Cylinder Pressure vs Temperature

[drunkdiver]

Hi,

Is there a formula which relates cylinder pressure to air temperature.

What I want to know is for example:

If I have 100bar of air at 20 deg C and heat it (or cool it) by 10 deg what would the pressure be ?

Cheers,

Rik

[Hickdive]

Dunno but AFAIK gas temp and press are in direct proportion but you must work in deg. Kelvin.

So, if you reduce or increase the temp by whatever %age of Kelvin, the pressure will change by the same %age. I think.

Don't take my word for it though.

[Leg Of Salmon]

PV=nRT

The state of an amount of gas is determined by its pressure, volume, and temperature according to the equation above

where

P is the absolute pressure [Pa],
V is the volume [m3] of the vessel containing moles of gas,
n is the amount of substance of gas [mol],
R is the gas constant [8.314 472 m3·Pa·K−1·mol−1],
T is the temperature in kelvins [K].

[divertim]

Generally use the 0.6 bar rule.

For every degree C in change, you lose or gain 0.6 bar. Charles Law.

You could if you like do P1 x V1 over T1 = P2 x V2 over T2, but that's far too complicated. Stick to the 0.6 rule.

So your 100 bar of air chilled by 10 degrees C would end up around 94 bar.

Tim

[Dave Crampton]

See Nigel Hewett's profile. His home page has loads of usefull stuff.

[diver-repairs]

As absolute zero is approximately -273.3 deg C and volume is proportional to the absolute temperature, if you keep the volume constant(well the cylinder won't grow that much) then for every 1 deg centigrade that the temperature goes up or down then the pressure will rise or fall by 1/273.3 of the original pressure:-

Try the maths:-

bottle pumped to 232bar but the temperature was found to have risen by say 20 deg c from 15 deg to 35 deg then on cooling back to 15 deg the pressure will drop by 20/273.3 x 232 =0.8488 x 20 =16.977 bar
so bottle pressure drops from
232bar to 215bar.

Does that seem to make sense?????

BrianC

[Captain Calamity]

Here's the formula:

P1/T1 = P2/T2

It works when the volume of a container is fixed e.g. a dive cylinder.

The temperature must be in degrees Kelvin for the formula to work (where 0 degrees Celsius = 273 degrees Kelvin). Convert to Kelvin by adding 273 to the temperature in degrees Celsius. Pressure is in Bar.

Example:

A freshly filled cylinder at 37 degrees C and 250 Bar is cooled to 17 degrees C in the sea. What will the cylinder pressure be at the lower temperature?

P1 = 250 Bar
T1 = 37 + 273 = 310 K
T2 = 17 + 273 = 290 K
P2 = ?

250 / 310 = P2 / 290

0.806 = P2 /290

290 x 0.806 = P2 = 234 Bar.

HTH

CC

[Gareth Burrows]

if you're gonna work it out using charles's law just remember to convert everything to absolute values first, and then convert back at the end. PErsonallyu, unless you need a more accurate answer, stick to the 1 degree=0.6 bar rule of thumb.

[Howard Payne]

F*ck me - I thought DIR got anal at times?

[uwila]

Originally Posted by Howard Payne

F*ck me - I thought DIR got anal at times?

Thanks for the kind offer, Howard, but just this once, I'll pass and stick to the maths