Al 80 vs HP100 useable amount of air

[Brett Hatch]

So the real point is: what is the true capacity when filling by water an AL80 and an HP100 (either in cuft or in liters, I don't care)?
Is this value stamped on the tank, as it happens here?

In the US, tanks are legally required to have their service pressure stamped on them. They also need to have a code that specifies the metal alloy and thickness, the manufacturer, the original (and current) hydro date. I don't believe that they are legally required to stamp the capacity (either internal volume, or the gas if it were to expand to 1 atm).

However, many manufacturers do print that on there anyway. Unfortunately, they get to choose to encode this however they want, and they often round up a bit. So Luxfer AL80's are marked with "80", but they actually hold 77.4 CF. I'm not aware of any manufacturers that stamp the internal volume on there, but some might. It would certainly make things easier, and that's something that SI stamping seems to do a better job of.

 

[scubadada]

...The pressure you see on the SPG translates into an amount of available gas which is just 11% larger than when using an AL80.
Not 25%...

No, if you have a HP100 and an AL80 filled to their service pressures, the HP100 will have 35% more usable gas if you dive them down to the same reserve volume. This is not rocket science, 87.1/64.5=1.35 Al 80 vs HP100 useable amount of air

 

[Angelo Farina]

No, if you have a HP100 and an AL80 filled to their service pressures, the HP100 will have 35% more usable gas if you dive them down to the same reserve volume. This is not rocket science, 87.1/64.5=1.35 Al 80 vs HP100 useable amount of air

I do not understand how 100 is 35% more than 80. It is 25% in my math...
And coming back to the OP.
He said that his normal usage is starting with the tank at 3000 PSI and ending the dive when the SPG shows 1000 PSI.
This means that he uses 2/3 of the total gas in an AL80, which is in reality 77 cuft, so 55.3 cuft.
Using an HP100 with the same initial and final pressure (3000 and 2000 psi), this corresponds to an amount of available gas equal to 58.7 cuft, which is an increase of just 6.12%.
If instead the initial pressure is raised to 3442 psi, and still thumbing the dive at 1000 psi, the HP100 provides 71.65 cuft, meaning an increase of 29.6%.
I am a bit tired, being 01:00, so please correct me if my calculations are wrong...

 

[JoeTPhilly]

Again the same wrong concept.
That is the theoretical maximum gas content.
But the real content is given by the tank volume multiplied by the pressure you read on the SPG after splashing down. This is often very different from the max rated pressure...
And the OP was specifically asking about how pressure shown by the SPG behaves on the two tanks.
This depends just on their real capacity, not on the max rated pressure.

Agreed. The metric system is easier for calculations but if one understands tank factors imperial isn't too hard. The rounded tank factor on an HP100 is 3. For every 100 psi is 3 cu ft. Studying the metric calculations helped me visualize everything though.

 

[Brett Hatch]

I do not understand how 100 is 35% more than 80. It is 25% in my math...

Angelo, it is 25% more gas in the tank, but we don't use all the gas in the tank. @scubadada is assuming that we want the same reserve volume in each tank. On the smaller tank, that is a bigger proportion of the total gas than it is on the bigger tank. Hence, the amount of usable gas (excluding some reserve gas), is more like 35% in this case. With that in mind, maybe it will be easier to follow the math from his post on page 2. Hope this helps

Edit to add. Say you are comparing a 240 bar 10L to a 240 bar 12L. The 12L has 20% more gas in it. But, if we want to reserve 2L times 240 bar = 480 liters of reserve gas, then we can only actually use 8L of the 10L tank, and only 10L of the 12L tank. Hence, the 12L tank has 25% more usable gas. This is exactly the same argument scubadada is making, just with SI units

 

[scubadada]

I do not understand how 100 is 35% more than 80. It is 25% in my math...
And coming back to the OP.
He said that his normal usage is starting with the tank at 3000 PSI and ending the dive when the SPG shows 1000 PSI.
This means that he uses 2/3 of the total gas in an AL80, which is in reality 77 cuft, so 55.3 cuft.
Using an HP100 with the same initial and final pressure (3000 and 2000 psi), this corresponds to an amount of available gas equal to 58.7 cuft, which is an increase of just 6.12%.
If instead the initial pressure is raised to 3442 psi, and still thumbing the dive at 1000 psi, the HP100 provides 71.65 cuft, meaning an increase of 29.6%.
I am a bit tired, being 01:00, so please correct me if my calculations are wrong...

Go back to my post #11, You dive an AL80 from 3000 psi, 77.4 cu ft down to 500 psi, 12.9 cu ft. You dive an HP 100 from 3442 psi, 100 cu ft down to 444 psi, also 12.9 cu ft. For the AL80 you used 64.5 cu ft. For the HP100 you used 87.1 cu ft. For me, 87.1/64.5=1.35. This is 35% more available gas when diving the 2 cylinders down to the same reserve volume.

I guess we must just think differently. You can adjust the numbers any way you please.

Thanks @Brett Hatch

 

[tursiops]

Using an HP100 with the same initial and final pressure (3000 and 2000 psi)

Why would anyone do this? The whole point of the higher pressure tank is to carry more gas.
Why would one end with the same pressure? The whole point is to reserve a certain volume of gas to breath.

 

[Belzelbub]

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.

Might be a typo, but the FH 7 came with a Z 2nd not a ZX. My daughters have ZXs on their regs, and there isn’t a ton of difference.

Zeagle also offers an upgrade kit (reasonably priced, IIRC) that incorporates some improvements that were done post Huish acquisition. May already have it on the 2nd you have on your 7.

Regardless, the Zeagles breathe great, and have for some time. I also regularly use Atomics, and I see no reason to switch.

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

Yeah, but despite what my grade school teachers told me, I often have a calculator in my pocket.

 

[scubadada]

Sorry to belabor the point. For a HP120 you would dive the cylinder down to 370 psi, also the 12.9 cu ft reserve volume, 107.1 cu ft breathed. This would be a 66% increase in the available gas between the HP120 and the AL80, 107.1/64.5=1.66.

I have to apologize, much of my diving is solo drift in SE FL. I generally to not have to pay attention to buddies, rock bottom... My buddy is a pony. I have free access to surface at any time unless I have light deco. In general, there's no particularly good reason I have to surface with 12.9 cu ft of reserve volume. However, I do understand the concepts.

 

[inquis]

it is misleading to label a tank by the total rated volume at nominal pressure

US labels by volume at rated pressure, elsewhere labels by volume at 1 atm. Both are arbitrary, but just as "standard".

I could say your tanks are misleading because they grossly understate the volume of gas. However, doing so would just indicate that I don't understand the "reference" pressure is different than my expectations.