How Cave Shops Calculate Cubic Feet

[jvanostrand]

Are Florida cave diving shops selling hot gas?

I returned to Florida caves with my transmitters and being the numbers nerd I am I decided to take a little time at the end of the week to calculate my gas usage. I was wondering if the fill card I had on file was enough to cover my week of diving. When I went to close my tab at the dive shop I already had my cu ft usage in a spreadsheet on my phone and it was much less than what the shop calculated. So I asked how they calculate it. The answer was something like:

We record your starting pressure, then fill your tank to rated pressure and record the cubic feet used. Then we let it cool and top it off recording that additional volume.

An example to illustrate:

You bring an AL80 in with 1500psi. They fill it to 3,000 and add 40 cu ft to your bill. It cools down to 2,700 in the water bath and they top off to 3,000 and add another 8 cubic feet to your bill. You end up with only 40 cu ft of additional gas, but they sold you 48.

Do other shops do this?

 

[Tracy]

I have never had them do that, also never had them cool tanks and top off. I don't ever remove my tanks from the vehicle in cave country. Just hook them up and fill away.

 

[formernuke]

Did you or they temp adjust?

 

[grantctobin]

Are Florida cave diving shops selling hot gas?

I returned to Florida caves with my transmitters and being the numbers nerd I am I decided to take a little time at the end of the week to calculate my gas usage. I was wondering if the fill card I had on file was enough to cover my week of diving. When I went to close my tab at the dive shop I already had my cu ft usage in a spreadsheet on my phone and it was much less than what the shop calculated. So I asked how they calculate it. The answer was something like:

We record your starting pressure, then fill your tank to rated pressure and record the cubic feet used. Then we let it cool and top it off recording that additional volume.

An example to illustrate:

You bring an AL80 in with 1500psi. They fill it to 3,000 and add 40 cu ft to your bill. It cools down to 2,700 in the water bath and they top off to 3,000 and add another 8 cubic feet to your bill. You end up with only 40 cu ft of additional gas, but they sold you 48.

Do other shops do this?

Pray tell which shop?

 

[Manatee Diver]

I believe CCDS is target pressure subtracting starting pressure times tank factor. Sometimes you get a bit more, sometimes a bit less.

That is also how Wayne told me to input my fills at Amigos.

 

[Manatee Diver]

Pray tell which shop?

I believe only DO actively sells fill cards.

 

[jvanostrand]

I believe CCDS is target pressure subtracting starting pressure times tank factor. Sometimes you get a bit more, sometimes a bit less.

That is also how Wayne told me to input my fills at Amigos.

My concern isn't about ideal gas vs real gas or minor rounding using tank factors. It's about 20% or so overcharge for gas. And I'd even be okay with that if the cost of gas was comparable. So a shop selling 12.5c/cu ft gas but overcharging by 20% is the same as a shop charging 15c with accurate volumes and I wouldn't care.

 

[Manatee Diver]

My concern isn't about ideal gas vs real gas or minor rounding using tank factors. It's about 20% or so overcharge for gas. And I'd even be okay with that if the cost of gas was comparable. So a shop selling 12.5c/cu ft gas but overcharging by 20% is the same as a shop charging 15c with accurate volumes and I wouldn't care.

You aren't the first report of unusually high volumes from DO. But I can't say it is widespread, because I think you are maybe the third time I've heard about it. But that might be selection bias as most of the people I know don't really patronize DO.

 

[NW Dive Dawg]

Maybe it's just me and I'm missing something..... but if I bring in a tank with existing 300psi or a tank with existing 1500psi..... then they charge the same price to fill the tank to full working pressure.

 

[Manatee Diver]

Maybe it's just me and I'm missing something..... but if I bring in a tank with existing 300psi or a tank with existing 1500psi..... then they charge the same price to fill the tank to full working pressure.

In cave country they charge by the cubic foot. Which allows to differences in tank sizes and starting pressures. So my LP108s cost more to fill than my LP85s.