METROLOGICAL PROPERTIES OF THE THERMAL-CONDUCTIVITY GAUGE

The hot-wire (Pirani) gauge is commonly regarded as a gauge with poor stability. In order to trace instabilities, both theoretical and experimental studies were performed. The heat losses from the wire by the relevant mechanisms were calculated from first principles. The heating power at all pressures is nearly proportional to the temperature difference between wire and ambient; thus a precise control of the temperatures is necessary. Surface properties of the wire play a crucial role: i.e. the radiative emissivity at vanishing pressure and the energy accommodation at the interesting intermediate pressures. Remarkably, the gauge response at intermediate pressures is not directly related to the response at zero and atmospheric pressure. Experimental data have been obtained for a sensor operated at constant mean wire temperature. The properties of the wire changed drastically by firing: The calibration results over a 1-yr period show good stability at high pressure (< 2%) but large variations (several per cent) at intermediate and zero pressures which are apparently due to changes of the surface properties.