Aldora Tanks

[ggunn]

Why is this? Is it because the central station hot fills to 3300 and by the time the tanks are used they have cooled down to 3000?

That indeed may be the case. Gas heats when it is compressed and the cylinders would have to be allowed to cool before being topped off to their max pressure. The fill station probably doesn't have the luxury of time to cycle every cylinder twice through their manifold.

 

[BRT]

That indeed may be the case. Gas heats when it is compressed and the cylinders would have to be allowed to cool before being topped off to their max pressure. The fill station probably doesn't have the luxury of time to cycle every cylinder twice through their manifold.

So then all the 3000# fills would be 2700. That is about right. I think the real reason is that all the aluminum tanks are filled to the same pressure.

 

[ggunn]

So then all the 3000# fills would be 2700. That is about right. I think the real reason is that all the aluminum tanks are filled to the same pressure.

Quite possibly they are, but it does not seem unreasonable to me that they are filled to 3300# and settle to 3000# when they cool. I could do the math with Boyle's and Charles' Laws but I am too lazy.

 

[BRT]

I think you are right. All AL tanks are filled to the same pressure and that pressure results in about 3000# when cooled.

 

[cvchief]

Quite possibly they are, but it does not seem unreasonable to me that they are filled to 3300# and settle to 3000# when they cool. I could do the math with Boyle's and Charles' Laws but I am too lazy.

Quitter.....

 

[boulderjohn]

Quite possibly they are, but it does not seem unreasonable to me that they are filled to 3300# and settle to 3000# when they cool. I could do the math with Boyle's and Charles' Laws but I am too lazy.

Boyle's Law won't help, and you can't use Charles' Law without knowing the actual temperatures involved. My experience filling tanks, though, tells me it is reasonable for an AL tank to drop 200-300 PSI as it cools.

But everyone who fills tanks regularly knows that, which is why everyone I know (including me) who fills tanks does so by overfilling the tank enough that it will drop to the desired final fill pressure. If you told me that you wanted your AL 80 filled to 3000 and I didn't have time to do a top off when it was cool, I would simply fill it to 3200 or so, thus enabling my customer to get what was requested. If I give you a hot fill at 3000 knowing it is going to drop to 2700, then I am cheating you.

 

[ggunn]

Boyle's Law won't help, and you can't use Charles' Law without knowing the actual temperatures involved. My experience filling tanks, though, tells me it is reasonable for an AL tank to drop 200-300 PSI as it cools.

But everyone who fills tanks regularly knows that, which is why everyone I know (including me) who fills tanks does so by overfilling the tank enough that it will drop to the desired final fill pressure. If you told me that you wanted your AL 80 filled to 3000 and I didn't have time to do a top off when it was cool, I would simply fill it to 3200 or so, thus enabling my customer to get what was requested. If I give you a hot fill at 3000 knowing it is going to drop to 2700, then I am cheating you.

But to get a cold fill of 3300# you'd have to fill it to ~3600# hot or wait for it to cool and top it off. Neither of those options may work for a large fill station.

 

[boulderjohn]

But to get a cold fill of 3300# you'd have to fill it to ~3600# hot or wait for it to cool and top it off. Neither of those options may work for a large fill station.

Why not?

 

[ggunn]

Why not?

If I were the owner of a fill station on Cozumel I would not like the idea of hot filling every tank with hundreds of psi over its rated capacity. Neither would I like the idea of having to deal with every tank a second time after a cooling off period.

For a small fill station only having to deal with a handful of tanks a day I guess it would not be that big of a deal to let them sit for however long it would take for them to cool down and then top them off, but for a station serving a dive location like Cozumel it would be a bigger issue.

 

[macado]

If I were the owner of a fill station on Cozumel I would not like the idea of hot filling every tank with hundreds of psi over its rated capacity. Neither would I like the idea of having to deal with every tank a second time after a cooling off period.

For a small fill station only having to deal with a handful of tanks a day I guess it would not be that big of a deal to let them sit for however long it would take for them to cool down and then top them off, but for a station serving a dive location like Cozumel it would be a bigger issue.

Slightly unrelated as this is Mexico and not US however per the DOT regulations it is legal to fill the tank over the service pressure provided the pressure decreases to the service pressure once the tank cools to room temperature.

My understanding is that tanks are rated at their maximum service pressure at 70° F.

It's pretty common all over the world (not saying it's right..) for large fill stations to fill to ~3200-3300psi with the anticipation that they will cool down to their rated service pressure.

For some incredibly dry government reading..
49 CFR 173.301a - Additional general requirements for shipment of specification cylinders.
49CFR 173.301a (c) and 49CFR 173.301a (d)