To convert a gage reading to psia (pounds per square inch absolute), simply add the atmospheric, or barometric, pressure to the gage reading.
One problem with adding the barometric pressure to the gage reading is that most barometers are not scaled in pounds per square inch. They are scaled in inches of mercury. The chemical symbol for mercury is Hg. When a measurement is in inches of mercury, it's usually labeled with "Hg.
Converting "Hg to psi requires a mathematical conversion. A column of mercury approximately 29.9 inches (759.5 millimeters) tall exerts 14.7 psi (1.01 bar) of pressure on the surface below it. Therefore, 29.9" Hg (759.5 mmHg) is equal to 14.7 psi (1.01 bar). Normal atmospheric pressure at sea level can be expressed as 14.7 psi (1.01 bar) or 29.9" Hg (759.5 mmHg).
Another unit commonly used for pressure measurement in a power plant is inches of water ("H2O). Because a volume of water weighs much less than an equal volume of mercury, a pressure of just 2.04 inches of mercury (51.8 millimeters of mercury) is equivalent to 27.71 inches of water (703.8 millimeters H2O). Therefore, inches of water is typically used to express small pressures. The conversion chart below shows a comparison of inches of mercury, inches of water, and pounds per square inch.
1 pound per square inch (1 psi) = 2.04 inches of mercury ("Hg) (1.01 bar)
1 inch of mercury (1" Hg) = 0.4912 psi (0.0339 bar)
1 pound per square inch (1 psi) = 27.71 inches of water ("H2O) (1.01 bar)
1" H2O = 0.0361 psi (0.0025 bar)