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.

 

[Pague259]

I just think it’s pretty funny that the same group (FL cave divers) that gave us DIR / GUE, and who have constantly reinforced the critical importance of consistent practices and adherence to standards, willingly ignores the juxtaposition of cave fills.

I think the other factor which makes it less dangerous than you would think to overfill is that a catastrophic failure would most likely only occur during filling, so if the tank survives the fill then it should be safe to dive.

 

[CT-Rich]

I think the other factor which makes it less dangerous than you would think to overfill is that a catastrophic failure would most likely only occur during filling, so if the tank survives the fill then it should be safe to dive.

A comforting thought for the expendable tank monkey filling tanks $10 an hour...

 

[Rilelen]

I think HP steels get pumped to 4000 psi but I am not 100 percent on that -- a Florida local will correct me if I am wrong.

I’m local, in cave country. HP steels and LP steels both get filled to 4000, which often cools to 3600 in summer. Sometimes a touch higher.

Pricing is per cf but typically you’re charged for target end pressure of 3600 (not the actual hot fill to 4000) minus your starting pressure. Aluminums are typically filled generously to 3300 (charged for 3000). They’re careful when filling aluminum and steel at the same time to close off the aluminums once they hit their limits; no one asks or cares if your steel is LP or HP.

 

[Pague259]

A comforting thought for the expendable tank monkey filling tanks $10 an hour...

Yeah I was careful not to say anything about the danger to the filler because I don't know if the burst disk would protect them or the valve would go into the roof, or even the worst case a tank rupture.

I have no idea how common each scenario is and how dangerous each one is either.

 

[Lorenzoid]

I have no idea how common each scenario is and how dangerous each one is either.

I'm not sure there is a single documented case of a properly hydro-certified tank exploding due to these "cave fills" in north Florida. The dive shops there have been doing this for, what, 35 or 40 years and tens of thousands of fills? Burst disks, sure, some have burst, as expected.

But some might say this may be an example of normalization of deviance. That is, just because tens of thousands of fills over decades have been done without incident does not mean a tank could not explode on the 100,000th instance of this practice.