How much air does an LP85 hold at 2400 PSI?

[CaveSloth]

I am training to eventually dive caves and one approach is to estimate your average depth during the dive and then conservatively use this along with your measured air consumption rate to determine multiple turn points for the dive including PSI, time, depth, distance, write these all on a slate and turn when the first one is hit.

 

[Lorenzoid]

@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.

Am I confused? I thought you just multiply water capacity in liters by measured pressure in bar to get the gas volume in liters?

 

[rjack321]

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 use 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 liters of air rater than liters of water.

Its liters of water because water is incompressible and there's no confusion about it being 11L at 1 ata. Metric tanks are filled with water at hydro time and are defined by water capacity because you can pour it out and measure it.

Liters of air immediately brings up (or should bring up) the question of "at what pressure"? You can't measure an air volume easily.

 

[Storker]

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

Allow me to disagree.

The "water volume" of a tank is its physical internal volume. It's a singular, non-confusable number. Multiply by pressure, and you have the actual surface gas volume (if we overlook the effect of compressibility). A 10L is a 10L is a 10L. No matter whether that 10L tank's service pressure is 200 bar, 232 bar or 300 bar. A 10x200 has a nominal gas volume of 2000 surface liters (70 cubic feet), a 10x232 is 2320 surface liters (82 cubic feet), a 10x300 is 3000 nominal surface liters, about 2700 real surface liters (95 cubic feet). I read my SPG, multiply my tank volume with my current tank pressure and know my remaining gas. No matter if the tank is a 70, an 82 or a 95. Because they all have the same internal ("water") volume.

 

[runsongas]

Am I confused? I thought you just multiply water capacity in liters by measured pressure in bar to get the gas volume in liters?

you have the problem that a 12.2 liter tank with a 184 bar service pressure will have lower capacity than a 11 liter tank with a 232 bar service pressure.

 

[Pedro Burrito]

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[CptTightPants21]

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?

Tank factors is a very simple chart. Once you have it, you have gas calculations on every tank imaginable and the ability to quickly compare tank volumes between yourself and your buddy. The factors are rounded .5 and the hp tanks line up with their LP size equivalent.

AL80 = 2.5
HP100/LP85 = 3
HP120,HP119,LP95 = 3.5
HP130,LP108 = 4
LP121 = 4.5

2x for doubles, Double AL80s = 5, HP100s = 6, etc

I find that people who complain about tank factors are usually the ones that haven’t been exposed to it in training and just don’t want to bother with it. You don’t even need to memorize the entire table, just memorize it for the tanks that you use and then actually apply it over a couple of dives. Once you start using it, there is no way you will go back to the proportions method when doing mental gas calculations.

 

[rjack321]

you have the problem that a 12.2 liter tank with a 184 bar service pressure will have lower capacity than a 11 liter tank with a 232 bar service pressure.

Are you defending imperial as "easier" or less confusing?? Liters x bar is about as clear as you get.

I mean a lp85 at 3800 holds more gas than a hp119 at 3300 too.

 

[kelemvor]

I just went and looked at my LP steels out of curiosity. They're stamped both with air volume "LP108" and water

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.

If you do it in PSI up front then you don't have to do math at any point during the dive. Unless you have a gauge rated in tank factors or something like that. Just memorize or write down your turn pressure and viola. At least, that's how we did it in my cave class.

 

[Storker]

Just memorize or write down your turn pressure and viola.

That's how I do it. I've calculated and memorized my min gas pressure at a couple of depths. Anything shallower than 10m is hunky dory as long as I stay above 50 bar, and from 10m to 30m I need to memorize just two (or perhaps three) data points to have enough data to stay within min gas. Even I can do that.