500 PSI – Low Pressure Tank vs. Hi Pressure Tank

[LStad]

Two divers both had 500 psi when they finished their dive. Diver #1’s tank was an 80 cf / 3000 psi high pressure aluminum tank, and Diver #2’s tank was a 93 cf / 2640 psi low pressure steel tank. Both tanks were full to capacity at the beginning of the dive. At 500 psi what is the difference in cubic feet between the two tanks? Who had more air? Please show your calculations.

 

[eelnoraa]

so,
80 x 500/3000 = 13.3cf for diver #1
93 x 500/2640 = 17.6cf for diver #2

I think the answer is too simple, what is real question?

 

[kombiguy]

4.3 is the answer.

 

[GrumpyOldGuy]

Diver 1 has 12.8 cu ft (500/3000*77)
Diver 2 has 17.6 cu ft (500/2640*93)

Is this a trick question or what?

 

[chrpai]

An AL80 (not really considered high pressure) is 77.4cf @ 3000 psi. So at 500 psi it's 12.9cf of gas. As for the 93cf tank, I know there are 95cf tanks. Assuming this is what you are talking about, that tank would be 95cf at it's + rating of 2640. (2400 + 10% ) So at 500psi it would have 18cf of gas.

air remaining in cf = current psi / servicepressure * cf at service pressure

 

[dfx]

Whose homework is that?

(And I didn't think AL80 was a "high pressure" tank...)

 

[HowardE]

An AL80 (not really considered high pressure) is 77.4cf @ 3000 psi. So at 500 psi it's 12.9cf of gas. As for the 93cf tank, I know there are 95cf tanks. Assuming this is what you are talking about, that tank would be 95cf at it's + rating of 2640. (2400 + 10% ) So at 500psi it would have 18cf of gas.

air remaining in cf = current psi / servicepressure * cf at service pressure

I'd agree... with the last 2 statements. It's probably a 95 CF tank. Faber has an LP 95 which is 95 cf at the +10... and a 3000 psi tank is not a High Pressure tank.

I always calculated it by doing = ((Capacity/Service Pressure = PSI per Cubic Ft) * (PSI in tank)) - Once you know the PSI/CF constant for that particular tank, you can calc the amount of gas at any PSI easily for that tank.

 

[eponym]

If this is a test question and not just a practical one, and you need to be spot-on accurate, don't forget to use absolute pressure (gauge pressure plus surface pressure).

The remaining air is actually calculated using 514.7 / 3014.7 (at sea level) rather than 500 / 3000.

The difference will be in tenths of a cubic foot so it's not important in practical terms. The total difference from the first two replies in the thread will be off by only one-tenth of a cubic foot.

Remember that analog pressure gauges can be off considerably, especially at the low end of their ranges.

 

[chrpai]

Yes, when doing calculations your answer will only be as good as the accuracy of your measurements. Some common capacities of "80's" are 77.4, 78, 78.2, 80, 80.6, and 82. Then there is the issue of the accuracy of the pressure gauge and even temperature of the tank.

So what is the purpose of the questions? Are we blending gases here or just wanting to demonstrate understanding of how much gas you have left?

 

[herbdb]

I used some LP rental tanks in Fla last year and the same question occurred to me during the DM brief when we were told to be on the boat with 500 psi.

I knew I would have more remaining air than someone diving a normal pressure tank! But not how much more. I decided that I would just not worry about how much more because of the potential inaccuracy of the SPG below 500 psi.