How much does outside temp affect PSI

[The Chairman]

Sure Andy,

a really hot fill can do this. If your air is 140 F when you first measure the tank and then cools to 70 F, you have your 70F drop! Also, you are using an analog and not a digital guage so what you THINK is a 400 psi drop may not be.

 

[H2Andy]

Fast filling in a truck bed using the long whips.....
P.S. Talked to Johnny this morning......Congrats !!

yeah... i think that might be it

(i assume the congratz is on the Apprentice? thanks!)

 

[steveann]

The air inside the cylinder will about equalize with the wall temp of the cylinder (mattering on how long the exposure and how steady the wall temp). If you took a contact pyrometer (read thermometer) you could tell what temp the tank was at, and yes it will be greater than air temp (sometimes much greater depending on color and environment.) Once you get in the water, the solar heating goes away, and the tank cools to water temp. This can give you that 75F change. Of course, the math isn't exact, as that uses ideal gases and air is anything but, but it still gets you a good ballpark idea. Also, don't forget you have to convert F or C to Rankine or Kelvin. Might make your temp difference a bit less.

 

[spectrum]

It can take about 4 hours to the air in a tank to equilibrate to ambient temp so even if the cylinder feels OK an hour after a fill there is still some heat exchange happening and with it pressure drop. The air only has minor convection happening so it moves to the walls in a very slow fashion.

These days I can easily have my rig at 90F in the sun and in 50F water in 10-15 minutes.

5 PSI / Degree F is the factor I generally hear as well.

Pete

 

[CompuDude]

We pretty much assume a 200 psi drop when the tanks hit the water. Moreso if you had your tank sitting in the sun on a hot day is not surprising.

I suspect the shop filled the tank normally and didn't pay attention to your notation. 3400 in a 3442psi tank is a distressingly normal occurrence.

 

[Jim Lapenta]

We pretty much assume a 200 psi drop when the tanks hit the water. Moreso if you had your tank sitting in the sun on a hot day is not surprising.

I suspect the shop filled the tank normally and didn't pay attention to your notation. 3400 in a 3442psi tank is a distressingly normal occurrence.

I do not understand what is distressing about it. I'd be pissed if I had hp tanks and the fill was any less than 3442. My 80's normally hit the water with 3500+. If you don't like full tanks just bleed some off. No big deal. And steel is much more forgiving than al on "overfills". I've got some steel 72's that just passed hydro and have had 2800 or so fills for the last year and a half. About 2-3 per month in fact. The original dates? 1955 and 56. Al tanks I don't like to overfill but than again I won't own an al tank.

 

[CompuDude]

I do not understand what is distressing about it. I'd be pissed if I had hp tanks and the fill was any less than 3442. My 80's normally hit the water with 3500+. If you don't like full tanks just bleed some off. No big deal. And steel is much more forgiving than al on "overfills". I've got some steel 72's that just passed hydro and have had 2800 or so fills for the last year and a half. About 2-3 per month in fact. The original dates? 1955 and 56. Al tanks I don't like to overfill but than again I won't own an al tank.

I'm not sure I follow you, or perhaps you didn't follow me? I said short fills were distressing. Makes me mad, too. That's one reason why I don't get my HP tanks filled at one of my favorite LDSs anymore... their compressor can't really go above 3300, although they occasionally squeak out 3400 if they spend a LOT of time topping it off.

I only own, and dive with, HP steel tanks. Caribbean vacations excluded.

 

[Jim Lapenta]

Sorry my bad, read your reply while rushing to get ready for work. My sincere apologies.