Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. 78 Math for HVACR Ritchie Engineering Co., Inc. – YELLOW JACKET Products Division 5.5.3 Temperature and Heat Two temperatures are used when measuring air temperature: dry-bulb (db) and wet-bulb (wb). Dry-bulb (db) temperature is the temperature of the air (or any material) without considering the amount of moisture present. Thus, dry-bulb temperature measures only sensible heat, which is heat that affects temperature change. The difference between dry-bulb temperature and wet-bulb temperature is called the wet-bulb depression. Wet-bulb (wb) temperature considers the amount of moisture in the air and will always be lower than the corresponding dry-bulb temperature, unless the relative humidity (rh) is 100%. At 100% relative humidity, the dry-bulb and wet- bulb temperatures will be the same. A wet-bulb thermometer uses a wick, moistened with distilled water, to measure the temperature. As the moisture on the wick evaporates, the moisture absorbs heat from the air, thus lowering the thermometer reading. This absorbed heat that evaporates the moisture is latent heat. Latent heat is heat that causes a change in a substance’s physical state. For example, it is the heat that causes water to evaporate. Latent heat does not affect temperature. It only affects whether a substance is solid, liquid, or gas. 5.6 Heat Measurement The measurement of heat energy is an important part of the HVACR trade, and it must not be confused with temperature measurement. Whereas temperature is a