Most valves are intended for duties for which steam-class fluid tightness is satisfactory. Tests for proving this degree of fluid tightness are normally carried out with water, air, or inert gas. The tests are applied to the valve body and the seat, and depending on the construction of the valve, also to the stuffing box back seat, but they frequently exclude the stuffing box seal itself. When testing with water, the leakage rate is metered in terms of either volume-per-time unit or liquid droplets per time unit. Gas leakage may be metered by conducting the leakage gas through either water or a bubble-forming liquid leak-detector agent, and then counting the leakage gas bubbles per time unit. Using the bubble-forming leakage-detector agent permits metering very low leakage rates, down to 1×10−2 or 1×10−4 sccs (standard cubic centimeters per second), depending on the skill of the operator.

Lower leakage rates in the atom class may be detected by using a search gas in conjunction with a search-gas detector.