Al 80 vs HP100 useable amount of air

[Wibble]

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?

Within 25%?

Seriously! How many SPGs actually show zero at 1 bar? Many of mine show 20+ bar unpressurised.

That's why we round up and are conservative. Even your estimation of your SAC needs to be higher than normal.

 

[inquis]

However, the fact that the "nominal" capacity differs from the real one is quite disturbing.

This plagues your tanks as well. You only have 2493 surface liters of air -- not 2600 -- in a 13 liter cylinder at 200 bar. At higher pressures, the Ideal Gas law is decreasingly accurate due to compressibility. That is why our AL80 only has 77.4 cf real capacity. Admittedly, we are inconsistent (our HP100 actually does hold 100 cf at rated pressure), which can be confusing for a new diver.

Read up on z-factors if you're interested to know the details.

 

[L13]

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).

Metric has a tank factor too.

The tank factor for a 12 liter tank is 12 liters/bar!

(Yes it is much easier to determine!)

 

[tursiops]

Metric has a tank factor too.

The tank factor for a 12 liter tank is 12 liters/bar!

(Yes it is much easier to determine!)

And the straightforward conversion from liter to cuft and from bar to psi give 2.9 cuft per100 psi.
Amazing!

 

[uncfnp]

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.

And this highlights another variable to consider when selecting a tank size, the extra gas may be of no benefit if you are unfortunate enough to be group diving and required to ascend when you still have plenty of gas. I try very hard to avoid this style of diving.

 

[L13]

And the straightforward conversion from liter to cuft and from bar to psi give 2.9 cuft per100 psi.
Amazing!

But usually the metric tank factor is stamped on the tank. The imperial tank factor isn't.

 

[spc751]

I will never deal with a shop that only fills my hp100 to 3000psi.