf tank at 3000psi then it should take me the twice the amount of time to empty a 80cf in 30 feet of water...

although I like Roakey's way of thinking also.

 

[The Chairman]

I agree with Walter and Warhammer here...

I own 4 different sizes of tanks, AL60, AL80, Twin Steel LP 63s, PST Steel HP120. Without using RMV, my comparisons between these are null... apples to oranges, if you will. Knowing the RMV, the precise pressure, the approximate depth AND the tank I will be using will give me the best estimation of how long I could stay under considering only air supply. Now I can compare apples to apples in different containers. Its the only way I have ever figured out my air consumption rate. I thought Walter's exposition to be lucid and straight forward!

 

[ScubaBaby]

Sorry the aircraft noise blotted everything out as it went STRAIGHT OVER MY HEAD!!!!!

 

[scubabunny]

I'm with YOU abby! I just know when my air is running low..math just ruins everything!

*passes abby the bowl of popcorn and sits back..watching*

 

[Walter]

Abby & Bunny,

What part did you not understand? This can be very easy if you take it one step at a time. I'll be happy to go over anything you are having trouble with.

Fishkiller,

If you depend on your aquarium method, you'll run out of air long before you expect. You are correct the 40 cu ft tank holds approximately half the air of the 80 cu ft tank. What you forgot is at 30 ft you'll be using approximately twice the air with every breath. That will put you emptying the 80 in about the same time as the 40 at the surface. Of course, most divers actually swim around on a dive increasing air consumption even more. Add that to the fact that all but the most experienced divers breathe faster at depth than at the surface. That 80 should give you less time at 30 ft than the 40 at the surface, not twice.

WWW™

 

[DameDykker]

Use an air integrated computer and a program (I use the Cobra) and all that i calculated for you in either cu. ft. or liters. Voila!