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Hi everyone, And here's the date I've promised. The compressibility of air-graph is in .jpg format, I hope that's not a problem to anybody. General equation for gas volume is pV/T=constant. In this p=pressure, V=volume, and T=temperature. Using this equation, we can see that in when a 300 bar tank is filled, and the temperature rises to let's say 47 deg C, the pressure reduces 37 bars when the tank is cooled down to 7 deg C in sea-water. This alone does not explain the rapid pressure decrease in a 300 bar tank. The formula p=V/T only applies to ideal gas. Real gases seldom agree to this in pressures in excess of 100 bars. For air, the easiest thing to do is to use compressibility factor Z from tables (see Sten's post and figure with this post). Z=pV/nRT, i.e. Z measures the deviation of the gas from ideal. Instead of pV/T=constant, we must use pV/TZ=constant. For air, |Z-1| <0.02 when the pressure is <200 bar and temp <10 deg. C. But when p>330 bar, and T>47 deg. C, Z=1.14, making the deviation 14% from ideal. This is well worth taking into consideration. Another example: 10 litre tank filled to 200 and 300 bar: V(1)= 200 bar*10L*1.00/1.10*1bar=1980 L V(2)=300 bar*10L*1.00/1.01*1 bar=2730 L So, adding the last 100 bars does not add 1000 litres of air, but only 750L. That's why the gas goes so fast in the beginnig of the dive. I'm a marine biologist, and as said, learned all this from my engineer buddies, especially Matti Leinio and Pekka Raty. I'll try to persuade them to put more of this stuff on the techdiver, if anybody's interested. Matti (a medical physics engineer and trimix diver) says that with helium, the compressibility factor is even bigger issue than with air. Anyway, I hope you guys find this useful. Good dives, Juha
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