How much air does an LP85 hold at 2400 PSI?

[CptTightPants21]

Without the unnecessary nonsense of tank factors, if the tank holds 85 cuft at 2640, then it holds
(2400/2640) * 85 = 77 cuft at 2400 and (3600/2640) * 85 = 116 cuft at 3600. Simple proportions, at least for an ideal gas, which tanks factors don't accommodate anyway.

I have to agree. The calculation is as simple as a calculation can be. Tank factors add a useless complication.

Really? Tank factors simplify everything down into quick 2nd grade math calculations. Two digit multiplication is always going to be faster and easier than division plus multiplication. You want to do the proportions method fine, but to say tank factors complicate things rather than simplify is just wrong.

 

[kelemvor]

I'm actually confused by this. Isn't the calculation you cited the exact same one that's used to determine "tank factor"?

They're close. But tank factors involve doing part of the calculation up front, leaving you to memorize a formula to glean the same information you otherwise could have obtained by the simple formula. It's an extra step. I can't imagine where that sort of thing came from. Maybe someone needed extra content for their technical course?

 

[JohnnyC]

The stamped rating of the tank is known as the tank's service pressure. During hydro the tank can be tested to see if the +rating can be stamped, allowing fills that are 10% higher than the service pressure. Some hydro shops never test for the + rating, so it's a good thing to ask about before having a hydro done.

I was trying to keep it simple for the OP. He's having some trouble understanding apparently, I figured it would be easier this way.

 

[CaveSloth]

The thing that is confusing is that an LP85 does not contain 85 CF of gas at its rated pressure, but only when filled to 10% over its rated pressure. The + sign should not exist and an LP85 without a + sign should be called an LP77.

 

[rjack321]

They're close. But tank factors involve doing part of the calculation up front, leaving you to memorize a formula to glean the same information you otherwise could have obtained by the simple formula. It's an extra step. I can't imagine where that sort of thing came from. Maybe someone needed extra content for their technical course?

Tank factors allow you to (most) easily match gas volumes in imperial units for dissimilar tanks.
Liters water capacity functions exactly like a tank factor but for metric users.

One number x pressure = volume

 

[Lorenzoid]

They're close. But tank factors involve doing part of the calculation up front, leaving you to memorize a formula to glean the same information you otherwise could have obtained by the simple formula. It's an extra step. I can't imagine where that sort of thing came from. Maybe someone needed extra content for their technical course?

As I understand it, knowing the tank factor is a trick that enables you to quickly calculate in your head what volume of gas you have, which tech divers need to know. The OP is a cavern diver, so he ought to get used to the concept.

 

[rjack321]

The thing that is confusing is that an LP85 does not contain 85 CF of gas at its rated pressure, but only when filled to 10% over its rated pressure. The + sign should not exist and an LP85 without a + sign should be called an LP77.

Manufacturers have always taken advantage of things like this when marketing scuba tanks.

An AL80 also only has 77cf at 3000psi.

 

[runsongas]

the rated pressure is 2640 psi when it leaves the factory with the + rating. subsequent hydros where you don't get the + rating is usually due to the recertifier not wanting to deal with + ratings. LP tanks are commonly overfilled anyways, so whatever.

 

[kelemvor]

Tank factors allow you to (most) easily match gas volumes in imperial units for dissimilar tanks.
Liters water capacity functions exactly like a tank factor but for metric users.

One number x pressure = volume

The problem with tank factor is it's not something that's stamped on the side of your tank. If you want to use it, you've got to calculate TF first. Then you can do the calculation for gas volume. I've never heard of tank factors other than on scubaboard. Maybe the similarity to metric is the key. If you're used to metric and you visit someplace with imperial tanks maybe you'd be more comfortable with them?

Where does the concept come from, anyway?

 

[Lorenzoid]

It IS confusing and ridiculous. I wish the US went with the system used in metric countries, where a tank's name is not something a manufacturer made up but rather the volume of water it would hold. Compare: "I just bought an LP 85" with "I just bought a 13L."