Calculate Remaining Air Suppy

[pll8on]

In my studies on calculating air supply, I could not understand how to calculate the following problem.
"A diver carrying twin 72-cubic-inch tanks descends to 85
feet to inspect a piling. The diver spends 10 minutes at that depth and uses up 20%
of the air supply. If the diver needs at least 5 cubic feet of air to inspect the bottom
of the piling at 130 feet, explain whether or not the dive can be completed."

I know Pata= depth+33ft/33ft or 118/33 = 3.57atm's and 144/3.57= 40.336 volumn
Calculating 20% of 40.336, equals 8.067 leaving 32.68 volumn of air. Beyond this I am stumped on what to do. I know the answer is 'NO' from my text book but need to understand the mathmatics of the solution. Please help. Thank you
Phil

 

[jham2081]

I'm afraid math isn't my strong suit, but I would assume you would use the following to figure out the rate he was using air (The diver spends 10 minutes at that depth and uses up 20% of the air supply) at 85 ft. Based off that (turn it into a SACR) and assume that air use rate at 130ft. Hope that made some sense, and helped. Sorry i can't help with actual numbers.

 

[DA Aquamaster]

A steel 72 holds 71.2 cu ft at 2475 psi

so 20% of 142.4 cu ft = 28.48 cu ft used in 10 minutes @ 85 ft.

This is 2.85 cu ft min @ 85 ft

Pata at 85 ft = (85/33) + 1 = 3.58 ATA

So we divide 2.85 cu ft/ min by 3.58 ATA to get the SAC rate of .796 at the surface (1 ATA). Then round it up to .80

Pata at 130 ft is (130/33) + 1 = 3.94 cu ft/min @ 130 ft.

So if you used 28.48 cu ft to 85 ft then descended to 130 ft at 60 ft/min in 45 seconds (45 ft / 60 ft in 60 seconds) and figured your consumption at the 130ft level you would use another 2.96 cu ft descening, then you use the 5 cu ft specified inspecting at 130 ft.

To that point in the dive you have used 28.48 + 2.96 + 5.00 cu ft = 36.44 cu ft.

Even if you figure the 30 ft/min ascent at the 130 ft use rate (4.33 minutes times 3.94 cu ft/min) is only another 17 cu ft. for a total of 53.44 used to that point in the dive.

Add a 5 minute safety stop at 15 ft (15/33) + 1 = 1.45, 1.45 x .80 = 1.16, 1.16 x 5 = 5.8 cu ft.

This brings us up to 59.24 cu ft used. A 500 psi reserve in the twin 72's is another 28.76 cu ft, but that still leaves 54.4 cu ft. available even at the end of the dive. I don't see a problem with gas management on that dive.

On the other hand, on a single steel 72 you would only have 11.96 cu ft left after the dive which would be less than the 14.38 cu ft available in a 500 psi reserve so the dive wopuld not be prudent.

 

[onfloat]

Here, read this by Delia Milliron

http://www.5thd-x.com/pdfs/battlefieldcalc.pdf

 

[Atticus]

In my studies on calculating air supply, I could not understand how to calculate the following problem.
"A diver carrying twin 72-cubic-inch tanks descends to 85
feet to inspect a piling. The diver spends 10 minutes at that depth and uses up 20%
of the air supply. If the diver needs at least 5 cubic feet of air to inspect the bottom
of the piling at 130 feet, explain whether or not the dive can be completed."

I know Pata= depth+33ft/33ft or 118/33 = 3.57atm's and 144/3.57= 40.336 volumn
Calculating 20% of 40.336, equals 8.067 leaving 32.68 volumn of air. Beyond this I am stumped on what to do. I know the answer is 'NO' from my text book but need to understand the mathmatics of the solution. Please help. Thank you
Phil

I'd be happy to help you with this.

Please verify that you stated the problem correctly. Specifically, I'm not familiar with 72 cubic inch tanks. Are you certain you don't mean cubic foot?

Also, as stated, since you know that the diver has consumed 20% of their air at the first depth, and you know how much air is needed for the second depth, you need only compare it to 80% of the size of the tanks (5 cf < or > 80% of tank size). I suspect you mean 5 minutes at 130 feet?

I regularly lead my students through calculations like this and would be happy to help you understand the needed equations.

EDIT: I posted this at the same time DA Aquamaster posted his response.

 

[andydiver06]

You need to calculate surface air consumption rate to calculate your gas requirement.
1 cu ft = 28.3168 litres ie. twin 72 = 28.3168*2*72 = 4077.6 litres
You consumed 20% in 10 minutes i.e your SACR = 4077.6*.2/10 = 815/10=81.5 (Your tank vol is constant so as you said air consumed is 20% of the total) this value is abnormal as it should be about 15-25 lpm Any way if you consume 81.5 lpm at surface you will consume 4.96 times at 130 f ( 39.6 m) ie. 4.96* 81.5 = 405 lpm
Next you need to understand the rule of thirds in planning 1/3 gas for descent+bottom time, 1/3 for ascent and safety stop, 1/3 for emergency reserve.
Hope I'm taking some sense .

 

[www.scubamazing.com]

I think these are the nicest replies I've seen.

When I start College Algebra, I'll come running to scubaboard.

Atticus, you gave a very considerate offer.

 

[onfloat]

Thirds?? All you really need is enough gas in reserve to safely ascend from your deepest depth for you and one other diver while making all stops. This could really be a half of all usable (minus the reserve)

 

[Packhorse]

Ah these things are sooo much easier to calculate using Bar and litres.

 

[Azza]

Ah these things are sooo much easier to calculate using Bar and litres.

Oh yeah