the present invention relates to an apparatus for monitoring physical parameters in an oil and / or gas well , the apparatus being connectable to a wellhead of the oil and / or gas well . the apparatus comprises a flange assembly configured with a through bore and an end termination , in which through bore a sensor and associated electronics are arranged . the sensor , including a first electronic circuitry , is connected to a second electronic circuitry via transmission devices that are passed through a pressure - tight element arranged in the through bore . the invention also relates to a wellhead for use with an oil and / or gas well with a plurality of casings defining a plurality of annili . the wellhead is configured with a plurality of through - holes , each leading into a respective annulus of the well , and each through - hole is connected to an apparatus as mentioned above .