Tank Pressure Rating definition

This might sound like a dumb question, but what exactly is the tank pressure rating?

Is this the max pressure to which the tank should be filled? This would mean that most Al80s would have a tank pressure rating of 3000psi.

Or is it the pressure to which it was filled, for example, if a Steel100 was filled to 3100psi?

If you're talking about "service pressure," it's the pressure to which a tank should be filled in order to yield it's specified capacity.

Let's say, for example, the service pressure for a given HP100 is 3442 psi.
If the tank is filled to 3442 psi, then it will contain 100 cubic feet of gas.
If the tank is filled to 3100 psi, then the tank would only contain approx. 90 cubic feet of gas.

As a rule, dive shops should not fill a tank above the rated service pressure.
In practice, in order to speed up the filling process, quite a few shops will "hot fill" a tank a couple hundred psi above service pressure so that when the tank cools, it will register the service pressure.
In areas featuring cave diving, some shops will do a "cave fill" on a low pressure steel tank. A "cave fill" greatly exceeds the rated service pressure of the tank, and there is an increased chance that the tank will rupture due to the excessively high pressure.

I think I've been calculating my SAC rate wrong. Haha. It's much lower than I thought. Good news, everyone!

What's your sac? Mines terrible, 40 mins =160 bar used at around 8 meters from 200 bar on an al80. May try some hp100 and see what my botto time is.

Aluminum tanks and special permit steel tanks hold their rated capacity at the service pressure stamped on the shoulder. The exception to this is the aluminum 80 as it only holds 77 cu ft, not 80 cu ft. It should have been called the aluminum 77.

3AA steel tanks however hold their rated capacity at 10 percent over the pressure stanped on the shoulder and the "+" stamped after the date of the last hydro test allows that fill pressure. Consequently the classic steel 72 holds 71.2 cu ft at 2475 psi and only 65 cu ft at the stamped service pressure of 2250 psi.

A tank is only overfilled if the pressure exceeds the allowable service or plus rated pressure once the contents and the tank cool to 70 degrees. Which is to say hot filling a 3000 psi tank to 3300 psi is not overfilling at all provided it does not exceed 3000 psi once it cools. Most shops seem to misunderstand this point but it's clearly defined that way in the DOT regs and the regs clearly reflect the realities of hot fills and that tanks will be stored in the sun, at high ambient temps, etc. which is one reason the margin between service pressure and burst pressure is so high.

The belief that a hot filled tank or a tank left in your trunk will rupture the burst disc is a myth as well. The burst disk assembly on a 3AL or 3AA tank must release at no less than 90% of the test pressure and no more than 100% of the test pressure. Since the test pressure of those tanks is 5/3rds the service pressure (5000 psi in the case of a 3000 psi tank) you'd need to fill an AL 80 to 4500 psi to get a recently replaced burst disc to fail. Even if it is a few years old, the 3300-3400 psi pressures it will see are still several hundred psi less than is needed to cause the disc to fail.

Cave fills in a 2400 psi 3AA steel tank are commonly 3600-3800 psi. That's still below the 4000 psi test pressure of the tank and a decade or two of service history doing that in cave country has not resulted in a single catastrophic tank failure. Thus while the practice is not technically allowed by the regs, it's not accurate to say it is "unsafe". It's also not accurate to say it is illegal as the DOT regs only apply to tanks used in interstate commerce. For awhile the some DOT inspectors tried to extend that definition to personally owned portable tanks that could potentially be used in interstate commerce but that is not currently the case.

I guess we know who to go to for questions re tanks

THANKS DA. It is nice to see others understand teh spec of working presure. I have heard such lame reasons for undrefilling tanks such as if it is in the sun it can not result in a presure over 3000. the crap of a 90% fill is a full tank is a lazyness excuse for the fill stations.