# Pab vs. Pg



## Blu1913 (Oct 12, 2006)

Can someone give me the simple version of the difference between Pabsolute and P gage? Im looking to understand it, not just know how to do it. Its just not clicking why there are two types of pressure... I understand gage is the pressure you would measure at a depth, but why do i need to care about absolute?


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## petergibbons (Oct 12, 2006)

Pabs = Patm + Pgage


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## MetroRAFB (Oct 12, 2006)

What he said. If you're concerned with deltaP it won't matter.


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## FusionWhite (Oct 12, 2006)

I had this same problem for a long time and it finally clicked for me one day. Ill try to explain it how I understand it but my reasoning may be a little screwy.

The definition of Pab and Pgage and Patm is that Pab = Pgage + Patm just for clarification.

Think of an open empty soda can sitting on your desk. The atmospheric pressure is equal inside the can and outside (14.7 psi). Now if you could seal the can the pressure inside the can is still equal to Patm and the pressure outside the can is still Patm. The absolute pressure (Pab) in the can is equal to the Patm. Gauge pressure equals ZERO according to the above equation.

Now lets say you increase the pressure in the can from 14.7 (which is equal to Patm) to 24.7 psi. Pressure outside is still atmospheric (14.7 psi) and there is a pressure gradient of 10 psi from the inside of the can and the atmosphere. This 10 psi gradient is the gage pressure. 14.7 psi of the total pressure in the can is a from the atmosphere and the 10 psi you added is the gage.

Pabs = 24.7

Patm = 14.7

Pgage = 10

That was probably a long and rambling explaination but thats how I remember what gage and absolute are.


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## DVINNY (Oct 12, 2006)

Fusion,

excellant post!!!!!!!!!!


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## Blu1913 (Oct 12, 2006)

Wow, massive props on the explaination :claps:

Completely makes sense now. Thanks a ton!

:thumbsup:


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