The influence of temperature on internal pressure in vacuum interrupters in a liquid nitrogen environment

Depending on the operating current and the ambient conditions, the operating temperature of vacuum interrupters (VIs) may vary. An increase in VI temperature can result from heat loss at electrical contact resistance points, whereas a significant decrease in temperature will especially occur if VIs are used in future applications in a liquid nitrogen (LN2) environment (e.g. in combination with high-temperature superconducting equipment). In both cases of increased and decreased VI temperature, the pressure inside the VI may vary due to the adsorption and desorption of bound residual gases and changing material properties. This paper addresses the influence of VI temperature on internal pressure and on the applicability of the magnetron method for the measurement of internal pressure. By means of experimental test series that varied the internal pressure in punctured (i.e. non-vented) VIs and the ambient temperature, we found that an increase in VI temperature leads to a limited increase in internal pressure due to desorption of bound residual gases. Correspondingly, immersing the VI into LN2 causes a significant adsorption of residual gases, resulting in a pressure decrease of up to several orders of magnitude. In both cases the magnetron method is generally applicable, although ignition behavior is considerably affected by adsorption in an LN2 environment.