The cases of liquid-filled gages are filled with a viscous lubricating fluid. Such gages maintain their accuracy much longer than conventional gages. They should be used in pulsating or vibrating services, such as the discharges of reciprocating pumps and compressors, and should be considered for other severe services. The fill liquid should be glycerine for ambient temperatures above 0°F and silicone oil for ambient temperatures below 0°F, or if the process fluid and glycerine are incompatible. Mineral oil (sometimes specified under the brand name “Kaydol”) is acceptable for services above 0°F. Liquid-filled gages should be filled until only a trace of a bubble is left in the face, to allow for thermal expansion and to show that the gage case is filled. The maximum temperature for liquid-filled gages is 150°F.
