Why is it hard to blend the exact % of O2 for a EANX fill ?

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When I did my gas blender course a few years ago the standard was supposed to be O2 within 1% and helium within 5%. I mix gas for my own cylinders adn don't do a lt of fills compared to a shop, but find it pretty easy to meet those paramaters. I think a shop should be able to fill accurately even if they are doing many fills in a short space of time.
 
Very few people fill factor in temperature differentials in filling because the math is essentially impossible to perform since the variables are far too great. If you fill slow enough, you can get VERY close, but it takes forever, read several hours to fill, and no one wants to do that, so they fill the O2 slow, and then fill the air as normal.
The simple and rather obvious solution to this is to let the tank cool down, and then top up the missing bar or psi. If you've done your math right you ought to be pretty spot on.

My club has a continuous blender, but I still top up. If not, I'd have some seriously short fills even when "overfilling" by some 10%. I don't bother with the mixer when I'm topping up, so I aim for 33% in the hot tank. That usually brings me within half a percentage point from 32 after topping up.

Now, PP blending into 300 bar tanks, or anything involving more than homeopathic amounts of He brings with it a whole 'nother can of worms called non-ideality or compressibility. Good luck getting close to the target if you use ideal gas behavior for those calcs
 
When I find a shop that fills in a water filled rubbish bin it's usually my last unless they have beer
 
The simple and rather obvious solution to this is to let the tank cool down, and then top up the missing bar or psi. If you've done your math right you ought to be pretty spot on.

My club has a continuous blender, but I still top up. If not, I'd have some seriously short fills even when "overfilling" by some 10%. I don't bother with the mixer when I'm topping up, so I aim for 33% in the hot tank. That usually brings me within half a percentage point from 32 after topping up.

Now, PP blending into 300 bar tanks, or anything involving more than homeopathic amounts of He brings with it a whole 'nother can of worms called non-ideality or compressibility. Good luck getting close to the target if you use ideal gas behavior for those calcs

that is obviously the ideal way to do it, but that means hooking the tanks up twice, and since that is where a good chunk of the money is, it's better to just overfill them while hot and let them cool off, at the cost of mix accuracy. +1% is good enough for any recreational depths. Once you get to 100m+ diving, then it starts to be a bit more critical, but at that point there is so much helium in the mix that you aren't filling those bottles fast unless it's out of a bank.
 
https://www.shearwater.com/products/perdix-ai/

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