Development of a Differential Microwave System to Measure Traces of Water in SF6

We report on the construction and evaluation of a prototype of a humidity meter based on a microwave differential circuit. This circuit has been developed to perform on-line monitoring of the humidity present in high power electrical equipment insulated with SF₆. The set up is built around two cylindrical tuneable microwave cavity resonators operating in the TE011 mode. One is filled with a pure sample of SF₆ and the other one with the contaminated sample under test, both are connected to a 180deg hybrid junction and are fed with a modulated carrier. Following demodulation done with lock-in detection implemented in software, we obtain a signal proportional to the concentration of contaminant in the gas. We have made measurements of the output signal for different samples of SF₆ contaminated with water having concentrations ranging from 20 to 400 ppm; we have determined that our circuit has a water vapour detection threshold of about 5 ppm at atmospheric pressure and room temperature