How high can you compress air before the volume equation becomes

[Potapko]

Non linear?
I have no idea if I am even asking this correctly but I'll try to explain what I mean. A couple of friends of mine came across some information claiming that if you compress air to a high enough pressure, you will no longer be able to calculate the amount of air you have in a tank. meaning that a 12l tank at 200 bar has 2400l of air in it. At 300 bar it has 3600l of air, at 400 bar it has 4800l of air.........

Can you "over-compress" air so that the simple equation is no longer valid?

I replied that if possible, it would certainly be at much higher pressure than we use in Scuba diving.

What say you experts?

thanks in advance,

 

[Bob DBF]

You would not have an intact SCUBA tank way before that particular problem arises. The ideal gas laws work on gasses, I would imagine as you compress it enough for it, or any of it's component gasses, to become a liquid all bets are off.

In addition, ideal gas laws work on ideal gasses of which air is not one, so there may be anomalies along the way if you were to perform a scientific experiment. It's all "good enough" for what we do, and if your friends decide to find out emperically, stand way back from the equipment.



Bob
-------------------------------
I may be old, but I’m not dead yet.

 

[Potapko]

You would not have an intact SCUBA tank way before that particular problem arises. The ideal gas laws work on gasses, I would imagine as you compress it enough for it, or any of it's component gasses, to become a liquid all bets are off.


Bob
-------------------------------
I may be old, but I’m not dead yet.

thanks, but I am really interested in finding out if anyone knows what that pressure might be where the change occurs

 

[cascas]

thanks, but I am really interested in finding out if anyone knows what that pressure might be where the change occurs

270 bar or 3915 psi ( more or less )

You would have to dive into the physics laws by van der Waals to figure it out exactly. ( not my cup of tea )

This is also the reason why in technical diving 300 bar tanks are rarely used. The benefits are really low. 220 bar tanks can easily be filled to 240 bar / 3480 ps or morei. That extra 30 bar is hardly worth the extra weigth of 300 bar steels or the extra weights you need to compensate for the lighter 300 bar composite tanks.


Equation of state - Wikipedia, the free encyclopedia

 

[Potapko]

270 bar or 3915 psi ( more or less )

You would have to dive into the physics laws by van der Waals to figure it out exactly. ( not my cup of tea )

Equation of state - Wikipedia, the free encyclopedia

That is actually valuable information. So if using 300 bar tanks, which are common, you cannot count on the fact that you have the gas you think you do. I thought it would be much higher. 350 bar or 400.

 

[cascas]

That is actually valuable information. So if using 300 bar tanks, which are common, you cannot count on the fact that you have the gas you think you do. I thought it would be much higher. 350 bar or 400.

Nope in my book 300 bar tanks are just useless.... Not just for the ( in the Netherlands) poor availability of 300 bar filling stations. And as I later added the fact that A there is no such thing as 300 bar and b the lesser wight needs to be compensated in case of composite tanks or you're stuck with tanks that are way too heavy...

 

[Potapko]

A few people here use the 300b steel doubles. they have double 8l that at 300b hold the same air as a pair of 200b double 12's. Plus are a whole lot easier to carry around at around a 5 kilo weight difference.

 

[VooDooGasMan]

My beauchat 140's are 6500 psi rated but to have a 1st & 2nd stage to be under that kind of pressure is hard on them, so really the lower the pressure the better, and my hp 100's pumped a bit are actually my favorite ones to dive.

 

[iain/hsm]

In addition to Van deer Waals there are a couple of other factors you need to consider.

Ignoring adiabatic compressive heat, the methods of compression and diving for a moment there is the real consideration as to spontaneous ignition temp of the oxygen content within the compressed air at very high pressure. The 20.9% partial pressure of oxygen in air at high pressure if you like.

Although we commonly compress inert gas such as nitrogen to 700 barg (10,000 psi) Due to the PP02 of Air a pressure of 350 to 414 barg (5000-6000psig) is about our considered limit for air compression.

At these pressure particulate concentrations and long chain hydrocarbons are also extremely dangerous.

As for so called pressure losses above 300 bar 4500 pig, it would be better as these pressure do not follow the P1 V1 T1 rules but to consider the contents of the cylinder as a molar mass of gas rather than pressure over volume.

Molar volume - Wikipedia, the free encyclopedia

Now back to diving, although regulators are made for these pressures they are mostly for use in the oil industry for pressurising seabed equipment already sitting at 3000 MSW to say 200 barg. The surface pressure required is therefore 500 barg.

For divers it is the pressure drop that is also critical, to effect a flow you need a press drop and in doing so you cool the gas, a second stage demand valve at the mouthpiece requires say 10 barg drive pressure, a 300 to 10 or 200 to 10 pressure drop can make all the difference in cold water between operating and freezing.

Hence why the required pressure dewpoint or water vapour content of the gas at higher pressures is much lower than at lower pressures. Say -55C at 200 barg and -65C at 300 barg.

Typical scuba regulators are not very well balanced at these very high pressures the hysteresis and spring rate is such that typical over pressure (superflow) occurs.

So in compressing air at very high pressure consider:
1. Oxygen ignition temperatures and adiabatic considerations with partial pressure.
2. Particulate contamination, oil mists, and deposits.
3. Weight of gas being delivered (molar volume) as well as just the pressure of gas
4. Pressure drop from storage pressure to required WP, Freezing, velocity
5. Water vapour dewpoint consideration.

Iain Middlebrook

 

[cascas]

A few people here use the 300b steel doubles. they have double 8l that at 300b hold the same air as a pair of 200b double 12's. Plus are a whole lot easier to carry around at around a 5 kilo weight difference.

I am afraid that your 8l @ 300b will hold somewhere between 2160 and 2400 l of gas @ 1bar. Most likely closer to 2160 than 2400. And that is due to the Vanderwaals forces.
Your statement would be true for ideal gasses while that what we breathe (either air or eanx ) is not.