Posted by admin on Wednesday, July 8, 2009 · Leave a Comment
I want to know, the difference between the pressure we see in the gauges attached with different machine like boiler, compressor etc and the pressure termed as Atmospheric pressure.
Most pressure gages (in power plants, etc.) contain Bourdon tubes with system pressure inside and atmospheric pressure outside. The pressure difference bends the Bourdon tube deflecting a needle on the gage. The gage face must be graduated (and calibrate) to indicate the system pressure that may be very high (above atmospheric) in a boiler or very low (below atmospheric) in a steam condenser. The pressure gages usually read PSIG (pounds per square inch gage) where zero psig is atmospheric. However vacuum gages usually read PSIA (pounds per square inch absolute) where 0 psia is a perfect vacuum. There are also differential pressure gages where liquid (or gas) pressure is connected to both sides of the Bourdon tube, perhaps the suction and discharge pressures of a pump or the inlet and outlet pressure of an air flow filter or strainer. The gage may be graduated to read psi or psid (pounds per square inch differential).
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