How much does outside temp affect PSI

[dlwalke]

So I went diving last weekend after having my tanks vip'ed and filled. I asked for a 3000PSI fill on the form that I left with the tanks, but when I got to the dive site and hooked them up, they read about 3400 which would be consistent with their max rating (PST E7100's). So maybe the filler didn't catch my notation and just filled them up to their max. On the other hand, they were also hot to the touch (like, you couldn't touch them for more than a couple of seconds without burning yourself) as they had been sitting in the bed of my pickup for the 2 hr drive out to the quarry I went to. Also, it seemed like I burned off the first few hundred PSI very quickly, which might be possible as it was the first few minutes of the first dive of the season and I would expect that, but then again maybe it was because the tanks quickly cooled down in the water. I usually compute my SAC rate after a dive, but I am guessing that I might get an incorrectly high reading this time as the change in PSI is not going to accurately reflect my true usage but is significantly contaminated by temperature fluctuations. So my question is....

About how much of a change in PSI would one expect taking a filled steel tank from out of the sun (I think it was upper 90s that day - does the tank get just that hot or does it absorb and retain heat and solar radiation, somehow getting even hotter than the air temp - OK, 2 questions) into water that was in the low 70s?

Thanks

 

[H2Andy]

here in florida i have seen steel tanks drop up to 400 psi going from hot sun to cold water. i am not sure about aluminum tanks.

you'll have to swag it ... the best way is to check after you're in the water for a few
minutes to make sure of the gas you actually have

 

[Charlie99]

The pressure of air in a closed container is roughly proportional to absolute temperature. At normal room temperatures that means about 1/300 change per degree C. For a 3000psi fill, that's about 10 psi/degree C or roughly 5psi per degree F.

To go from 3000 to 3400psi degrees the temp would have to change about 40C or about 75F.

It's common to see a 200psi or so drop as sun-warmed tanks get cooled by the water at the beginning of a dive.

 

[Jay_SMART_Diver]

Not to mention how fast the shop fills the tanks, if the fill is fast the air heats up and the tanks get hot so what you think is a 3000 psi fill may be 2800 psi or less when the tank cools.

 

[H2Andy]

hmmm.... well, the water in the springs is about 70 degrees, and let's say outside
temperature in the sun is about 100, so that's only a 30 degree change

is there anything else that could account for the 400 psi drop? i am pretty sure
i have seen this kind of drop on my doubles

 

[Charlie99]

Temperature sensitivity of your bourdon tube SPG, or

hoovering as you first descend.

-----------

I'll have to remember to see if my SPG does have any significant temperature coefficient. A simple test would be to first dip it, and it alone, into the water and see if the indicated pressure changes.

 

[Rich Courtney]

hmmm.... well, the water in the springs is about 70 degrees, and let's say outside
temperature in the sun is about 100, so that's only a 30 degree change

is there anything else that could account for the 400 psi drop? i am pretty sure
i have seen this kind of drop on my doubles

Fast filling in a truck bed using the long whips.....

This time of year it's a better idea to take the doubles off the wing and backplate (or not!) and stick 'em in the water, plus if you're in a hurry it doesn't help....

I always fill my doubles in the "ambulance', but I do it in the early morning when I'm opening up the shop and no one's here yet, that way I can go nice and slow. I hit the water every evening with at least 3400psi and that's plenty for me. Need more? Take a stage.....

Safe diving,

Rich

P.S. Talked to Johnny this morning......Congrats !!

 

[dlwalke]

To go from 3000 to 3400psi degrees the temp would have to change about 40C or about 75F.

and

well, the water in the springs is about 70 degrees, and let's say outside emperature in the sun is about 100, so that's only a 30 degree change. Is there anything else that could account for the 400 psi drop?.

So a 75 degree drop seems unreasonable at first glance, but that sort of speaks to my other question. Namely, although the difference in air temp to water temp was only about 20 degrees, I am thinking it might be incorrect to assume that the tank temp is no warmer than the air temp...for the same reason that walking on grass with your bare feet in summer is not a problem, but walking on blacktop will blister your feet. So now, I'm wondering how much my tanks and the air in them would have heated up. In any case, I think I will drop a couple of hundred PSI from my sac calculation to get a ballpark SAC rate.

Thanks again

 

[H2Andy]

hoovering as you first descend.

yeah well, no need to get personal here ...

i mean, i deny that!

 

[dlwalke]

Not to mention how fast the shop fills the tanks, if the fill is fast the air heats up and the tanks get hot so what you think is a 3000 psi fill may be 2800 psi or less when the tank cools.

True enough although in my case, the tanks were filled many hours earlier so thats not an issue in this particular instance