Abstract: A high voltage switching apparatus 10 is described containing an arc-quenching gas containing sulphurhexafluoride in a vessel 12 that is monitored by a gas monitoring device 20. A temperature transducer 34 is attached to the exterior of the vessel 12 to sense the temperature of the gas. A gas line 24 leads from a control panel 18 to an enclosure 22 of the device 20. The device 20 has a front display panel with a digital communication terminal 42 for connecting to a computer to load and download data to and from the device 20 relating to the density of the gas within the vessel 12. The device 20 includes a data input module 44, a control module 46, a relay module 48, a display module 50 and a DC power supply module 52 mounted in an enclosure 22. The enclosure 22 has an internal heater 56 for maintaining the temperature within the enclosure above 20.degree. C.