simple calculation

h90 · Sep 12, 2008

Diving in cool water I wanted to explain a new diver why the air is even more cold and how to calculate it.
I could before....Lets say: 200 bar and you get on the surface 1 bar in your mouth. No heat exchange (for making it simpler).
Air at 200 bar=15 degree celcius
Air at 1 bar=??
I think I knew that 2 years ago but now....brain empty....someone know??

Blackwood · Sep 12, 2008

off the top of my head you're asking about the perfect gas law (Pv=nRT). Note that gas at high pressure isn't considered perfect, but you may get a decent approximation.

h90 · Sep 12, 2008

Blackwood:
off the top of my head you're asking about the perfect gas law (Pv=nRT). Note that gas at high pressure isn't considered perfect, but you may get a decent approximation.

yes, considering that it is perfect enough at 200 bar.
How would you use it in this case?
(or lets say open the tank on surface, how cold is the air coming out).

Thanks
Michael

fppf · Sep 12, 2008

(P1)(V1)(T2)=(P2)(V2)(T1)

But remeber all units must match, pressure must be absolute, and temperature must be in Kelvin.
(200/1)(288)= 1.44 K or -271 C
Of course this would be assuming volume stays equal, and that this is 100% adiabatic exspansion. Which is not possible. The gas does not have enough specific heat to maintain that temperature.

Teamcasa · Sep 12, 2008

Forget the math and physics class, just tell them that a gas becomes cooler as it expands just as it heats when compressed. Show them (on the surface) by opening a tank valve (without a regulator) and release some air, the valve will get cold. Them remind them that a freshly filled tank is always warmer that normal.

h90 · Sep 12, 2008

fppf:
(P1)(V1)(T2)=(P2)(V2)(T1)

But remeber all units must match, pressure must be absolute, and temperature must be in Kelvin.
(200/1)(288)= 1.44 K or -271 C
Of course this would be assuming volume stays equal, and that this is 100% adiabatic exspansion. Which is not possible. The gas does not have enough specific heat to maintain that temperature.

That's not correct because the volume changes...
That was exactly my problem....

ianr33 · Sep 12, 2008

A perfect gas would not get colder when it expands,but perfect gases do not exist in reality.

A real gas gets colder because when it expands intermolecular attractions have to be overcome.The energy to overcome these attractions has to come from somewhere and that somewhere is the temperature of the gas.

Charlie99 · Sep 13, 2008

ianr33:
A perfect gas would not get colder when it expands,but perfect gases do not exist in reality.

Are you sure about that?

I'm not a thermodynamic physicist but I'm under the impression that the adiabatic heating and cooling of air is very close to that of an ideal diatomic gas.

ianr33 · Sep 13, 2008

Upon further consideration it seems that you are in fact correct Charlie and that I have some light bed time reading (and comprehension!) to do !!

Adiabatic process - Wikipedia, the free encyclopedia

Bottomfeeder Brodie · Sep 14, 2008

any recommended literature on this subject, gentlemen?

simple calculation

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