A better understanding of psia vs. psig 1

[westex]

Fill a scuba tank up to 10psig (sea level, outside on a boat) and put that scuba tank into a decompression chamber and pressureize the decompression chamber to 24.7psia. If you were living inside the decompression chamber with the scuba tank that was filled with 10psig, and you put a pressure gauage (psig) on the tank. What would that psig read that you just put on the scuba tank? Would it be 0 because the ambient air pressure (decompression chamber 24.7psia) is equal to the scuba tank pressure (10psig)?

Thanks Wes

 

[Compositepro]

yes.

 

[westex]

So in other terms a psig gauge shows the difference of the ambient air pressure and what ever the pressure port pressure is.

 

[Compositepro]

Correct.

 

[zdas04]

In most common pressure gauges there is a bent tube connected to the needle on the dial. Internal pressure tries to straighten the tube out. The amount that it succeeds is the reading on the pressure gauge. Atmospheric air is on the outside of the tube and is resisting the movement. The gauge naturally reads the difference between the pressure in the tube and atmospheric pressure.

David

 

[westex]

Well I know of some people that tell me this is all WRONG. Would like some more info to help those along. Please if you can think of more examples, it would be greatly appreciated. Thanks Wes

 

[JohnGP]

You need to get the same "some people" to explain why it is "wrong". David has explained how a pressure gauge typically measures - a gauge won't see or show a differential pressure if the pressure it is sensing is the same as the pressure surrounding it.

If the tank starts off with an internal pressure of 27.4psia, it is not going to discharge into an atmosphere of 24.7psia.

Cheers,
John

 

[JohnGP]

Oops, dyslectic again - of course if the tank was at 27.4psia, it could discharge into a 24.7psia atmosphere, but not if its contents were at 24.7psia.

 

[DRWeig]

Westex, click here:

Pressure Measurement

Good on ya,

Goober Dave

 

[zdas04]

I think you should have your bar buddies put their extensive middle-school educations to the test and try it. Most dive schools have a decompression chamber.

The barometric chamber simulates atmospheric pressure so if you put your scuba tank with 10 psig into the chamber and pressured the chamber up to 23.1 ft of water or 14939 ft below sea level (24.7 psia), the gauge on the tank would read zero and if you open the valve you would not get any organized flow (there would be some gas exchange, but the flow would be equally as likely to be predominantly in as out if the gauges were both well calibrated).

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[chicopee]

A decompression or hypobaric chamber is a chamber where the pressure is reduced below atmospheric pressure. In your original post you are probably refering to a hyperbaric chamber not decompression chamber.

 

[Compositepro]

A decompression chamber is a hyperbaric chamber used to decompress divers from the pressure at diving depth to surface pressure slowly so that they do not get the bends (decompression sickness).

 

[cloa]

They would increase the air pressure to around the level of the depth that you dived to and then slowly reduce it to allow the Nitrogen in your blood to slowly undissolve and be eliminated by your body in a normal rather than bubbles of nitrogen in your blood stream which give the bends.

 

[JohnGP]

It's amazing how some threads take on a life of their own.

 

[iken]

Another view, is once any tank is "filled" to any pressure above atmospheric pressure, then the tank will always be fill (unless you "suck" the contents out). So even when your gaige states you are empty, in fact you still have a full tank, only your tank contents are at the same pressure as your surroundings, and without this differential in pressure, you will get no flow.