How much air does an LP85 hold at 2400 PSI?

[Storker]

Are you saying the amount of gas in the tank changes if I take the tank to depth?

No, of course not. I thought you knew my posting history better than suggesting something like that.

My tanks hold 10L of gas each, no matter their pressure. But the volume of that gas at ambient pressure depends on both tank pressure and ambient pressure. PV=nRT, P1V1=P2V2.

If I have 110 bar in my tank, I have 1000 bar·L (plus the 10 bar reserve) of breathable gas. That's 1000L on the surface, but only 500L at 10m. And at 30m it's 250L.

 

[Diving Dubai]

Perhaps with the exception of that last division by 12 or 15, I can do those calcs in my head.

While you're correct, you do make things complex for yourself

On a 10l tank 1bar=10l, 12l 1bar=12l etc

So my rule of thumb for calcs:

If I'm using a 15L, then I'll round my SAC up to 15/min or 1 bar/min at the surface

So from 30m I would figure it out thus

30m to SS = 3min + 3min SS = 1min ascent = 7min

Sac = 15 x 4 ATA so 4 bar per min, thus 7 x 4 = 30 bar min gas required (all rounded up and no allowance for reducing pressure) exc reserve

If I'm approaching Deco at 30m , then I calc gas consumption at that depth (not 10M) just because I like contingency and instantly know based on gas, and overall dive time, by how much I can over stay.

All nice easy maths

 

[tursiops]

My tanks hold 10L of gas each, no matter their pressure

Perhaps this is the semantic problem. A liter is a measure of volume. You have 10L tanks. But the amount if gas you can stuff in them depends on the pressure of the gas. You know all this, but insist on conflating the quantity of gas you have stuffed into the tank with what its volume might be if released at depth. Take that 1000 bar-L tank to 10m; you have not changed the quantity of gas in it. However, when you turn the valve, that quantity is compressed by a factor of 2, so fewer liters are available to you. All I'm arguing is that you can't say the tank has 1000 L at the surface but only 500L at 10m, which us what I think you said.

 

[runsongas]

I've showed you mine, what about you showing me yours?

0.5cf x 3 x 2.5 = 3.75 cf
0.5 x 1 x 4 = 2 cf
0.5 x 3 x 1.5 = 2.25 cf
total = 8 cf
16cf if buddy needs to be accounted
with al80 80cf tank, you need roughly 1/5 of the tank or 600 psi
with hp100, you need roughly 1/6 of the tank or 575 psi

you had a math error btw, at 20 liters consumption, you would need 150 liters on your initial ascent, total of 320 liters for yourself

 

[kelemvor]

Gas is a liquid, easy to measure volume.

They do NOT know how many cuft you got without knowing your tank size and working pressure. They measure pressure at beginning and end of fill, then calculate (P(end) - P(start))/2640 * 85. They know the working pressure from the stamping on the tank, but have to guess at tank size....like everybody on this thread is doing. 85 is a very good guess, but I would not bet my house on it.

And how did you account for dissimilar tank sizes?
If I have 95s and you have 108s both filled to 3600, what is your turn pressure?

2400. Same for my buddy. Although my buddy has less gas than me, he's far far better on air consumption. Unless he has a leak I'm going to be the one turning the dive. Why complicate it beyond the first person to hit their third (or sixth if that's what you're into) turns the dive?

Gas is a liquid, easy to measure volume.

They do NOT know how many cuft you got without knowing your tank size and working pressure. They measure pressure at beginning and end of fill, then calculate (P(end) - P(start))/2640 * 85. They know the working pressure from the stamping on the tank, but have to guess at tank size....like everybody on this thread is doing. 85 is a very good guess, but I would not bet my house on it.

The shops I frequent charge flat rate for fills (except for helium, which I don't use) so I'm not really in the know on this subject. Are you telling us that it's common for shops to be charging by volume of gas sold rather than fill and they aren't using a mass flow meter to accurately determine how much gas that they actually dispensed? Something like this, or any of the cheaper models. Seems like a recipe for some trouble if they ever get a disgruntled customer.

 

[rjack321]

2400. Same for my buddy. Although my buddy has less gas than me, he's far better on air consumption. Unless he has a leak I'm going to be the one turning the dive. Why complicate it beyond the first person to hit their third (or sixth if that's what you're into) turns the dive?

When you have used 1200psi of 108s and your buddy who's got the better consumption has used 1200psi of 95s... You are both turning <now>. If you run OOA at maximum penetration your buddy now has 1200psi to get you out and 1200psi for him.

1200psi is 42cf in his 95s.
Except you used 1200 of 108s or 48cf to reach that point.

You both drown.

 

[kelemvor]

When you have used 1200psi of 108s and your buddy who's got the better consumption has used 1200psi of 95s... You are both turning <now>. If you run OOA at maximum penetration your buddy now has 1200psi to get you out and 1200psi for him.

1200psi is 42cf in his 95s.
Except you used 1200 of 108s or 48cf to reach that point.

You both drown.

I guess I forgot that. Just so you don't think I'm a suicidal moron, I'll disclose that I'm not diving caves anymore due to a spinal fusion I had that limits my neck mobility and therefore ended my cave diving. Interestingly, my buddy typically dove Faber lp95's and I typically dive Worthington lp108's so that was indeed the perfect example.

 

[Lorenzoid]

. . . Are you telling us that it's common for shops to be charging by volume of gas sold rather than fill and they aren't using a mass flow meter to accurately determine how much gas that they actually dispensed? Something like this, or any of the cheaper models. Seems like a recipe for some trouble if they ever get a disgruntled customer.

With the price of helium, a flow meter like that might not be a bad idea!

 

[rjack321]

I guess I forgot that. Just so you don't think I'm a suicidal moron, I'll disclose that I'm not diving caves anymore due to a spinal fusion I had that limits my neck mobility and therefore ended my cave diving. Interestingly, my buddy typically dove Faber lp95's and I typically dive Worthington lp108's so that was indeed the perfect example.

Nah no worries man.

Doing dissimilar tank size calculations with oddball starting pressures is really what "tank factors" are for. I honestly couldn't tell you if 3300 in 95s is the same or more or less than 3600 in 85s without doing the math.​

 

[tursiops]

The shops I frequent charge flat rate for fills (except for helium, which I don't use) so I'm not really in the know on this subject. Are you telling us that it's common for shops to be charging by volume of gas sold rather than fill and they aren't using a mass flow meter to accurately determine how much gas that they actually dispensed? Something like this, or any of the cheaper models. Seems like a recipe for some trouble if they ever get a disgruntled customer.

I've never know a dive shop anywhere in the world that uses a gas flow meter like your link. It might be what is needed. It surely is expensive or they'd tell you the price!