Al 80 vs HP100 useable amount of air

[JoeTPhilly]

How can you do the same instantaneous calculation with an AL80 at 2950 PSI? The math is much more complex, as you do not know the tank volume and the pressure is expressed in weird units, so you cannot simply multiply the two numbers...

AL80 rounded tank factor is 2.5 x 30 units = 75 cubic feet.

 

[tursiops]

AL80 rounded tank factor is 2.5 x 30 units = 75 cubic feet.

For those not used to Imperial shortcuts:

Tank Factor = tank max capacity/max pressure x 100 = 77.4 cuft/30 = 2.58 cuft/100 psi.​

"Unit" means how many "100 psi" are there...so 30 in a 3000 psi tank.​

If the tank were half full then there would be 15 such "units."​

Rounding down the tank factor makes the calculation more conservative.

A HP100 has a Tank Factor of 100/34.42 = 2.9

So you can remember your tank volume in liters, and multipy by bars to get the volume of gas (in liters) in your tank
OR
You can remember the Tank Factor, and multiply by your psi/100 to get the volume of gas In cufic feet) in your tank.

Not really very different.....

 

[inquis]

How can you do the same instantaneous calculation with an AL80 at 2950 PSI?

That tank is "nearly full".

Seriously though, if doing anything other than doodling around in OW with Minimum Gas as my ascent indicator, I would know I need at least X PSI from the planning. If 2950 psi is greater than X, there's "enough".

 

[Angelo Farina]

It is really interesting to see these work-arounds employed in Imperial units...
I have never heard of a "tank factor".
Is it impressed over each tank, as the liters are impressed here?
And what is that "magic number" 34.42?
I am discovering an entirely new way of computing the capacity of a tank...
I was stuck with our very simple approach (liters x bars).

 

[lowwall]

Or (for an NDL dive) skip all the calculations and start your ascent when you hit 800Psi/55bar.

 

[Angelo Farina]

Or (for an NDL dive) skip all the calculations and start your ascent when you hit 800Psi/55bar.

That's my approach. very simple and independent on the tank size...

 

[lowwall]

I am discovering an entirely new way of computing the capacity of a tank...
I was stuck with our very simple approach (liters x bars).

The shortest calculation for US-style tanks is actual filled capacity x filled pressure/service pressure. For example an HP100 filled to 3000psi has:

101.3 x 3000/3442 = 88.3 cu ft

The service pressure is stamped on the tanks. Unfortunately the actual capacity is not. Instead we get the manufacturer and a model name. The model name is normally a prefix to indicate tank type followed by a nominal filled capacity. As we just saw while the Faber HP100 has a nominal capacity of 100 cubic feet, the actual capacity at rated pressure is 101.3 cu ft. Meanwhile, the Worthington X7-100 holds 99.5 cu ft (at 3442psi) and the Pressed Steel E7-100 really is 100 cu ft.

Likewise, the AL80 is a nominal capacity with an actual capacity of 77.4 cu ft at the rated pressure of 3000psi.

The nominal capacity thing is, IMO, completely stupid. The only way to determine actual capacity short of taking the valve out and filling it with water is to hope you can find the manufacturer specs somewhere.

 

[Angelo Farina]

The shortest calculation for US-style tanks is actual filled capacity x filled pressure/service pressure. For example an HP100 filled to 3000psi has:

101.3 x 3000/3442 = 88.3 cu ft

The service pressure is stamped on the tanks. Unfortunately the actual capacity is not. Instead we get the manufacturer and a model name. The model name is normally a prefix to indicate tank type followed by a nominal filled capacity. As we just saw while the Faber HP100 has a nominal capacity of 100 cubic feet, the actual capacity at rated pressure is 101.3 cu ft. Meanwhile, the Worthington X7-100 holds 99.5 cu ft (at 3442psi) and the Pressed Steel E7-100 really is 100 cu ft.

Likewise, the AL80 is a nominal capacity with an actual capacity of 77.4 cu ft at the rated pressure of 3000psi.

The nominal capacity thing is, IMO, completely stupid. The only way to determine actual capacity short of taking the valve out and filling it with water is to hope you can find the manufacturer specs somewhere.

Thanks for the great explanation. Now everything is quite clear.
However, the fact that the "nominal" capacity differs from the real one is quite disturbing.
Add to this that often during manufacturing there is a quite significant tolerance.
When inspecting all the tanks of the diving ceter where I was working, they were all Faber steel tanks, with nominal capacty of 10 or 15 liters. Regarding the 10 liters, the REAL capacity stamped on each tank varied wildly. It was going between 9.2 and 10.7 liters. Each tank was different, also the weight was different (which is also stamped on tanks).
So, if in the US the real capacity of a tank is not stamped on it, how can the diver make accurate calculations?
I understand that for most shallow recreational dives knowing the exact capacity is not that important.
But I am an engineer, and I was used to always make the calculation very simply, by reading the real capacity of each tank by the stamped value in liters, and multiplying by the real pressure measured with the SPG at the beginning of the dive.
Not knowing the real capacity of the tank, and having to rely on some average value found on the Internet, is very disturbing to me.

 

[Wibble]

Ali80 volume is 11.1 litres. Multiply by working pressure of 207 bar gives 2297 litres of gas. No idea what that is in cubits per square donkey.

However, it's the volume when you dive that counts. If your gauge says 190 bar, then you've got 2109 litres.

Then there's the real world which isn't accurate. So use 11 litres and round down.
11 x 190 = 2090 litres, or around 2000 litres.

Is your gauge accurate though? Few are.

 

[lowwall]

Is your gauge accurate though? Few are.

Excellent point. And even if it is accurate, how closely can you read an SPG underwater? I guess the digital gauges give an exact number, but what are the tolerances?