Pressure / Temperature

[j.w.]

<font color='#000080'>Before we start and I end up with lots of abuse about temperature and how it effects pressure, I know that it does. Mainly due to the number of dissapointing fills that I have had and its been put down to &quot;lucky you got more than 200 bar on a day like this&quot; when questioned.

What I want to know is, is there some way or guide line to follow if you are prepping your gear the night before and notice your cylinder pressure lets say 329 bar @ 13 deg.How much will it have dropped when you are on the dive site in the morning?
I don't know what the temperature was when the cylinders were filled ( the obvious get out for many, if only we had known...)

It goes without saying it will make no difference to the dive we will do I'm just curious, I'm sure when we kit up tommorrrow the room temp will be a distant memory.
Is there a bar per degree drop.

Come on answers on a post, will let you know how you got on.

John.

[Phil Ennis]

<font color='#0000FF'>You lose/gain roughly 0.6bar per degree change. &nbsp;

[Juz]

<font color='#32CD32'>Hi,

IIRC it's 0.6 bar per degree Centigrade.

Juz

[bang-on]

<font color='#0000FF'>jw,
the drop in bar presssure in your cylinders between room temp and dive site is very small,however the issue tends to be having 232 bar in your cylinders at the compressor when they are filled and then 200 or so when its time to dive.
this is caused by the heat produced when your cylinders are filled and the warmer the air the higher bar pressure is shown but as the cylinder and fill cool and the air contracts the pressure in the cylinder falls and your down 30 bar.
so get your fills stick your cylinders in the boot go and have a wonder,then put a gauge on the cylinder when you get back and then take it back to the compressor to be topped off back to 232 bar.

cheers
barrie

[Janos]

Pressure is directly proportional to temperature.

Except that:

1) You have to work in Absolute Temperature (Kelvin) To convert Celcius to Kelvin add 273. Eg 27 degrees C is about 300 K

Hence a 1 degree drop at 27 deg C changes the pressure by a factor of 1/300, which (assuming 200 bar in the tank) is 2/3 of a bar which is (200 x 1/300) = 0.66 bar (as previous posters said) You get a smaller drop in pressure for more empty tanks, and a higher drop for higher pressures of tanks

2) Also, things go screwy over around about 250 bar, and the laws become non-linear (ie pressure is no longer proportional to temperature)

Laters,
&nbsp; &nbsp;Janos

[Digger]

<font color='#000080'>Well, that's confused hell outta me! Janos, you never cease to amaze. Personally, I go for 250/260 hot fills, and 240 cold ones. Chances of a cold fill these days are getting smaller, as dive centre want to fill faster and faster. Worrying really. Slow and steady wins the race. We all know that.

[Helen F H]

<font color='#000080'>Its charles law if you want to look it up for a simpler explanation.

[Janos]

Originally Posted by [b

Quote[/b] (Digger @ April 03 2004,02:07)]Well, that's confused hell outta me!

Bugger. I must be some sort of anti-explainer. I take stuff you understand and talk at you until you don't. &nbsp;

Hmmm. I'll bring graph paper and a pen to the Easter trip. I bet you're looking forward to that now.

But basically, a temperature change of 1 degree will cause a change in pressure of 0.33%. Ie 0.8 bar if you've got 250 bar in your tank, 0.66 bar if you've got 200 bar in your tank, 0.16 bar if you've only got 50 in to start with.

Above 250 bar things go screwy, and it's not .33% anymore.

Laters,
&nbsp; &nbsp;Janos