500 PSI is 500 PSI?

[rednose83]

I'm in a little bit of an argument with my brother's SO. Which thought is correct?

Opinion #1-
500 psi is a different amount of gas depending on the size of the tank the diver is using.

Opinion #2-
500 psi is the same amount of gas regardless of tank size.

[inventor]

Which has more air at 14.7 PSI? Your house or the Superdome? The Superdome has a greater volume, so it holds more air. Regardless of the pressure, a larger vessel will hold more air than a smaller vessel, at the same pressure.

To clarify, at equal pressures, the larger tank holds more air.

I'm sure most here will verify this. If your instructor has an issue with this, I suggest he post that question on a tec thread or forum.

[inventor]

And wet sand weighs more than dry sand. Promise this won't be a screed.

[rednose83]

My argument is that it is different amounts of air depending on the tank size. Otherwise, people wouldn't bother diving with larger tanks- what would be the point? Why dive a tank with 80 cu ft vs 63 if you are going to have the same amount of air regardless?

Her argument is that it doesn't matter the tank size because PADI says 500 psi and they wouldn't set one barrier if it was different amounts.

[inventor]

Originally Posted by rednose83

My argument is that it is different amounts of air depending on the tank size. Otherwise, people wouldn't bother diving with larger tanks- what would be the point? Why dive a tank with 80 cu ft vs 63 if you are going to have the same amount of air regardless?

Her argument is that it doesn't matter the tank size because PADI says 500 psi and they wouldn't set one barrier if it was different amounts.

Not a valid argument. Your reg will stop providing you air at a certain pressure, not according to how much mass is left in the tank. There is more gas in a cold tank at 500 PSI, than a hot tank at 500 PSI. I believe it is a safety measure that this pressure gives you a certain amount of time to get where you need to go. Like aircraft have to have a 45 minute IFR reserve, so they can divert if neccessary.

I'm out on this thread.

[clayton]

You need to clarify the word amount with the words VOLUME and PRESSURE those are the differences.

[captain]

Assuming both are 3000 psi tanks.
80 cu/ft tank at 500 psi = 13.3 cubic ft
100 cu/ft at 500 psi = 16.6 cubic feet

80 divided by 3000 = .026 cu/ft per 1 psi, .026 cu/ft X 500 =13.3 cu/ft
100 divided by 3000 = .033 cu/ft per 1 psi, .033cu/ft X 500 = 16.6 cu/ft

[db83]

PV = nRT where R is the gas constant.
At constant pressure (P) and temperature (T), the greater the volume of the container, the greater the amount (n) of gas.

[FishFood]

So what is the argument here? When to turn a dive?

[comet24]

"psi" stand for "pounds per square inch". It's a measure of pressure. It is not a measure of volume.

If you have two different sized tanks with the same psi the tank with the great volume will have more gas.