working pressure

[Cave Bum]

ScuBase uses this also to determine SAC. Here is a break down.

For aluminum cyclinders it's as follows:

Since an 80CF aluminum really only holds 77.6 CF you can either call it a 78CF which is confusing or say it's a 3100 psi rated pressure cylinder which makes it come out correctly but still shows it as an 'aluminum 80'. I prefer the latter.

Other then 80CF cyclinders, aluminums are their working pressure, usually 3000 psi rated pressure. (This includes 'Super 80's' and 100's)

For LP Steel cyclinders, stamped pressure plus 10%:

Steel 72's are 2250 plus 10% equals 2475 psi rated pressure.

Steel 85's, 95's, 104's, 108's, 112's and 121's are 2400 plus 10% equals 2640 psi rated pressure.

High Pressure Steels are their rated pressure, some are 3442 psi rated pressure and some are 3500 psi rated pressure, check the cylinder.

Now if you take the tank size divided by it's rated pressure you get a factor for cubic feet per psi:

80 CF divided by 3100 psi equals .0258 cubic feet per psi. That way you can calculate how much gas you used.

An Example:
Say you breathed a tank down from 3000 psi to 800 psi. You dropped 2200 psi times that .0258 factor means you used 56.76 cubic feet.

Here is a reference out of my Bankmixer.xls Spreadsheet:

Size Singles Doubles
Aluminum:
13 CF 0.0044 0.0088
20 CF 0.0066 0.0133
30 CF 0.0100 0.0200
40 CF 0.0133 0.0267
63 CF 0.0210 0.0420
80 CF 0.0258 0.0516
100 CF 0.0303 0.0606

LP Steel:
20 CF 0.0076 0.0151
46 CF 0.0174 0.0348
66 CF 0.0250 0.0500
72 CF 0.0238 0.0476
85 CF 0.0322 0.0644
95 CF 0.0371 0.0742
104 CF 0.0394 0.0788
120 CF 0.0455 0.0909
121 CF 0.0458 0.0917
135 CF 0.0496 0.0992

HP Steel:
80 CF 0.0229 0.0457
100 CF 0.0286 0.0571
120 CF 0.0343 0.0686

If you want the whole spreadsheet (for free) you can download it here. (The tank size reference and Cubic Foot calculator are on the last tab at the bottom of the spreadsheet)

Be safe and have fun in the water! Bruce

 

[Blackwood]

Thinking of gas used in terms of psi might work if you and your buddies all use the same tank and never change, otherwise, thinking in terms of volume makes more sense.

(This whole issue is avoided when metric measurements are used.)

nonsense

 

[rongoodman]

Why "nonsense"? In metric, tank sizes are specified in liters. The amount of gas in a pressurized tank, in liters, is tank size(liters)*pressure(bar). That's it--there is no need for the idea of working pressure.

 

[Tigerman]

Without knowing the working pressure, you can't know the amount of gas in a cylinder just by knowing the pressure it's filled to. My LP85s hold 85 ft3 at 2640 psi(2400+10%). If they come back filled to 2500 psi, they don't have 85 ft3 of gas in them, but I wouldn't know that unless I knew the working pressure. Thinking of gas used in terms of psi might work if you and your buddies all use the same tank and never change, otherwise, thinking in terms of volume makes more sense.

(This whole issue is avoided when metric measurements are used.)

Yes you can. All you need to know is the volume of the tank and the pressure.
Without being assed to do conversions, if you have a 11 lliter tank filled to 100 BAR start you have 1100l of air at the start.
If you have 50 BAR end pressure, thats 550 liters at the end of the dive, youve used 550 liters.
If you want to convert it to imperial, go ahead but its all math.. An AL80 is 11,1 liters if I recall correctly. It doesnt matter if you work in metric or imperial as long as you know the tank cappacity in volume rather than cuft of air at max pressure..

 

[Blackwood]

Why "nonsense"? In metric, tank sizes are specified in liters. The amount of gas in a pressurized tank, in liters, is tank size(liters)*pressure(bar). That's it--there is no need for the idea of working pressure.

You need all the same information.

If I tell you I breathe 15L/min on the surface and ask you how many BAR I should expect to consume at over a 5 minute interval at 30m, you need to know the relationship between volume and pressure in my tank. Maybe that's 11.9L/bar. Maybe it's 100CF/3442PSI. Either way, you need to know it.

The math is the same. The physics is the same. Sure, on "metric" tanks you are told how much it will hold at one atmosphere (i.e. as empty as you can get it without a vacuum), and on "imperial" tanks you are told how much gas will come out of it when emptied in a one atmosphere environment. But either way, the same information is given: volume-to-pressure.

 

[Tigerman]

You need all the same information.

If I tell you I breathe 15L/min on the surface and ask you how many BAR I should expect to consume at over a 5 minute interval at 30m, you need to know the relationship between volume and pressure in my tank. Maybe that's 11.9L/bar. Maybe it's 100CF/3442PSI. Either way, you need to know it.

The math is the same. The physics is the same. Sure, on "metric" tanks you are told how much it will hold at one atmosphere (i.e. as empty as you can get it without a vacuum), and on "imperial" tanks you are told how much gas will come out of it when emptied in a one atmosphere environment. But either way, the same information is given: volume-to-pressure.

No. If you breathe 15L/min on the surface I just need to know the volume of the tank to know how many bar that is.. If its a 15L tank, thats 1bar/minute at the surface. If its a AL80 its 1,35 BAR/min at the surface.
If I know 15L/min is 1,25 BAR youre using a 12L bottle.
If youre using 1,8 BAR/MIN out of a 10L bottle I know thats 18L/Min.

If you dont know what that translates to at 30m, theres some nice charts in the manuals..

You never need to know more than 2 of the variables, there only are three. Pressure, Volume of the Tank and Volume of (compressed) Air.

 

[Blackwood]

No. If you breathe 15L/min on the surface I just need to know the volume of the tank to know how many bar that is.. If its a 15L tank, thats 1bar/minute at the surface.

Same can be said for imperial.

One is rated at 1 bar (synonymous to: how much water it will hold), and one is rated at full pressure. The sum total of the information is exactly the same.

L/bar, CF/PSI; same concept, different units. If you understand one, you understand the other. It's that trivial.

 

[Tigerman]

Yes and for neither of them you need to know the working/max pressure of the tank.
You need the volume of the tank and the pressure. Or to calculate the consumption, start and end pressures. As long as you know the tanks volume you dont need to know its working pressure.
Guess thats the upside to working with tank volume rather than capacity rather than content of air at working pressure - You always know the exact volume of the tank without remembering, asking or calculating that an AL80 is 11,1 l/0.39 cu.ft.