How much air does an LP85 hold at 2400 PSI?

[Bowers]

Seems impacting the ceiling is enough to cause total gas loss. Sobering thought.

strong argument for sidemount

 

[JohnnyC]

Seems impacting the ceiling is enough to cause total gas loss. Sobering thought.

Do you have any idea how hard you would have to impact a cave in order to damage an isolator to the point of failure? I've seen doubles fall off the tail gate of a pickup truck and smash isolator first into the ground, not damage. Chances are if you hit the cave that hard you've knocked yourself unconscious in the first place.

They're not fragile faberge eggs. I worry so little about isolator issues that I'd rather dive backmount if in any way possible for a variety of reasons, having access to all of my gas in almost every single failure scenario being one of them.

As for losing gas from a leak, you can hear it. Over the sound of your breathing, over the sound of your scooter, through a hood, you can hear it. If a leak is significant enough to be of concern, you can most definitely hear it.

 

[CaveSloth]

I recently weighed my twinset before and after filling. Here is the data I collected:

Before filling:

83.9 lbs @ 1150 PSI

After filling:

90.9 lbs @ 3550 PSI

That 3550 PSI was the reading 45 minutes later after entering the water at Ginnie Springs so the tanks had a chance to cool down a bit. Immediately after filling it was more like 3750.

I tried computing the tank sizes from this data but got some wild numbers that can't be correct. Have a crack at it if you would like to help me 100% determine what kind of tanks I have.

I will keep measuring my tanks before and after filling and post more data here as I collect it. I am using a digital bathroom scale and balancing the twinset on it the best I can on flat ground.

 

[JohnnyC]

What’s 13 x 245?

 

[tursiops]

I recently weighed my twinset before and after filling. Here is the data I collected:

Before filling:

83.9 lbs @ 1150 PSI

After filling:

90.9 lbs @ 3550 PSI

That 3550 PSI was the reading 45 minutes later after entering the water at Ginnie Springs so the tanks had a chance to cool down a bit. Immediately after filling it was more like 3750.

I tried computing the tank sizes from this data but got some wild numbers that can't be correct. Have a crack at it if you would like to help me 100% determine what kind of tanks I have.

I will keep measuring my tanks before and after filling and post more data here as I collect it. I am using a digital bathroom scale and balancing the twinset on it the best I can on flat ground.

I would have expected 2400psi pressure drop in doubled LP85s to drop the weight about 12.4 lbs.

2x85/2640 x 2400 x 0.08 = 12.4 lbs​

But they only dropped 7 lbs.
So:
(a) your pressures are in error (not likely, and not that much);
(b) your weight change is in error (possible, but not that much);
(c) they are not LP85s.

For LP85s, you used 155 cu ft. At a RMV of 0.7 cuft per minute, in Ginnie (average depth maybe 60 ft), your dive would have lasted about 78 mins. How long did it last?

 

[CaveSloth]

Note: these measurements were taken before and after filling, not diving.

That fill was the first fill after a prior dive trip a couple of weeks earlier and I didn’t keep track of the starting pressure or average depth of that dive.

The dive at Ginnie was rather random because I was diving with my gf who is new to diving and so we surfaced a lot. Again, didn’t keep track.

Both employees at the shop both said my tanks are 85s.

I’ll keep measuring!

 

[Bob DBF]

I would have expected 2400psi pressure drop in doubled LP85s to drop the weight about 12.4 lbs.

2x85/2640 x 2400 x 0.08 = 12.4 lbs​

But they only dropped 7 lbs.
So:
(a) your pressures are in error (not likely, and not that much);
(b) your weight change is in error (possible, but not that much);
(c) they are not LP85s.

For LP85s, you used 155 cu ft. At a RMV of 0.7 cuft per minute, in Ginnie (average depth maybe 60 ft), your dive would have lasted about 78 mins. How long did it last?

Using your formula, the gas would be within a pound if one of the tank valves were turned off.


Bob

 

[NAM001]

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.

This goes back to I think ww2 days. the standard was made then in regards to the + marking and its use.

 

[NAM001]

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."

then you have the problem when you have a 13L/184 or a 13l/160 still the same complaint just a bit easier in math to use it.

 

[NAM001]

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.

The war wanted tanks ffor what ecer reason adn they were too small for their mil spec. so they aauthorized existing steel tanks to be over filled by 10% to meet their needs. the practice has never gone away. Back then tanks were really LP tanks well under 2000 psi. Perhaps even 1500 or less. the R&D involved was too long of a time and it was simpler to just authorize a + 10% to get the volumn or PSI they needed for their existing systems.

I would imagine that to make a tank (if the l,p85 was really in need in WW2) by todays standard, ther red tape would require a special permit to create,,, from the start,,, a lp85 tank that was still a 13L with the same specs,,, or stay with the existing standards of making a true lp77 or any 3AA tank with an overfill authorization. The design is based on the 2400 capacity and not the 2640 capacity.

Then you can look at an old lp72 and that is i think the rating at the plus psi. for sure if it is the newer tanks then you would think that the lp 72 is at 2475 with the + rating. . Someone chime in on this please becasue LP 72 just don't seem to fit the mold. lp 72's are marked as 2250 with the plus on the tank. would it make any difference if the tank was a 3A vs 3AA when it comes to the plus and working pressure?