Check my math? (SAC)

[divermike1011]

So I started reading threads about SAC and since I had no idea what mine was I thought I'd try and calculate it, and I'm just not sure if I did it right, though the number I got seems to be about right. Here is what I used[this was done while sitting on my couch]:

I breathed from an AL 80 for 30 minutes. Initial pressure: 1500 psig. Final Pressure: 850 psig. Total consumed: 650 psig.

I calculated from those numbers that I used 22.67 cu. ft. of air in 30 minutes or a resting SAC of 0.76 cu. ft. per minute.

I'm bouncing back between metric and imperial lately for work, so while I am pretty sure I got my maths correct if anyone is bored and once to double check, thanks!

Michael

 

[Charlie99]

I breathed from an AL 80 for 30 minutes. Initial pressure: 1500 psig. Final Pressure: 850 psig. Total consumed: 650 psig.

I calculated from those numbers that I used 22.67 cu. ft. of air in 30 minutes or a resting SAC of 0.76 cu. ft. per minute.

1500-850 = 650psi.
650/3000*77.4 = 16.77 cu ft,. (AL80 is really 77.4 cu ft at 3000psi).
16.77cu ft/30 min = 0.56 cfm.

 

[divermike1011]

Damn Charlie, that was fast! Thanks, and I feel a lot better about having a SAC of .56 vice .76

Michael

 

[g1138]

Unless you were diving on the surface, you still have to add your depth into the equation.
Divide 0.56 by [(depth/33FSW) +1]
to take into account the ata you were at when breathing.

This will give you the "Surface" Air Consumption rate you had at depth.
You can use your avg depth or max depth. I tend to go with the latter for simplicity's sake.
Plus it's gives more of a moral boost =]

BTW who gave you the low fill?

 

[Bubbletrubble]

So I started reading threads about SAC and since I had no idea what mine was I thought I'd try and calculate it, and I'm just not sure if I did it right, though the number I got seems to be about right. Here is what I used[this was done while sitting on my couch]...

Unless you were diving on the surface, you still have to add your depth into the equation.
Divide 0.56 by [(depth/33FSW) +1]
to take into account the ata you were at when breathing.

@g1138: I think divermike1011 accounted for the "depth" at which the gas was consumed, don't you?

 

[g1138]

@g1138: I think divermike1011 accounted for the "depth" at which the gas was consumed, don't you?

I don't see the math, maybe he did.
I do know someone with a SAC of 0.23 on average


edit: oh wait, I see what he meant.
I though he just calculated it on his couch.
opps

 

[Charlie99]

The "sitting on a couch" scenario isn't a reliable or realistic as doing the measurement while underwater, and then normalizing back to surface conditions.

Another way of looking at RMV or SAC is that the units are really CFM/ata rather than just CFM.
Expressing consumption as 0.55cfm per ata makes it pretty clear that, if you are at 2ata (33' depth) that your consumption of 0.55cfm/ata results in 1.1cfm actual consumption.

 

[divermike1011]

I know I'll have to add 1 ata per 33 ft/10 m, I had just been reading Doppler's Tech Blog, which I had followed from a link on another SAC thread here on SB. Since I couldn't get in the water yesterday I figured I'd give the calculations a shot (which I obviously failed at, lol) and see how I did resting at the "surface."

@G1138 It was a tank I had lying around I hadn't had refilled post dive, didn't want to burn air off my full tank, so I just used that one.

Michael

 

[TSandM]

Divermike, you didn't really fail . . . I suspect that you didn't account for an Al80 only having 77 cf in it when it's at its working pressure.

One really quick way to do this is to work with what's called "tank factors". If you sit down and calculate the cubic feet of gas in a tank at a given pressure and multiply times 100, you get the CF in 100 psi. For an Al80, that's close enough to 2.5 to use that number, given that SAC rates are influenced by so many variables. So you can then figure the gas you used in 100s of psi (in your case, 6.5, and multiply times the tank factor (2.5) and that gives you your cf used. It's just a little easier than doing the division by 3000 or 2640 or 3442 over and over again.

 

[divermike1011]

Thanks TSandM, that is far easier!