Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. 80 Math for HVACR The joule (J) is a small unit of heat in the SI system. It is the amount of heat necessary to change the temperature of one kilogram of water by 1°C is 4187 joules or 4.187 kilojoules (kJ). 5.7 Pressure Measurement Pressure is defi as force per unit of area: fined P = F ÷ A. Based on this equation, if the area remains the same, more force will result in greater pressure less force will result in lower pressure. If the area is increased, the same amount of force will result in less pressure. If the area is decreased, the same amount of force will mean greater pressure. Pressure exerted can be measured in pounds or newtons. The unit of area can be measured in square inches or square meters. A square inch (in2) is an area that is one inch by one inch. Likewise, a square meter (m2) is an area that is one meter by one meter. P A F F = P F = P A = P A = P Goodheart-Willcox Publisher Square Units 1″ 1″ 1m 1m Goodheart-Willcox Publisher Vacuum is the absence of atmospheric pressure, and it is routinely measured by HVACR technicians. Vacuum can be measured in negative psi, but this is seldom done, as it is more accurately measured in inches of mercury (in. Hg) or microns (μm). 5.7.1 US Customary The most common pressure measurement used in the United States is expressed in pounds per square inch (psi). It is the pounds of force applied on a given area. For very small pressure levels and vacuum, a measurement in inches of water column (in. WC) may be used. This is also sometimes called inches of water gauge (in. WG). 1 in. WC = 0.036 psi 1 psi = 27.7 in. WC A pressure gauge may read in absolute pressure or gauge pressure. Gauge pressure (psig) reads 0 psi when open to atmosphere and is written as psig, meaning pounds per square inch gauge. Absolute pressure (psia) reads atmospheric pressure as positive pressure. Therefore, absolute pressure is the sum of gauge pressure plus atmospheric pressure. It is written as psia, meaning pounds per square inch absolute. Most pressure gauges in HVACR work display gauge pressure. Vacuum measurement in HVACR often uses inches of mercury (in. Hg). For inches of mercury, the scale starts at atmospheric pressure (0 in. Hg) and ends at a perfect vacuum (29.92 in. Hg).