Control system for immersion heater.

摘要

A capacitive proximity switch sensor is disposed adjacent wall means isolating the sensor from liquid in a receptacle having an immersion heater disposed therein. The proximity switch senses liquid level in the container and is operative through a relay in series with the heater to cut off power to the heater in the event liquid drops below a predetermined level. In one embodiment for use with a metallic liquid receptacle, the wall means comprises protective plastic casing about the sensor and disposed interiorly of the metal receptacle for sensing liquid. In another embodiment for use with a metallic liquid receptacle, the wall means comprises a nonmetallic tube parted to the lowest level of the receptacle and extending exteriorly thereof with the sensor adjacent the tube for sensing liquid level in the tube. In a further embodiment for use with a nonmetallic liquid receptacle, the wall means comprises the structural wall of the receptacle with the sensor disposed exteriorly thereof and closely adjacent for sensing liquid level. An optional feature includes an electrically operated drain valve energized by an operator actuated control switch with a delay timer for preventing opening of the drain valve until lapse of a predetermined time after the heater is cut off to permit the heater to cool before the liquid is drained from the receptacle.