Which HP cylinder would hold same cf as LP95?

[scubadiver888]

I'm looking to learn the math of cubic feet to PSI.

I like the idea of using low pressure tanks because they are easier on my gear. I also like the idea that using a high pressure tank will eliminate the chance someone accidentally mistakes my LP for an HP fills my LP95 with 3000+ PSI.

I've read that the LP95 will hold 95cf if it is overfilled to 2400 PSI + 10% or 2640 PSI.

I've read that an HP119 filled to 3442 PSI holds 119cf but if you underfill it to 2640 PSI you get 95cf.

If I assume the PSI and cf ratios are linear the math does not work out. For example 3442 / 119 = 28.92 but 2640 / 95 = 27.79. Are the numbers wrong? Or is the math not so straight forward. My gut says the ratio is logarithmic.

[rox@ucf11]

you divide the tank capacity in cu/ft by the capacity in PSI so a LP95 cuft tank would be

95(cuft)/2650(psi)=.0358(cuft/1psi) *100 = 3.58(cuft/100psi) also know as the tank factor

So your numbers were right... to begin with, but your math was wrong.

As to overfilling your LP95 to 3000 I wouldnt worry about it at all, in Cave country it is not super rare to have a LP cylinder filled to 4200, and 3600 is standard and very very common, you dont hear about tanks exploding, or failing hydro so...

I believe the hp/lp tanks are the same the reason for hp tanks being more expensive is that the manufacturers have to pay DOT every 2 yrs for a reexemption in order to have a rated high pressure. If you look for example at a Faber HP130/LP108 they are very similar.

[in_cavediver]

One more point, there is no identical tank from LP to HP. The metals used are different with different tensile strengths.

That said, an LP95 is very close to an HP119. An LP104 is close to an HP130, the LP85 is close to the HP100.

Last bit, if you do go LP tanks - look up cave fills. I accidently get 3600psi in my LP104s every time I go to North Florida. Its even a cool 3600 at that!

[rox@ucf11]

I have heard from alot of people that they are the same metal....but of course cant be absolutely sure. I believe in the TDI tank inspector manual it did indeed say that HP tanks required a DOT exemption every two years.

[CWSWine]

Just little more info

True Capacity Are

LP95 = 93.3 cu feet
HP100 = 99.5 cu feet
HP119 = 123 cu feet
HP120 = 120.6 cu feet
HP130 = 131.4 cu feet

[Splitlip]

Originally Posted by scubadiver888

I'm looking to learn the math of cubic feet to PSI.

I like the idea of using low pressure tanks because they are easier on my gear. I also like the idea that using a high pressure tank will eliminate the chance someone accidentally mistakes my LP for an HP fills my LP95 with 3000+ PSI.

I've read that the LP95 will hold 95cf if it is overfilled to 2400 PSI + 10% or 2640 PSI.

I've read that an HP119 filled to 3442 PSI holds 119cf but if you underfill it to 2640 PSI you get 95cf.

If I assume the PSI and cf ratios are linear the math does not work out. For example 3442 / 119 = 28.92 but 2640 / 95 = 27.79. Are the numbers wrong? Or is the math not so straight forward. My gut says the ratio is logarithmic.

simple ratios
psi/cf=psi/cf will get yo close enough if you forget things like an AL80CF is really 77.??? LOL

And if you don't listen to people like my AOW instructor who tried to explain it to me. Tried talking from **** he meorized. He got it all jacked up. I am a degreed engineer. WTF! I tried to listen to him. But the hell with that.
Very simple, no voodoo. Math rules.

[Splitlip]

As to OP. LP 95 is 95 for 5 years at least at + service pressure. HP 100 is 100. Close enough.

Pump up a LP tank, see my "equation" above. Pump up or under fill a HP. See above.

[scubadiver888]

Thanks for everyone's reply.

The reason I wanted the math was to see how short I could fill an HP to get the right cubic feet for a dive. Basically, if I have an HP100 and I don't need 100cf can I short fill the tank and reduce the stress on my regulator.

If I can easily do a dive on an AL80 then I could fill an HP100 to 2700 PSI (100/3442 ~= 0.029, 2700 * 0.029 = 78.3cf) and it would be easier on my regulator plus I'd have better buoyancy characteristic then the AL80.

I've also noticed most the shops I found are only selling HP now. They say to just short fill an HP if you want LP.

[in_cavediver]

Originally Posted by scubadiver888

Thanks for everyone's reply.

The reason I wanted the math was to see how short I could fill an HP to get the right cubic feet for a dive. Basically, if I have an HP100 and I don't need 100cf can I short fill the tank and reduce the stress on my regulator.

If I can easily do a dive on an AL80 then I could fill an HP100 to 2700 PSI (100/3442 ~= 0.029, 2700 * 0.029 = 78.3cf) and it would be easier on my regulator plus I'd have better buoyancy characteristic then the AL80.

I've also noticed most the shops I found are only selling HP now. They say to just short fill an HP if you want LP.

I wouldn't worry about 'stressing' your reg. There is a saying in cave diving that gas is life. If you have something to breathe, you can solve most any problem. This is equally true in rec diving. It doesn't hurt to have 20cft in your tank when you surface. (actually, look up Rock Bottom Gas planning and that will tell you how much you should have left). There is no rule saying you have to breathe it all to be a good dive.

Again, with the regs. The first stages are designed to take tank pressure down to around 130psi or so. Unless you have a very old reg, all can easily handle 3500psi. Some will say to go DIN at 3500 but even that is somewhat debateable if truly needed. DIN does go higher with some tanks using fill pressures up to 4500psi. I wouldn't think twice about using any of my first stages (DIN) on a 4500psi tank. If you go HP - 3500ish - go DIN on your reg.

So, choose the tank that has the right capacity, buoyancy characteristics and wallet characteristics for you. Then, fill it to as close to its rated capacity (or cave capacity) as you can qet with your fill station and be happy. Leave underfills for deco mixes, trimix blends (without booster), transfills (without booster) or other more unique situations you aren't likely to run into.

[skdvr]

Here is a excel spread sheet that I have for my particular tanks. I have it printed out and laminated in my gear box so I can easily look and see how much gas is left in any of my tanks. All you have to do is chage the actual volume amount in yellow and the rated fill pressure and it will change all the number below it....

Phil