Transfill whip math

[Litefoot]

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?

 

[inquis]

Nope, those volumes are at different pressures. Instead, you should use the normalized volumes (vol / rated pressure)

 

[johndiver999]

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

 

[inquis]

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.

 

[inquis]

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.

 

[inquis]

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!

 

[Divin'Papaw]

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?

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.

 

[rhwestfall]

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.

 

[halocline]

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?

My eyes tend to glaze over when I see written equations, so for a practical matter in this case I would just watch a gauge and try to fill the Al19 to it’s rated pressure. If you have a full HP100 (3442) you should be able to get the AL19 to close to 3000PSI.

If you want to understand what’s actually going on with the pressures/volumes and have trouble with equations like I do, you can pretty close by figuring it out; start by understanding that a cubic foot of gas (volume) is represented by a quite different amount of pressure in each tank. For the HP100, you have 100cft at 3442 PSI, so divide 100 by 3442 and you get each PSI is .029 cft. For the AL19, you have 19 cft at 3000 PSI, so each PSI is .0063 cft. Or, in the HP100 each cubic foot is 34.42 PSI, where in the AL19 each cft is roughly 158 PSI.

Assuming you start with a completely full HP 100 and a completely empty AL19, if you are transferring 19 cft from the HP 100, that is about 654 PSI in the HP 100. That would leave you with 2788 in the HP 100. But that can’t happen because the AL19 would then be at 3000. You can’t just split the difference (2894) because the tanks are different sizes, and that means they fill at far different rates. This is where the equations come in handy because all that can be factored in. But, since we’re just wasting time trying to reason it out…you can get pretty close by estimating. We know 19cft won’t work, let’s try 18. That drops the 100 to 2822 PSI. 18 cft in the AL19 is 2844 PSI. That’s pretty close.

These figures are not exactly accurate for a variety of reasons that someone more knowledgeable than I am could explain. One factor is that we’re talking about PSIG, as opposed to absolute PSI, and that the tanks are not ‘empty’ at 0 PSIG, they each have air at atmospheric pressure which equals the liquid volume of the tanks.

All of this assumes equal temperature, which I’m sure you realize is not going to be the case as you’re filling.

I have no idea if this long winded post is helpful, and I’m sure that if I’ve made a mistake, some kind soul will correct me.

 

[Litefoot]

Of course we haven’t touched on transfer rates and its effect on temp and pressure between the two bottles or the effect of altitude on our gauges. Fun discussion, but I think the best suggestion here is….to see if the shop will top off your pony bottle.