Pressure Question?

[ScubaTodd77]

I have a Sherwood/Catalina 80 cubic ft, compact neutral tank that is rated at 3300 psi...however, when I have the tank filled, they only fill it to 3000 psi. My question is why don't they fill it to 3300? Would that cost more? Do I need to tell them that it's rated at 3300? It would be nice to have the extra air.

 

[rstofer]

You probably have to point it out. Shops fill Al 80 tanks by the hundreds and they are almost all 3000 psi.

It shouldn't cost more, they probably fill 3442 psi HP steel tanks and so, presumably, have the equipment to deal with 3300 psi tanks.

I have a '+' rated steel 72. I make sure to point it out when I take it in even though the shop already knows about it. They arranged the hydro! Still, it never hurts to remind them to get it full.

Richard

 

[herman]

I agree with Richard, most likely they just missed it, they likely fill mostly AL-80s. Try a piece of bright tape with "HP Tank Please Fill to 3300PSI" placed near the valve.
Another possible reason is the person who filled it , hot filled it. If I am in a hurry I usually fill my 3000psi 80's to 3200 and end up with around 3000 when they cool down. It's quicker to overfill a little than to wait for a slow fill.

 

[DA Aquamaster]

The average fill in a 3000 psi tank is probably a lot closer to 2800 due to hot fills and a mistaken belief by many fill operators that they cannot exceed the service pressure at any time. The DOT regs require that a tank not be filled to more than the services pressure when the tank is at room temperature. (70 degrees).

So you can fill a 3000 psi tank to 3200 psi at normal tank fill rates and it will then drop to about 3000 psi when the tank cools to room temp. As long as the tank is at 3000 psi when cool, it is no more over filled than if it were slow filled to 3000 psi when cool then taken outside and set in the sun on a hot day - the volume of gas in the tank has not changed, just the temperature and pressure, the tank is still "full", not "over full".

But many shops refuse to do that resulting in room temperature pressures around 2800 psi unless they come back and top the tank off after it cools. 2800 psi in an AL 80 is only 72 cu ft, not the 77.4 cu ft you get when it is full at 3000 psi, so it is enough to make a difference.

As Herman suggests, the same thing may be happening with your 3300 psi tank.

 

[Rick Inman]

I would love to watch you try to get some young shop guy to fill an al80 to 3500 so it will cool to 3300, no matter what it says on the tank. That would be fun.

 

[DA Aquamaster]

There in lies the problem...

 

[Ouvea]

It's actually a valid point. Some dive shops do not pay attention to the + sign next to the work pressure stamp. Make it a habit of pointing it out to the fill operator and you'll be in business. I ran into this with my Faber 72cft @ 3000PSI + 10%.

As for overfilling or not overfilling, I always ask for a slow fill w/o overfilling. It's just my preference, although I do know if overfilling by a few hundred PSI actually damages the tank.

Claude

 

[DA Aquamaster]

A few hundred PSI over the service pressure for the hour or two it takes to cool to room temp will not harm the tank and is WELL within the test pressure (5/3rds of the service pressure - or 5000 psi for a 3000 psi tank.) Do you worry about the tank sitting in the sun on the beach or boat deck when full? If you do, don't.