How much air does an LP85 hold at 2400 PSI?

[kelemvor]

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.

I've always thought the same thing. I suspect the answer is that it's "marketing." That's just a suspicion. The +/10% "overfill" is completely standard. I've never met any shop that would be concerned with filling a + rated tank to the + rated pressure. Many shops will fill well beyond that + rating if it's a steel (vs aluminum) tank.

 

[Lorenzoid]

Where does the concept come from, anyway?

I believe tech divers came up with it as a shortcut way to calculate how much gas they have in cubic feet in real time, without a calculator. As rjack mentioned, you need to know volume if you're diving with a buddy who has a different tank size than you, so you can talk in terms of volume, not psi. "Dissimilar tank matching," they call it.

 

[CaveSloth]

They routinely fill my tanks to 3600 in cave country and last time I got 3800.

 

[Lorenzoid]

They routinely fill my tanks to 3600 in cave country and last time I got 3800.

So here's your quiz You have an LP 85 filled to 3600 psi. Roughly how many cubic feet do you have?

 

[CaveSloth]

ROUGHLY because I am sure it is not exactly 85cf at 2640.... At 3800 I would have:

3800/2640*85 (times 2 tanks) = 244.6cf

 

[rjack321]

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?

Because you need to calculate a minimum volume to reserve for ascent aka "rock bottom" You have that volume based on how long your ascent will take. But how do you convert it to psi pressure while on the back of a boat or the side of a rib? You have 85s I have 95s. The minimum gas we need to reserve is the same but the pressure to actually have that will be different.

In metric its easy L x bar = volume
In imperial its not as easy because 85cf (95, 119 etc) is tangled up with pressure and not a water capacity volume. The tank factor removes pressure from the equation so all you have to do is multiple or divide one number (x psi) to figure out what you need to know.

 

[Storker]

So here's your quiz You have an LP 85 filled to 3600 psi. Roughly how many cubic feet do you have?

What's the service pressure at which the tank holds 85 surface cubic feet?

 

[kelemvor]

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 12L."

I like metric, although I'm more comfortable with imperial. However, I don't think there's any rule in the metric system that says you have to measure by water volume vs gas volume. I think that's just something people are used to doing. It always sounded bizarre to me. Measure tank volume using something you don't ever want inside your tank? Or measure tank volume by how much it will hold of the thing it's actually going to be used for holding.

IMO the non-confusing way to measure tank volume in metric would be cubic liters of air rather than liters of water.

 

[runsongas]

@Lorenzoid

water capacity isn't really that much better because of varying service pressures. and once you calculate the capacity in liters, it isn't that intuitive.

 

[Lorenzoid]

ROUGHLY because I am sure it is not exactly 85cf at 2640.... At 3800 I would have:

3800/2640*85 (times 2 tanks) = 244.6cf

Sure, but here's where knowing your tank factor comes in. Since your tank factor is 3.2, you could round down to roughly 3. You know you have 38 units of 100 psi, and for every unit of 100 psi you have 3 cubic feet. So just multiply 38 by 3. You can do that in your head, while floating in the spring basin preparing for a dive.