A practical system for flow measurement in rotating seals using the injection and sampling of a tracer gas is presented. Carbon dioxide or helium is injected as a tracer into a labyrinth seal at a controlled rate and gas samples are extracted downstream for concentration measurement. Test results from a rotating labyrinth seal rig were obtained over a range of seal pressure ratios and rotor speeds in order to determine the conditions which assure optimum tracer gas mixing. Seal leakage rates calculated by tracer gas concentration are compared to venturi flow measurements.

M'hen the pressure is equal to 760 iiini. the boiling point is indicated. I ani greatly indebted to Llr. Edward 0. Heuse for assistance ren-Other applications of the moving drop of mercury are being planned. dered in connect ion \vi t h this investigation.