I’m moving gas from an HP100 to an AL19 pony. We’ll round the 19cf pony up to 20cf for simplicity. So I would expect that the HP100 would lose 20/100 or 20% of its volume of air when I transfer to the pony bottle which is 3000*.20 or 600 psi. So should expect the two cylinders to equalize at 2400 psi?
Transfill whip math
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Nope, those volumes are at different pressures. Instead, you should use the normalized volumes (vol / rated pressure)
johndiver999
Contributor
Don't sweat the math. Typically you will not have a drained pony bottle - usually just a little low. If it is empty, you can first whip from a used tank and get a few hundred psi, then go to the full tank for top off.
The HP 100 should start at 3500 psi, hopefully. If you are smart you will get whip with a gage and you can see what the pressures are in each tank during the transfer process by turning the respective valves off.
I think the math is 100/119*3500 if starting from zero. 2917
The HP 100 should start at 3500 psi, hopefully. If you are smart you will get whip with a gage and you can see what the pressures are in each tank during the transfer process by turning the respective valves off.
I think the math is 100/119*3500 if starting from zero. 2917
The equalization pressure is a weighted average of the two cylinder pressures. The weights are the tank factors (denoted T below) which are normalized capacities.
Pe = (Tb * Pb + Td * Pd) / (Tb + Td)
where "b" is for the "bank" and "d" is for the destination. T is the tank factor = capacity per psi (100/3442 for the HP100 and 19/3000 for the AL19).
You will get more accurate answers with this formulation if you use the ideal capacity, though (especially when the bank is below about 2400 psi). The HP100 is quoted including compressibility, but the AL19 isn't (but it is rounded):
T(HP100) = 106.7 / 3442
T(AL19) = 18.9 / 3000
You can also use the water volume as the tank factor if you happen to know it since the units ultimately cancel out. It's conveniently stamped on the cylinder outside the US market.
Pe = (Tb * Pb + Td * Pd) / (Tb + Td)
where "b" is for the "bank" and "d" is for the destination. T is the tank factor = capacity per psi (100/3442 for the HP100 and 19/3000 for the AL19).
You will get more accurate answers with this formulation if you use the ideal capacity, though (especially when the bank is below about 2400 psi). The HP100 is quoted including compressibility, but the AL19 isn't (but it is rounded):
T(HP100) = 106.7 / 3442
T(AL19) = 18.9 / 3000
You can also use the water volume as the tank factor if you happen to know it since the units ultimately cancel out. It's conveniently stamped on the cylinder outside the US market.
The easy route is to use the Gas Blender Toolkit, specifically the Topoff Tool from its menu.
It will be more accurate than the above expression, since it understands when compressibility is a factor and when it's not.
Accuracy aside, though, it's just incredibly simple to just pick your tanks & pressures and it will give you the answer.
It will be more accurate than the above expression, since it understands when compressibility is a factor and when it's not.
Accuracy aside, though, it's just incredibly simple to just pick your tanks & pressures and it will give you the answer.
A simplified expression, specific to the HP100 and AL19:
Pe = 83% * P(HP100) + 17% * P(AL19)
For P(HP100) = 3000 psi and P(AL19) = 0, Pe = 2490 psi
Or use 0.8x and 0.2x if that's easier to remember... which is what you guessed in the OP!
Pe = 83% * P(HP100) + 17% * P(AL19)
For P(HP100) = 3000 psi and P(AL19) = 0, Pe = 2490 psi
Or use 0.8x and 0.2x if that's easier to remember... which is what you guessed in the OP!
I just fill my pony bottles once a year at VIP. I never have to fill them again until the next VIP. I have 2 AL40s that I use this way throughout the year. I don't ever transfill. If you don't have to use your pony, which you shouldn't, it shouldn't ever be an issue. If you DO need it, then you'll be glad you had it and you'll also be happy to pay for that one fill to get it back to full pressure. Just my 2 cents. YMMV.
ADDITION: I keep different gas in my pony than my back gas. My back gas tanks all have nitrox, typically 36%, but I keep air in my pony. So that also contributes to why I never transfill them. Plus my pony bottles are on the large side at AL40s. I have them from my tech diving days, am very comfortable slinging them, and they provide plenty of gas for a recreational pony at any depth. I also use them to shoot my DSMB at the end of most dives!
Regardless, you shouldn't have to fill your pony much, even an AL19, unless you are practicing or had to use it in an emergency.
sidebar/ramblings:
A shop here, as long at the pony is in hydro and vip, will fill/top it up for free. They value those implementing safety.
Having my own compressor, it doesn't matter to me.
I'll typically finish my last dive of the day by using my pony at the safety stop. Keeps "muscle memory" exercised, and also verifies equipment is functioning correctly.
Anyhow, yes, the link to the above resource is quite useful.
A shop here, as long at the pony is in hydro and vip, will fill/top it up for free. They value those implementing safety.
Having my own compressor, it doesn't matter to me.
I'll typically finish my last dive of the day by using my pony at the safety stop. Keeps "muscle memory" exercised, and also verifies equipment is functioning correctly.
Anyhow, yes, the link to the above resource is quite useful.
