Abstract: A sensor mechanism for measuring a property of a fluid which must not be allowed to escape into the environment. A sensor housing provides a chamber from which a sensing element may be extended into contact with the fluid to be measured, and into which the sensing element may be retracted prior to being sealed off from the fluid. When the sensing element is in its retracted position, the closure of a valve seals the chamber off from contact with the fluid. A purging inlet and a purging outlet allow the interior of the sensor housing to be purged of traces of fluid trapped therein by the closure of the valve, prior to the time that the sensor housing is disassembled for the servicing or replacement of the sensing element. An improved sealing arrangement allows the interior of the sensor housing to remain sealed off from the environment without the use of sliding seals. An improved cleaning arrangement allows the sensing element to be cleaned without removing it from the sensor housing.