One of the most critical parameters monitored and controlled in a power plant is pressure.
Pressure is defined as force per unit area and is commonly measured in pounds per square inch (psi)
The force of one pound per square inch (1 psi, or 0.07 bar) against a large object can be several hundred pounds of actual force
The air in the atmosphere exerts pressure on all objects due to the weight of the air
Pressure can be measured using many different units. Psig stands for pounds per square inch gage and is often referred to as gage pressure. Gage pressure is pressure measured relative to atmospheric pressure. It uses atmospheric pressure as its zero reference.
Psia stands for pounds per square inch absolute. Psia measurements use zero psi as their zero reference. To calculate absolute pressure one must add atmospheric pressure to the gage pressure.
Barometric pressure is normally measured in inches of mercury ("Hg). Other pressure measurements may be reported in inches of water ("H2O). Any pressure measurements can be converted to other units by using the proper conversion factors.