Wassup with Cave Fills?

[Tracy]

Not that this is at all important, but I'm just curious. Do they measure the pressure before the fill, and then after the fill, and subtract the first measurement from the second and then charge by that measure of cubic feet? And is the second measurement when hot or cold? Or do they just fill it and charge according to the final pressure without reference to how much air was already in the tank? Or do they have something that meters air as it goes in?

It is based on starting and ending pressure.

 

[boulderjohn]

Not that this is at all important, but I'm just curious. Do they measure the pressure before the fill, and then after the fill, and subtract the first measurement from the second and then charge by that measure of cubic feet? And is the second measurement when hot or cold? Or do they just fill it and charge according to the final pressure without reference to how much air was already in the tank? Or do they have something that meters air as it goes in?

It is based on starting and ending pressure.

...and it really has to be that way.

A set of doubles of trimix can cost you hundreds of dollars, and if you are diving thirds into flow and exiting with the flow, you will probably have at least half of that gas left. It would be criminal for the shop to top it off and charge for the full amount in the tanks.

In contrast, it you return with 500 PSI of air or nitrox in your AL80, it would be silly for a shop to measure it and deduct that amount from your $10 fill cost.

 

[CT-Rich]

The flip side of this is the short fill. I check my tanks before I leave the shop and leave them over night so they can get a proper fill. On more than one occasion I have gotten my HP back with 3000-3100 and have asked them to top it off. They know me now and it seldom happens, because I check them before I leave.

Buying air is like buying ice cream. Nobody complains about the scoops being too generous... In a competitive local market, I could see the sense in giving the locals a generous fill.