Air is not an ideal gas?

[JB]

I wonder if someone has an answer to this question, I'm sure there's a physicist on the board.
Logically it seems to me that the higher the pressure in a cylinder the less volume you get per bar of pressure. We conveniently describe the relationship of volume and pressure as linear, but it can't be. There must be a point of diminishing returns. Perhaps within the range of 1 - 300bar air may behave in a linear fashion, I don't know. I can't find any simplistic explanation, there is a constant for each gas that seems to vary with pressure and temperature, something like you can fit less N2 into a cylinder than Air for the same pressure and temperature.
I suppose why I'm thinking about this is perhaps the advantage of a 300 bar cylinder is not all it seems to be at first glance. I notice on my dives I have an enormous SAC in the first few minutes, I thought due to cooling of the cylinder and BC infaltion, and then settles to around 10 l/min for the rest of the dive, (ends up at 11.6 average), but maybe it is loosing the top few bars quickly due to a smaller volume of gas/bar?
Overall even if it is so, does it make a meaningfull difference?

 

[ianr33]

This might help (or then again it might not!)
http://www.scubaboard.com/forums/advanced-scuba-discussions/344058-real-thirds-2.html

I don't believe the non ideality of air introduces any meaningful errors at diving pressures.

High SAC rate at the start of a dive is most likely due to tanks cooling (especially if they have just been filled) and the diver not having settled in to the dive.

Go cave diving in Florida in the winter and your tank pressure can actually increase for the first few minutes. (Cold tanks going into 68F water) Would air integrated computers report a negative SAC, and hence infinite Air Time remaining?

 

[diverrick]

"does it make a meaningfull difference?"

No. not in real life..I would atribute your higher SAC rate during the first few mins to the effort it takes to get set up and into the water, plus cooling effect of water on a usually warmer tank. Once in youget in and calm down, you exert yourself less, Etc., hence the SAC drops off some.
I bet if you jumped in and swam the whole dive against a current you would maintain the high SAC rate throughout the dive.

 

[elan]

I noticed the same effect too. It did not seem to be the cooling as I often dived in the conditions where the temperature of the water mathed that of the air - say 41- 41 F, you do not really spend much gas for inflation as for example if you need 10lbs of the lift to maintain your buoyancy it's roughly 5 littres at the surface, and 20 at 30m which is really nothing if you compare to the tank volume less than 1/100 of an HP100 -about 40 psi


Check the factor here http://en.wikipedia.org/wiki/File:Compressibility_Factor_of_Air_250_-_1000_K.png

As you can see the compressibility factor changes a lot (about 6-7%) in the area 120 - 240 bar.

 

[CuNanoTube]

Well, there are no "Ideal" Gasses. If you want precise more you will need to use a Van der Waals equation and pull data off of a chart. Of course the precision of your measuring equipment is probably not great enough that to make a difference.

 

[JB]

Check the factor here File:Compressibility Factor of Air 250 - 1000 K.png - Wikipedia, the free encyclopedia

As you can see the compressibility factor changes a lot (about 6-7%) in the area 120 - 240 bar.

Thanks for that, it is exactly what I was looking for. Seems at average room temperature, the compressibility of air drops by about 10% from 200 to 300 bar.

 

[Crush]

Real gases are less compressible than are ideal gases at high pressures. For air at pressures above 3000 psi there are in fact fewer molecules of mix than your pressure gauge would lead you to believe if you think PV=nRT. Also, real gases attract one another more than do ideal gases (which don't interact), making the gas more "compact" at lower pressures. At pressures below 3000 psi for air (or 3333 psi for EAN 36) there is more air in your tank than you would lead to believe based upon PV=nRT and your pressure gauge. However, these effects are far smaller than the inherent uncertainty in your SPG. For more details see http://www.scubaboard.com/forums/basic-scuba-discussions/355067-how-much-air-tank.html