tank pressure?

[baywatch106]

If you had your tank filled to 3400 psi at an altitude of 1100 ft and then went on a dive that ws at an altitude of 3500 ft would the pressure in the tank read different when hooked up to your spg?
My reason is that I took my hp 3500 120 cu ft tank to my lDS to get topped off since it was hot filled. The shop said he filled it to 3400. I went on a dive at the higher elevation and my spg said 3000 psi. thats what my spg said before I took it to the shop.

Any ideas?

[Rainer]

Originally Posted by baywatch106

Any ideas?

Sure, stop getting hot fills and you won't end up with such low fills. Atmospheric pressure played no role here.

[rox@ucf11]

I think the change in altitude probably played very very little role here it was the hot fill you got which cooled to 3000 oh yeah and damn that much have been a HOT fill to cool 400 psi

[BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)]

Change in atmospheric pressure could only account for 14.7 psi even if you took your tank to the moon, but temperature has a big effect.

Recall the ideal gas law from chemistry...

PV = nRT

since n and R are constants, the relation implies that

P1 * V1 / T1 = P2 * V2 / T2.

Since your tank's volume is constant for all practical purposes this reduces to

P1 / T1 = P2 / T2

and re-arranging we get

P2 = P1 * ( T2 / T1 )

These temperatures are absolute temperatures though. Since you probably want temperatures in fahrenheit we need to substitute the identity

T = (F + 459.67) into the above equation.

Doing so, we get:

P2 = P1 * ( (F2 + 459.67) / (F1 + 459.67) )

As an example, a 3400 psi fill at 120 degrees F (not particularly hot) should be just 3048 psi after it has cooled to 60 degrees F.

I don't know the temperature change the gas in your cylinders experienced, but it doesn't seem out of line at all - and is probably within the tolerance of your pressure gauge anyway.

//There, now that I have proved I'm a geek, my man card is definitely gone. But at least Lullubelle likes me.

[baywatch106]

maybe he didn't even fill it ,since I dropped off my tanks and picked them up the next day.

[Rainer]

What was it filled to when you dropped it off?

[BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)]

Originally Posted by baywatch106

maybe he didn't even fill it ,since I dropped off my tanks and picked them up the next day.

You can avoid that problem by checking your tanks before you leave the shop. It's more hassle but a lot less hassle than getting to a dive site you've been looking forward to for a month and finding that they filled your high pressure tank to 2200. Yes, I learned this the hard way.

[IrishSquid]

Originally Posted by BouzoukiJoe

.....//There, now that I have proved I'm a geek, my man card is definitely gone. But at least Lullubelle likes me.

What? Hell, I'm impressed! I vote you get it back. Then again, I may now lose mine.

[RoyN]

Originally Posted by IrishSquid

What? Hell, I'm impressed! I vote you get it back. Then again, I may now lose mine.

I lost mine few years ago. I'm still living if that counts.

[clararhb]

I agree, invest in your own gauge. There are few LDS's that I trust. If it is hot then it sits or I drop it in the pool for a bit. Unfortunately I have to go through this.