Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. 76 Math for HVACR HVACR technicians typically use an electronic scale to measure the weight of the refrigerant added to or removed from a system. The scales can measure in pounds and ounces or grams and kilograms. Mastercool Inc. In air-conditioning and refrigeration systems, the weight of the refrigerant (called the charge) is often critical. While large systems have the ability to operate with slightly more or less refrigerant than is required, small systems are often referred to as critically charged systems, because the refrigerant charge must be exact. According to the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Guideline K and the United States Department of Transportation (DOT), the approved method for fi lling refrigerant cylinders to 80% capacity is based on the following formula. refrigerant capacity = 0.8 × WC × SG + TW where 0.8 = 80% of capacity WC = water capacity of the cylinder (normally in pounds) SG = specifi gravity of the refrigerant when recovered at 77°F fic TW = tare weight (the weight of the cylinder when empty, normally in pounds) AHRI Guideline K is an industry guideline. Thus, it is recommended but not required to be followed. However, DOT regulations are the law. Since DOT recognizes Guideline K, cylinders to be transported must adhere to this method of filling capacity. Consult the Federal Register (49 CFR 173.304a) for complete details. Specifi c gravity (SG) is defi as the ratio of the density of a substance to the fined density of a reference substance, which is usually water. Water has a specifi density fic of 1. Theoretically, if the specifi gravity of a substance is less than 1, it is less dense fic than water, and it will fl oat in water. If the specific gravity of a substance is greater fi than 1, it is denser than water and will sink in water. For example, at 77°F (25°C) R-134a has an SG of 1.21, R-410A has an SG of 1.06, and R-404A has an SG of 1.05. Thus, all are denser than water.