Al 80 vs HP100 useable amount of air

[scubadada]

Hi @Richard Dayan

You can choose whatever reserve pressure/volume you are comfortable with. For comparison:

An AL80 has a gas volume of 77.4 cu ft at 3000 psi. At the commonly used end pressure of 500 psi, there is a reserve volume of 12.9 cu ft, 64.5 cu ft used.

A HP steel 100 has 100 cu ft of gas at 3442 psi. It would have the same 12.9 cu ft reserve at 444 psi with 87.1 cu ft used.

In my example, the HP100 has 35% more available gas

I have breathed cylinders down to around 200 psi on a few occasions and have never noticed an increase in the effort to breathe

 

[Angelo Farina]

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.

 

[Belzelbub]

F8 is a great regulator, you should never notice a difference in breathing from over filled to near empty.

Exactly. I have a Flathead 7. Same basic 1st stage as the F8, but a different 2nd. My second has had the upgrade kit that came about after Huish acquired Zeagle. Upgrade kit and F8 were designed by Atomic. So more or less same reg set as the OP. If anything the OPs should be better.

If I weren't to check my air pressure, I would absolutely not notice by breathing alone that I was getting low on air. On at least one occasion, I've breathed it down below 500 psi and there was no noticeable difference in breathing performance.

 

[inquis]

This is trivial in SI units, quite tricky in imperial units.

Not tricky at all. The rated pressure of the HP tank is stamped right on the side (3442 psi). If it's only filled to 3000 psi, then it only holds 100/3442*3000 = 87 cf.

(No, this doesn't fully address compressibility, but neither does what you described.)

 

[lexvil]

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.

Funny thing is I understand the crappy imperial perfectly, the metric part of your post may as well be written in Greek

 

[lexvil]

Exactly. I have a Flathead 7. Same basic 1st stage as the F8, but a different 2nd. My second has had the upgrade kit that came about after Huish acquired Zeagle. Upgrade kit and F8 were designed by Atomic. So more or less same reg set as the OP. If anything the OPs should be better.

If I weren't to check my air pressure, I would absolutely not notice by breathing alone that I was getting low on air. On at least one occasion, I've breathed it down below 500 psi and there was no noticeable difference in breathing performance.

The FH 7 and F8 first stages only differ in the trim chrome bits, the second stage has a few upgrades but the old ZX seconds breath very well, I have both.

 

[MarkA]

Not tricky at all. The rated pressure of the HP tank is stamped right on the side (3442 psi). If it's only filled to 3000 psi, then it only holds 100/3442*3000 = 87 cf.

(No, this doesn't fully address compressibility, but neither does what you described.)

Using the figures from you and Angelo for a partially filled cylinder.

Imperial. 100/3442*3000 = 87 cf
Metric. 13*200 = 2600L

Although the Imperial calculation is not difficult, the metric one looks less likely to produce errors.

My 10L UK cylinders have stamped on them: 232BAR, 10.0L. I have never seen a US cylinder. Do the HP100 cylinders have 100cf stamped on them?

 

[scubadada]

I find all this math unncessarily complex as using crap imperial units...

...The american way of considering tanks always filled at their nominal pressure is misleading, as this does not happens in practice...

The OP is from the US and posted in imperial units. I am from the US and posted in imperial units. I'm glad you feel superior. I often post in both imperial and metric units to enhance international communication

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I nearly always dive AL80s in the US and when diving land-based or on a liveaboard outside of the US as that is all the gas I need. In my experience, these cylinders are usually filled to an average pressure of greater than 3000psi/207 bar. Here is a table of all my fills for 2021 and 2022. Average fill pressures were all above 3000 psi/207 bar. The lowest fill pressures are not bad. With the exception of Florida, HP steel tanks are not available on these trips. Perhaps if I used HP cylinders, the fill pressures would not look as good. Sorry you get poor fills.

 

[tursiops]

OP, what is your question, actually?
Does a 100 cuft tank have more air in it (if full) than an AL80 tank (if full)? YES.
How much more? 29%. (100/77.4)
Is all that additional gas available to you when you dive? YES.
Can you get an HP100 filled to be full? MAYBE.
Even if only filled to 3000 psi it still has more gas than an AL80.

 

[inquis]

Do the HP100 cylinders have 100cf stamped on them?

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