PSIG refers to pound/force per square inch gauge, while PSI measures the pressure relative to a vacuum. If you want to convert a figure from PSIG to PSI, you would need to add 14.7psi to your PSIG figure, which will give you your PSIA result.
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0 psia is used as a reference point for pressure measurements because it represents the absence of pressure, equivalent to a perfect vacuum. By using 0 psia as a reference, pressure measurements are taken relative to this baseline to accurately determine pressure differentials in various systems and processes.
0 mmHg is equivalent to 0 psia because both represent a pressure of zero relative to absolute vacuum. However, 760 mmHg is equal to 14.7 psia, not 0 psig. 760 mmHg is equivalent to atmospheric pressure, which is the reference point for psig.
You must add the atmospheric pressure.
To convert bar to psia (pounds per square inch absolute), you need to add the atmospheric pressure in psia to the pressure in bar. 1 bar is approximately equal to 14.5038 psia (standard atmospheric pressure at sea level).
Boyle's Law says that PV is constant for ideal gas at a constant temperature. The pressure used should be the absolute pressure, not the gage pressure. Ge the absolute pressure should be obtained using : P = PG + PE where PG = gage pressure ( kPag , psig, etc. ) PE = barometric pressure ( kPaa, psia, etc. ) P = absolute pressure ( kPaa , psia, etc. ) ( PG + PE ) ( V ) = Constant for constant temperature The g in kPag and in psig indicates gage pressure.