# Psia and Psig



## Calixico (Aug 15, 2009)

Is it safe to assume that whenever a pressure problem is given in psia or psig, the formula for pressure= (P=rho*h), remains the same. So if i want to convert psig to psia, what is the method to do so? I know that p(atm)=p(abs) + p(vacuum). But what is absoulte P in simple terminology? I get a little worried whenever I see the question shifting from psia to psig, even though I understand the concept of psai and psig, but not p(abs).

A little guidance here on this matter is very much appreciated. Just don't want to give away easy points on the exam over carelessness....

Thank you..


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## buick455 (Aug 15, 2009)

Spyderman said:


> Is it safe to assume that whenever a pressure problem is given in psia or psig, the formula for pressure= (P=rho*h), remains the same. So if i want to convert psig to psia, what is the method to do so? I know that p(atm)=p(abs) + p(vacuum). But what is absoulte P in simple terminology? I get a little worried whenever I see the question shifting from psia to psig, even though I understand the concept of psai and psig, but not p(abs).
> A little guidance here on this matter is very much appreciated. Just don't want to give away easy points on the exam over carelessness....
> 
> Thank you..


It depends on the altitude but at sea level it is PSIG = PSIA (absolute) - PSI (atmosheric) 14.7 lbs/in2

*Reference MERM:* Pressure of Fluid: 14-3 (14.1), Air pressure at Altitudes: 20-2 (20.1)


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## HVACstevie (Aug 18, 2009)

Spyderman said:


> Is it safe to assume that whenever a pressure problem is given in psia or psig, the formula for pressure= (P=rho*h), remains the same. So if i want to convert psig to psia, what is the method to do so? I know that p(atm)=p(abs) + p(vacuum). But what is absoulte P in simple terminology? I get a little worried whenever I see the question shifting from psia to psig, even though I understand the concept of psai and psig, but not p(abs).
> A little guidance here on this matter is very much appreciated. Just don't want to give away easy points on the exam over carelessness....
> 
> Thank you..



Don't over think it. Unless stated otherwise the questions on the exam are going to be at standard conditions. In most cases you're most concerned with the difference in P. So it doesn't matter as long as you always subtract PSIA from PSIA or PSIG from PSIG.


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