How much air in a low filled HP80

[kramynot2000]

I have a couple of HP 80 and have come across the problem other's have where an LDS only fills them to 3000 pounds. I'm curious about what this equates to in a lp tank. Would this be the same as a 72 filled to 3000 pounds or something like that? I just don't know how this formula works.

Thanks,
Tony

 

[Scubaroo]

HP tank = 3500 psi

The relationship between pressure and volume is (ideally) linear

80 / 3500 * 3000 = 68.6cf gas

Does this help at all?

 

[kramynot2000]

That's exactly what I was looking for. Its less that what I hoped for. I guess I better shop around for shops that give me a complete fill.

Tony

 

[scubafanatic]

.......also remember you'll lose 300-400 psi of pressure if your shop does a 'hot-fill'. Typically my low-pressure steels lose that much by the time my tanks cool down from the dive shop fill. So if you walk out the door with 3000 psi, you probably only have 2700 psi by the time you hit the water....that's why my shop fills mine to 3300-3400 psi, since they always drop down to 2900-3000 psi by the time I'm at the dive site......

Karl

 

[sheck33]

scubafanatic once bubbled...
.......also remember you'll lose 300-400 psi of pressure if your shop does a 'hot-fill'. Typically my low-pressure steels lose that much by the time my tanks cool down from the dive shop fill. So if you walk out the door with 3000 psi, you probably only have 2700 psi by the time you hit the water....that's why my shop fills mine to 3300-3400 psi, since they always drop down to 2900-3000 psi by the time I'm at the dive site......

Karl

True, when a tank cools you loose pressure but you dont loose air. A lot of people think they somehow have less air after a tank cools from a hot fill to a somewhat lower pressure.

 

[wetbrain]

I've used HP tanks for quite a few dives, and I have noticed that the pressure-volume relationship is fairly non-linear above about 3200 psi.

This means that the last 300 psi represents less air than the same pressure change from 2000 to 2300 psi, for example.

What does this mean for your question? Basically, that an HP 80 filled to 3000 psi contains more air than the 68 cf the linear calculation predicts. How much more? Hard to say....

 

[dmdalton]

sheck33 once bubbled...


True, when a tank cools you loose pressure but you dont loose air. A lot of people think they somehow have less air after a tank cools from a hot fill to a somewhat lower pressure.

Yes and no.

Please note: this is only an example, there is a specific formula if you are that nerdy, that will allow you to determine how much the volume of the gas in your tank will vary with changes in temeprature. The following is just an example of how it works:

An AL 80 holds 77.7 cu ft of air at 3000 psi. If you get a hot fill and the tank reads 3000 psi @ 110 degrees f then you have 77.7 cu ft of air in the tank at THAT temerature. If, when your tank cools to 70 degrees f it reads 2500 psi then at 70 degrees you have 64.75 cu ft of air. When you jump into the quarry and the temp drops to 40 degrees and the psi on your spg reads 2000 then you only have 51.8 cu ft of air from that 77.7 cu ft hot fill.

Gas volume and pressure varies directly with temperature. Increase temp, volume and therefore pressure in a fixed container (tank) both increase. Drop the temp and they both decrease.

Bottom line: Hot fills suck, and will make you think you suck down more gas than you actually do!

Dave D

 

[omar]

Air at 3200psi will have LESS volume than what would be calculated by the ideal gas equation. For an idea on how much go here:
http://www.atomox.com/gas_volume.html

How to figure it out go here:
http://www.atomox.com/mixing.html

omar

 

[sheck33]

what i was trying to say is the following, if i fill a tank to say 3800 PSI, close the valve, take it to the beach and then i attach my SPG it might give me a 3500 PSI reading, you can use any formula you want but the amount of air, as in the amount of air molecules, is the same.

Dmdalton, your statement is incorrect:

When the temperature rises the pressure rises in a scubatank, the volume however does NOT change, the volume is fixed since the tank is a RIGID metal container.


Also dont forget that the SPG is a pressure gauge and NOT a content gauge.

 

[omar]

The Pressure, Volume, Temperature relationship (combined gas laws) is:

(P1 * V1) / T1 = (P2 * V2) / T2

A change in temperature will have corresponding change in pressure for a scuba tank (the volume will not change).

However, the volume (free air delivery) for a tank at 3000 psi versus 3500 psi with both tanks at the same temperature, will be less. This is where the hot fill equals a short fill comes from.

omar