Saturday, November 17, 2012

MECHANICAL ENGINEERING: Thermodynamics - Atmospheric, Barometric, Gage, Absolute, and Vacuum pressure


Atmospheric pressure --> Patm

= 14.7 psi

= 101.325 kPa

= 760 mmHg

= 760 torr

= 29.92 inHg

= 1013.25 millibars

= 1.01325 bar


When gage pressure is above atmospheric:


Pabs = Patm + Pgage


When gage pressure is below atmospheric (vacuum):


Pabs = Patm - Pgage


where:

Pabs = absolute pressure

Pgage =  gage pressure

Patm = atmospheric pressure


1. An oxygen cylinder has a pressure of 2400 psig at 70 F. What is the absolute pressure of the oxygen inside?


find:

Pabs = absolute pressure of oxygen inside the cylinder


given:

Pgage = 2400 psig


solution:

gage pressure is above atmospheric:

Pabs = Patm + Pgage

Pabs = 2400 + 14.7

Pabs = 2414.7 psia



2. The gauge at the vacuum pump reads 7 in. of Hg. What is the absolute pressure?


find:

Pabs = absolute pressure at the vacuum pump


given:

Pgage = 7 inHg


solution:

gage pressure is below atmospheric (vacuum):

Pabs = Patm - Pgage

Pabs = 29.92 - 7

Pabs = 22.92 inHg


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