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baywatch106
03-18-2009, 09:12
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
03-18-2009, 09:39
Any ideas?

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

UCFKnightDiver
03-18-2009, 09:41
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)
03-18-2009, 10:25
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.:smiley27:

baywatch106
03-18-2009, 11:06
maybe he didn't even fill it ,since I dropped off my tanks and picked them up the next day.

Rainer
03-18-2009, 11:09
What was it filled to when you dropped it off?

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
03-18-2009, 11:17
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
03-18-2009, 18:55
.....//There, now that I have proved I'm a geek, my man card is definitely gone. But at least Lullubelle likes me.:smiley27:
What? Hell, I'm impressed! I vote you get it back. Then again, I may now lose mine.
:smiley36:

RoyN
03-18-2009, 19:54
What? Hell, I'm impressed! I vote you get it back. Then again, I may now lose mine.
:smiley36:

I lost mine few years ago. I'm still living if that counts. :smiley2:

clararhb
03-18-2009, 20:08
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.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
03-18-2009, 20:17
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.


WTF??

Rainer
03-18-2009, 20:18
Don't smoke weed and then post.

Doug B
03-18-2009, 21:45
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.:smiley27:


I thought PV = nRT was Boyl's law.... man it's been too long since I did all of that stuff in school. The sad part is, I was very good at it, but haven't used it in 17 years.

What was that super long equation from Thermodynamics....hmm... I need to look that up. I really liked Thermo 1 and 2. Didn't care for heat transfer though.

BouzoukiJoe A.K.A. wrecker130 AKA Chuck Norris AKA joeforbroke (banned)
03-18-2009, 22:18
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.:smiley27:


I thought PV = nRT was Boyl's law.... man it's been too long since I did all of that stuff in school. The sad part is, I was very good at it, but haven't used it in 17 years.

What was that super long equation from Thermodynamics....hmm... I need to look that up. I really liked Thermo 1 and 2. Didn't care for heat transfer though.

If you don't use it you lose it, unless you're weird like me. I haven't really used this stuff since 1982.

If I recall Boyle's law is a special case of the ideal gas law that assumes a constant temperature. It wouldn't be applicable to this problem.

baywatch106
03-18-2009, 22:26
my tank was at 3000 psi when I took it in for a top off. So either it never got topped or there is a small leak in my valve and over a week it leaked 400 psi.

IrishSquid
03-19-2009, 17:31
What? Hell, I'm impressed! I vote you get it back. Then again, I may now lose mine.
:smiley36:

I lost mine few years ago. I'm still living if that counts. :smiley2:

Hey Roy,
Little side track here. Are you the same RoyN that was really creeped out by my temporary avatar on SB when they was having that problem with the server upgrade? It was the really disgusting Squidward from SpongeBob. :smiley20:

Techdiver
03-20-2009, 21:06
You most definitely got a hot fill. The air you breath gets thinner or less dense the higher you go. When people have a hard time breathing at 10,000 feet it's not lack of Oxygen in the air, its lack of pressure that causes this. I'm not sure what effect it has on the pressure in your tanks. I believe it would have very little effect at the altitude you can dive at.

ColdPass
03-23-2009, 11:11
at the 1100 ft elevation you got your fill at, the atmospheric pressure would be roughly 14.2 psi, and at 3500 feet, roughly 13 psi. So your gauge, if it was that precise, would reflect a 1.2 pound higher pressure at the elevation you dove at. (gauge pressure actually is showing you the difference between pressures in the tank and the atmosphere surrounding the gauge.)


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.:smiley27:

Flatliner
03-23-2009, 12:47
my tank was at 3000 psi when I took it in for a top off. So either it never got topped or there is a small leak in my valve and over a week it leaked 400 psi.

Correct assessment. Just food for thought, but is it possible the tank valve could have gotten a little bump in transit at some point and lost a little air? I know the way I pack, it would be a real possibility...

divinginn
03-23-2009, 21:43
I would bet it is a combination of a hot fill and cold water. I usually lose about 300psi from a hot fill on my hp tanks.

kat
03-23-2009, 23:53
my tank was at 3000 psi when I took it in for a top off. So either it never got topped or there is a small leak in my valve and over a week it leaked 400 psi.

If you took it with 3000 psi and you came back with 3000 psi, I don't think he touched your tank. Maybe it was accidental, but be mindful in the future. That happened with my husband one time. He told the lds and got a free fill the next time.

WD8CDH
03-25-2009, 09:40
A gauge is slightly effected by ambient pressure but it would be much less than a needle width from sea level to 10,000 feet (less than 5 psig change). You just got a hot fill or an under fill. Or....your gauge and/or the LSD gauge are wrong.