Al 80 vs HP100 useable amount of air

[Angelo Farina]

If a divemaster on a 'normal' guided recreational dive is requiring clients to surface at 1000PSI, then you need to find another divemaster. Jeesh, this is not rocket science. Maybe there is something unusual about the environment you're diving in.

From what I understood 1000 PSI is the pressure where the DM calls the end of dive: time to start ascending and swimming back to the boat.
It is not unreasonable...
When I was working as a divemester the rule was that it was time to ascend as soon as when one of the divers was at 100 bars.
We did not care of the size of the tank, which was ranging brteeen 10 and 20 liters (twin set). The assumption was that divers using larger tanks were air hogs, requiring them.
So the rule was the same for everyone: at 100 bars, wathever the tank size, it was time to end the dive and to start ascent and deco procedures.
This ensured that most divers emerged with at least 50 bars.
Some divers, indeed, did not meet this goal and surfaced with just 20-30 bars.
Each organization has different rules and customers are supposed to comply with them.
Thumbing the dive at 1000 PSI, wathever the tank size, seems something quite reasonable for me.

 

[NW Dive Dawg]

Al 80 vs HP100 useable amount of air​

I almost can't believe that this thread even went more than one page. If a 100HP tank is filled to normal working pressure...... and an 80cf tank is filled to normal working pressure....... then the 100cf tank has 20cf more of usable air. Am I missing something here?

 

[inquis]

You missed the opportunity to bash US units.

 

[boat sju]

 

[Jcp2]

Al 80 vs HP100 useable amount of air​

I almost can't believe that this thread even went more than one page. If a 100HP tank is filled to normal working pressure...... and an 80cf tank is filled to normal working pressure....... then the 100cf tank has 20cf more of usable air. Am I missing something here?

I believe that some people were in areas that could not get a true HP fill, so then one would be diving an expensive LP85, which is still better from a buoyancy standpoint than an AL80.

 

[MikeHoncho]

I find all this math unncessarily complex as using crap imperial units.
Let's convert to metric, where everything is easier.
A so called HP 100 here in Italy is a steel cylinder with a real capacity of roughly 13 liters, whilst an AL80 is just 11 liters. But being made in aluminium, the external volume of the two tanks is substantially the same. Which means that the AL80 is much more buoyant, which is bad.
The other difference is the max rated pressure, a value which is simply ignored by most filling stations. In theory the HP100 can be filled at 232 bars, whilst the AL80 should not be filled over 207 bars.
In practice in most filling stations, after cooling, you must be happy if you get 200 bars.
This means that the HP100 contains a total of 2600 liters of air (13×200), whilst the AL80 contains just 220 liters of air (11×200), when both are filled at 200 bars.
In both cases you should consider an hard reserve of 50 bars, and start ascending at 100 bars.
If diving at 40m depth, where the total pressure is 5 bars, the standard consumption of 20 liters/minutes becomes 100 liters/minute.
So the HP100 will last 13 minutes (same number as liters) and the AL80 will last 11 minutes.
In both cases you will have enough air for ascending and making a short deco stop.
In conclusion the HP100 gives you slightly less than 20% more bottom time than an AL80. And, apart the need for less weight, it does not change substantially your dive plan: ascend when the pressure gauge indicates 100 bar (half of the initial 200 bar fill).
You will get slightly longer bottom time, that's all.
The american way of considering tanks always filled at their nominal pressure is misleading, as this does not happens in practice.
Better to know the real capacity of your tank in liters, multiply by the pressure shown by the gauge (in bars), and know the real amount of air you have.
This is trivial in SI units, quite tricky in imperial units.

I don’t find this any easier than American math.

 

[Angelo Farina]

I don’t find this any easier than American math.

As I did already explain, my point was not about units.
It was about the American way of labelling tanks based on their rated total nominal capacity (tank volume multiplied by the rated max pressure multiplied for same weird factor).
Here in Europe we label the tanks based on their internal volume, indipendently by the max rated pressure.
And we evaluate the available gas at the beginning of each dive multiplying the tank internal volume by the pressure (in bars) read on the SPG.

 

[tursiops]

As I did already explain, my point was not about units.
It was about the American way of labelling tanks based on their rated total nominal capacity (tank volume multiplied by the rated max pressure multiplied for same weird factor).
Here in Europe we label the tanks based on their internal volume, indipendently by the max rated pressure.
And we evaluate the available gas at the beginning of each dive multiplying the tank internal volume by the pressure (in bars) read on the SPG.

Fine, let's do that with the HP100 (12 l, 237 bar fill pressure) and the AL80 (10.6 l, 207 bar).
The available gas at the beginning of the dive is 2832 l vs 2192 l. That is 29% more gas in the HP100.
The main difference is because the HP100 has a higher rated fill pressure, not because the internal volume is a bit bigger.
Sure, you can underfill (183 bar) the HP100 so it also starts with 2192 l; then the two tanks are identical in the amount of gas in them.
Why would you only partly fill the HP100?
I think you are not used to tanks with different rated fill pressures..

 

[Angelo Farina]

Why would you only partly fill the HP100?
I think you are not used to tanks with different rated fill pressures..

Because I worked in diving centers where all tanks were always filled at 200 bars, wathever the rated pressure.
Some were rated at just 170 bars (my own Aralu twin tanks, 9.5+9.6 liters). Some were rated at 200, some at 232, some at 250 bars (13 liters Faber steel tanks).
It was materially impractical to fill the tanks one by one, resetting the automatic pressure switch of the compressor differently for each tank. We had a multiple hoses setup filling 6-8 tanks simultaneusly...
And still now I see diving centers doing the same, 200 bars for every tank.
But my point was another: not trusting the rated nominal total amount of gas: it is much simpler and safer to evaluate the real total amount of gas by MEASURING the real pressure in your tank, and multiplying by the tank volume.
If I know that my tank is 12 liters, and I measure a pressure of 192 bars, then I know that I have 12×192=2304 liters.
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...

 

[tursiops]

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...

True, but maybe not really relevant. My SPG tells me pressure; all I really need to know is what fraction that pressure is of a full tank (e.g., 3000 psi). Does it help me to know how many liters I have in hat tank? Only if I also know my SAC and my depth, so the calculation is much more complicated than just how many liters are in the tank; I also need to know how fast it is going away (at my depth and work-rate) and what that means in time left....which is exactly what the Gas-Time-Remaining display shows me on my wirst with no additional calculations required.

As to filling all tanks to the same pressure.....that is not a good practice. Best to avoid any dive operations that do that.