Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. 72 Math for HVACR Technical Terms (Continued) meter (m) meter per minute (m/min) micrometer micron (μm) millibar (mbar) milligrams per cubic meter (mg/m3) millimeter (mm) ounce (oz) parts per million (ppm) pascal (Pa) pound (lb) pounds of steam per hour pounds per square inch (psi) pressure Rankine scale sensible heat SI system specifi gravity (SG) fic square inch (in2) square meter (m2) take-offs tare weight temperature therm ton (t) ton of refrigeration torr US Customary system weight wet-bulb depression wet-bulb (wb) temperature yard (yd) 5.1 Introduction to Measurement For HVACR technicians, the proper use and understanding of measurement is necessary for satisfactory performance in the trade. Errors in measurement, which can then lead to errors in calculation, often result in lost time and money for the service technician and the employer or building owner. Technicians routinely encounter measurement tasks. Linear measurements are used for determining duct dimensions and tubing and piping length. Volume measurements and calculations are necessary when determining heating and cooling requirements for conditioned spaces and water capacities for tanks and piping. Refrigerant system charge is measured by weight, and temperature, heat, and pressure are commonly used measurements in the field. In the United States, two systems of measurement are encountered on a daily basis. The US Customary system, also referred to as the inch-pound (IP) system, consists of centuries-old measurement units, such as the inch, the gallon, and the pound. The International System of Units (SI) is commonly called the SI system or metric system. It is based on standard units for length, weight, and volume with prefixes in fi powers of ten. This system was designed to replace multiple units of measurement with values that could be used worldwide. 5.2 Linear Measurement Goodheart-Willcox Publisher Linear measurement is used to determine the length, width, or depth of an object or the distance between two points. Linear units are especially used in installation work.