# Thread: PSIA PSIG Can someone help explain the Difference

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## PSIA PSIG Can someone help explain the Difference

I guess the title says it all. I understand that it's pounds square inch. But the a and g have me confused.

When you read your digital gauges do you use PSIA, or PSIG.

Thanks Jay Parker

2. psig.. gauges...

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PSIA = PoundsSquareInch Absolute, which means it includes atmospheric pressure equivalent to 14.7psi at sea level

PSIG is just PSIgauge gauge pressure not including the 14.7 absolute value

4. a is for absolute, as in from perfect vacuum.

G takes atmospheric pressure out of the equation.
so, G means you open it up and it reads zero
A means you open it up and it reads approximately 14.7 psi.... (atmospheric pressure)

5. PSI "A" = Absolute, where "0" is lowest point a pressure can be.
PSI "G" = Gauge. where "0" should be equal to the surrounding pressure of the gauge.
So if you work at different altitudes you should always reset your gauge to "0"
The difference between between "A" and "G" is the pressure around the gauge. Normally 14.7psi

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PSIA (A=absolute)is gauge pressure plus atmospheric pressure which varies based on elevation but has a standard value of 14.7 psi

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THANKS!
So if you zero out your gauges you use PSIA and if you don't you use PSIG??? Is this sensible thinking or am I totally confused.

8. Originally Posted by JAYLUPARK
THANKS!
So if you zero out your gauges you use PSIA and if you don't you use PSIG??? Is this sensible thinking or am I totally confused.
To make thinks clear.
when you zero your gauges and it reads zero with nothing connected to it, then you are reading PSIG.

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Thanks Babar

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Originally Posted by JAYLUPARK
THANKS!
So if you zero out your gauges you use PSIA and if you don't you use PSIG??? Is this sensible thinking or am I totally confused.
I am fresh out of school so I maybe wrong. If your hoses are not connected to anything they should read 0 PSIG which is equal to 14.7 PSIA. I think that's right ,could somebody correct me if I am wrong?

11. Think of it this way.

You are stood on the ground and you are 6 feet tall.
You stand on a chair that is 2 feet tall.
The top of your head would be 8ft from the ground. So if the ground is the reference point your are 8 feet tall. This is called absolute.
But you as a person have not changed in size only the chair has changed. So if you were to measure your height from the chair you would still be 6 feet tall.
Using the second method you discount having to measure the chair height. This would be called "gauge".
So by zeroing your gauge, you are removing the height of the chair.(in real terms the pressure around the gauge)

12. Originally Posted by AFoster
I am fresh out of school so I maybe wrong. If your hoses are not connected to anything they should read 0 PSIG which is equal to 14.7 PSIA. I think that's right ,could somebody correct me if I am wrong?
This true at sea level only "normal or standard"
Imagine you are working at ski resort high up in the Rockies. Atmospheric pressure is much lower than at sea level, your gauge if set at sea level would be reading wrong. This is why it is important to zero your gauges when you use them. (because you do not always know what the atmospheric pressure is)

13. Originally Posted by barbar
This true at sea level only "normal or standard"
Imagine you are working at ski resort high up in the Rockies. Atmospheric pressure is much lower than at sea level, your gauge if set at sea level would be reading wrong. This is why it is important to zero your gauges when you use them. (because you do not always know what the atmospheric pressure is)
PT charts are based off psig at sea level, so if you zero your gauges at altitude, your PT relationships will be off.

It is best to calibrate your gauges with a known refrigerant pressure, ie., a drum of refrigerant at a known stable temperature, and basically ignore zero.

If you have Digi-Cool gauges you can switch the scale to psia, pull a deep vacuum on the manifold, then hit the zero button and switch back to psig.

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