The rhometer: a continuous impurity monitor for liquid metal circuits

The rhometer uses the change in resistivity with impurity content of liquid metals, mainly sodium and sodium-potassium alloy, to measure the amount of impurities, such as oxide and hydride, etc., down to a few parts per million. It is a `non-contact¿ or electrodeless instrument, in that the liquid metal through a toroidal pipe which forms one turn of a 50-c/s transformer. The output from this transformer is fed to a phase-sensitive rectifier, and changes in resistivity are then indicated on a standard chart recorder. A special circuit compensates for changes in resistivity caused by changes in temperature of the liquid metal. Operating experience with the rhometer in the primary coolant circuits of the Dounreay Fast Reactor is described in detail, and some possible future trends in the further development of the instrument are briefly discussed.