Dumb down true air Capacity!!!

[happy-diver]

I am a hillbilly too but here in the southern hemisphere that means people from up north
because the closer you live to the hotness of the equator, the slower the brain functions

With your own compressor just plan your dive with the sac rate set at high, double disc
your valves, squeeze some extra air in, and dive whatever tank you like, conservatively

Unless you are one of those divers that won't get out of bed for less than 100 metres

Factors schmactors!

 

[Subcooled]

I have searched the forum over for a good way to figure the true air capacity of a scuba cylinder. I am attempting to figure this out, I am not a rocket scientist and don't clam to be. Can someone please assist me with the calculations. Please don't site laws without explanation. One of the main problems I'm finding is with a cylinder that is in liter water volume. ie Lp50 is 7.8L but most of the formulas I find are either straight up metric liters to bar or inches to psi. Thank you in advance.

Exact capacity is affected by two things:
1) physical volume V
2) pressure P

The capacity is not a simple product if PV when ptessure exceeds 230 atmospheres.
Reason: compressing air becomes more difficult as pressure increases. A related term: compressivbility

Let the volume be constant.
300 ATA means 270
270 ATA means 250
250 ATA means 240
240 ATA means 240

Please note that this is (obviously!) only a rule of thumb!

 

[PBcatfish]

I have Lp50s, OMS Lp98's and am getting a set of Lp85s. I fill my own tanks just trying to decide what the advantage of one tank pressure is over the other and just want to be able to do the math.

"advantage of one tank pressure is over the other"

High pressure tanks usually hold the most air in the smallest volume.
High pressure tanks often hold the most air for a given weight of the tank
High pressure tanks don't always get filled completely at all dive shops due to pressure limits on some compressors.
High pressure tanks require more energy to put the same number of CF into a tank.
High pressure tanks are usually the most negatively buoyant.
Some older regulators are not rated for the pressure in a HP tank.
High pressure tanks tend to be expensive

Low pressure tanks usually are a little negatively buoyant
Low pressure tanks can be used for blending gasses supplied from high pressure tanks
Low pressure tanks usually weigh a little more than a HP tank for the same volume capacity
I can still run my 1960's vintage regulator on a LP tank.
LP tanks are kind of expensive too.

Aluminum tanks tend to be floaty
Aluminum tanks are usually 3,000 psi, now considered standard pressure
Aluminum tanks tend to be less expensive.

After saying all that, I still like old school steel 72cf 2250psi tanks for their low weight, reasonable volume capacity & good buoyancy characteristics. I leave a J-valve on that kind of tank, just to catch the attention of the guys filing the tanks & encourage them to notice that this is not a modern HP tank. old steel 72's are hard to replace. I also circle the fill pressure on modern LP tanks.

Since you are filling your own tanks, it may be worth considering that filling at a lower pressure will put less wear & tear on your compressor.

 

[ofg-1]

Well, if I dumb it down to just the basics, doesn't matter the size or type of tank, you will have air in your tank until you don't.

 

[Tracy]

old steel 72's are hard to replace.

Really?
Can I interest you in 100 or so?

 

[PBcatfish]

Really?
Can I interest you in 100 or so?

I have a hard time finding usable ones in my area. I'd be interested in picking up one or two more.

 

[Subcooled]

High pressure tanks usually hold the most air in the smallest volume.

In europe we have 18/15/12/10/7/5 litres (=liquid volume) and either 300 or 232 ATA.

7 litres, 232 ATA = 232x7 = 1624 litres of air = 81 minutes on the surface (SAC: 20 L/min)
5 litres, 300 ATA = 270x5 (compressibility considered) = 1350 litres of air = 67 minutes on the surface

HP = Most air in smallest volume? False!

Please show a counter example with U.S. tanks!

High pressure tanks often hold the most air for a given weight of the tank

The most air = largest volume of 1 ATA gas = liquid volume times pressure reduced by compressibility.
Now, the amount of air grows with the growth of volume.

Dry weight or weight in the water? The latter, only, is relevant!

High pressure tanks don't always get filled completely at all dive shops due to pressure limits on some compressors.

And because compression causes heat and thus an increase in the pressure.
As the gas cools down, pressure will drop.

High pressure tanks require more energy to put the same number of CF into a tank.

Of course. Compressibility.
Whatever you squeeze, at first it will be easy, but later it will just suck.

High pressure tanks are usually the most negatively buoyant.

Thicker walls, yes. Great for drysuit divers!

Some older regulators are not rated for the pressure in a HP tank.

Sir, you are quite old, aren't you?
Please inform me if this is an U.S. problem!

High pressure tanks tend to be expensive

Out there!

Low pressure tanks usually are a little negatively buoyant

A little, perhaps, while HP tanks are adequately.
- I am a drysuit diver

Aluminum tanks tend to be floaty

Yeah

After saying all that, I still like old school steel 72cf 2250psi tanks for their low weight, reasonable volume capacity & good buoyancy characteristics.

History is not always bad. Bright individuals - and solutions - have existed since time immemorial.

 

[PBcatfish]

In europe we have 18/15/12/10/7/5 litres (=liquid volume) and either 300 or 232 ATA.

7 litres, 232 ATA = 232x7 = 1624 litres of air = 81 minutes on the surface (SAC: 20 L/min)
5 litres, 300 ATA = 270x5 (compressibility considered) = 1350 litres of air = 67 minutes on the surface

HP = Most air in smallest volume? False!

Please show a counter example with U.S. tanks!

The most air = largest volume of 1 ATA gas = liquid volume times pressure reduced by compressibility.
Now, the amount of air grows with the growth of volume.

Dry weight or weight in the water? The latter, only, is relevant!

And because compression causes heat and thus an increase in the pressure.
As the gas cools down, pressure will drop.

Of course. Compressibility.
Whatever you squeeze, at first it will be easy, but later it will just suck.

Thicker walls, yes. Great for drysuit divers!

Sir, you are quite old, aren't you?
Please inform me if this is an U.S. problem!

Out there!

A little, perhaps, while HP tanks are adequately.
- I am a drysuit diver

Yeah

History is not always bad. Bright individuals - and solutions - have existed since time immemorial.

I spoke of generalities, hence I used words like "often" and "usually". I knew that there were specific instances where those assertions would not apply.

I was attempting to help the original poster by offering points for him to consider, that were related to the question that he had asked.

 

[PBcatfish]

...
Sir, you are quite old, aren't you?
Please inform me if this is an U.S. problem!
...

I might be considered old by some standards. The same can be said of the Magna Carta, certain bottles of Pauillac wine or a Stradivarius violin. The same can also be said of some dive gear that I still own & use.

One of my older regulators says Cousteau-Gagnon on it. That may have come from France originally. That was manufactured when 2,250psi was the standard service pressure. The pin gauge on it is fully extended at that pressure.

The majority of regulators that I bought new in the early to mid 1980s are rated for 3,500psi service pressure. This was back when a 3.000 psi aluminum tank was considered to be a high pressure tank. Since tank pressures can sometimes exceed the stamped ratings, the 500psi buffer would certainly seem prudent. I still commonly use these regulators with aluminum tanks. My equipment was well chosen, is quite well maintained and has stood the test of time.

In more recent years, when I purchased 3,442 & 3,500psi rated tanks, I also purchased a new regulator that had a higher pressure rating, as I wanted to maintain my margin of design safety.

I will stand by my opinion that while my antique Cousteau-Gagnon regulator is still acceptable for use with a 2,250psi tank, I would not attempt to use it with my more-modern higher-pressure tanks. I don't even like to use a 3,500 psi regulator on a 3,442 pst tank, that may have a 10% overfill. That kind of good judgment would be a hallmark of wisdom, which sometimes comes with age.