Any problem leaving a filled tank in the car?

[Bob DBF]

When a burst disc fails, I would bet it has more to do with the condition of the burst disc than a tank threatening pressure.



Bob
---------------------
I may be old, but I’m not dead yet.

 

[reo]

Let's figure a worst case scenario.
Let's make the following assumptions:
Interior temperature of the car = 160 degrees Fahrenheit
Tank was filled to 3300 at 70 degrees Fahrenheit (a very good fill!)

What is the pressure in the tank at 160 degrees Fahrenheit?

Using the idea gas law where volume is kept constant:
P₂ = P₁(T₂/T₁)
P₂ = the pressure at 160 F (619.67 Rankine - the absolute temp we use for gas pressure calculations)
P₁ = 3300 (the pressure at 70 F (529.67 Rankine)
T₁ = 529.67 R (70 F)
T₂ = 619.67 R (160F)

P₂ = (3300psi(619.67/529.67))
P₂ = 3860.73psi

Worst case, your tank pressure goes up to 3861 psi. Since your tank is probably hydroed to 5000 psi (assuming it's an AL-80), you're OK, provided your burst disk holds up (burst disks for AL-80s are usually rated to 5000 psi).

As far as pressure is concerned, you are safe. Since you live in an apartment complex, You should be concerned if your tank is visible to people looking inside your car. I know a couple of people who have left their scuba gear in their car and lost it due to theft.

I'm planning diving tonight after work, and my tank is laying on the floor of my car behind the driver's seat. Since I'm north of Seattle, I'd be surprised if the car temp gets up to 70.

Ron

 

[WhiteLightning]

Ron, thanks for the math to back up with most of the comments have been!

 

[nimoh]

I still gotta go on the other side of the argument from most of the posters, but perhaps my opinion is based on false assumptions.

I have always been told that a tank has a limited number of fills before it will fail hydro due to the expansion and contraction. If you leave a full tank in a car where the temperature drops at night and heats up considerably in the sun the next day, then I think you are reducing the life of your tank.

 

[Bob DBF]

I still gotta go on the other side of the argument from most of the posters, but perhaps my opinion is based on false assumptions.

I have always been told that a tank has a limited number of fills before it will fail hydro due to the expansion and contraction. If you leave a full tank in a car where the temperature drops at night and heats up considerably in the sun the next day, then I think you are reducing the life of your tank.

Theoretically speaking there is a limit to the number of cycles the tank will endure before the metal will loose its elasticity, which is what the hydro checks. If a steel tank is kept in good condition and used in it's working pressure range they will outlast you. I have a couple from the '70's and have seen them from the '50's, you can find older tanks in other occupations but mass produced SCUBA tanks didn't start all that long ago.

The reason you don't see a lot of older SCUBA tanks is that they were lower pressure and lower volume than what we use today so they were repurposed or scrapped.


Bob
-------------------------------------
I may be old, but I’m not dead yet.

 

[Peter_C]

I have always been told that a tank has a limited number of fills before it will fail hydro due to the expansion and contraction.

YES that is true. The requirement for a tank is that it is capable of 10,000 cycles before it fails, and it must fail to pass. So don't plan on getting to much over 10,000 cycles from your tank...enjoy those 10,000 dives per tank. We have 11 tanks so I got a little to work with.

 

[WreckDiver1321]

I've left mine in the car for a week during the summer once. They were hot as hell when I finally did move them, but they hadn't blown up.

Downside was, after expansion and contraction from the change in temps, both tanks had only 2400.

 

[Splitlip]

Let's figure a worst case scenario.
Let's make the following assumptions:
Interior temperature of the car = 160 degrees Fahrenheit
Tank was filled to 3300 at 70 degrees Fahrenheit (a very good fill!)

What is the pressure in the tank at 160 degrees Fahrenheit?

Using the idea gas law where volume is kept constant:
P₂ = P₁(T₂/T₁)
P₂ = the pressure at 160 F (619.67 Rankine - the absolute temp we use for gas pressure calculations)
P₁ = 3300 (the pressure at 70 F (529.67 Rankine)
T₁ = 529.67 R (70 F)
T₂ = 619.67 R (160F)

P₂ = (3300psi(619.67/529.67))
P₂ = 3860.73psi

Worst case, your tank pressure goes up to 3861 psi. Since your tank is probably hydroed to 5000 psi (assuming it's an AL-80), you're OK, provided your burst disk holds up (burst disks for AL-80s are usually rated to 5000 psi).

Ron

Agree with you. However, is a faulty burst disk more likely to fail at 3000 psi or 3900 psi? For me it's not worth risking it. Also I wonder if the tank is in the car for several days where it heats up, cools then heats up again.
I won't do it.

 

[reo]

Agree with you. However, is a faulty burst disk more likely to fail at 3000 psi or 3900 psi? For me it's not worth risking it. Also I wonder if the tank is in the car for several days where it heats up, cools then heats up again.
I won't do it.

A faulty disk is more likely to fail at 3900 than at 3000

All kidding aside, you do bring up a good point--faulty burst disks. I'm not sure how often you should have the burst disk replaced. Once a year seems too often. Is it OK to have it replaced and the valve rebuilt only when the tank is in for hydro?
Hopefully someone will chime in with the answer.

Ron

 

[beaverdivers]

We have a similar question usually from students.

Will the air freeze?