Microwave Methods for Cryogenic Liquid and Slush Instrumentation

Microwave instrumentation is being developed to measure densities and flow rates of nitrogen or hydrogen in either the single-phase liquid state or the liquid-solid (slush) state. The dielectric behavior exhibited by these fluids is the basis for using microwave systems for the measurement. These methods can also be applied to the measurement of fluids at ordinary temperatures. The three density instrumentation techniques discussed here require that the microwave signal be propagated through a column of cryogenic fluid (from the top to the bottom of a cryostat, for example). All of the fluid in the column influences the behavior of the microwave signal, thus the sample is representative of the fluids at all levels within the cryostat. A Doppler flowmeter is also discussed. Its main advantages are that no moving mechanical parts are involved and no probe need be installed in the flow stream. The flowmeter may give simultaneous density and velocity measurements so that the mass flow rate can be determined.