tube that reaches to the bottom of the cylinder, making it possible to remove liquid without turning the cylinder upside down. Some manufacturers require special recovery cylinders for use on their recovery units. The cylin- ders contain built-in safety devices to stop the recovery unit when the cylinder is full. The sensing devices are located inside the steel cylinder and consist of solid-state thermistors and float switches. The reliability of these automatic shut-off devices is questionable. All recovery cylinders should be inspected carefully and placed on an accurate scale before being filled. The technician should monitor the recovery process and stop the recovery when the required recovery vacuum has been achieved, when the scale shows the cylinder is full, or when leaks prevent recovery to the proper vacuum level. Recovery cylinders are considered full at 80% of total cylinder capacity. The capacity rating of a recovery cylinder designates the maximum amount of liquid refrigerant (in pounds) the cylinder can safely hold. The capacity rating is stamped on the cylinder and sometimes reads “W.C.” for water column. Cylinder manufacturers use water instead of refrigerant to determine cylinder capacity by weight. Never exceed a cylinder’s weight capacity rating for any reason. All liquids expand when heated. The expanding liquid inside a cylinder creates hydrostatic pressure. each recovery cylinder permanently and clearly. Always carry at least one refillable recovery cylinder for each type of refrigerant commonly used. Otherwise, when using only a single recovery cylinder, it must be emptied and evacuated before each use. Caution Damaged Cylinders Do not use dented, rusted, gouged, or damaged cylinders. Examine the cylinder valve assembly for leakage, damage, or tampering. Refillable cylinders must be retested and recerti- fied every five years from the test date stamped on the cylinder shoulder, in accordance with DOT Title 49 CFR, Section 173.34(e) and 173.31(d). Do not fill a container that is out of date (five years or older). Return it empty and have it retested. As shown in Figure 15‑10, recovery cylinders have two valves. One valve is marked “liquid” and the other “gas.” The liquid valve has a dip Manchester Tank Figure 15‑9. Recovery cylinders are easily identified by their color coding: yellow top and gray bottom. L I Q UID OP E N GA S O P E N Liquid Gas Hole (for gas) Dip tube (for liquid) Goodheart-Willcox Publisher Figure 15‑10. Recovery cylinders have two valves: one marked “liquid” and the other marked “gas.” 268 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.