Electrode structure for a high temperature capacitance probe for sensing turbine speed

A probe for sensing the passing of a rotor blade in the hot turbine section of a gas turbine engine comprising:

• a housing constructed of high temperature resistant material having an open receptacle constructed therein;
• an insulating member constructed of high temperature resistant insulating material secured in the receptacle of the housing and adapted to receive and position at least one pair of electrodes within the housing, said insulating member constructed to insulate the electrodes from the housing;
• at least one pair of electrodes constructed of high temperature resistant material mounted in locked position within the insulating member and further comprising:
• an electrode pin constructed to fit into the insulating member and being positioned thereby to extend outward from the housing;
• an electrode tip secured to the outer extremity of the electrode pin and extending transverse thereto to form an electrode having a substantially T-shaped cross section, said electrode tip being entirely surrounded by an air gap thereby increasing the over surface path between the electrodes and between the electrodes and the housing;
• a cable extending through the housing and containing a conductor connected to each electrode;
• and
• means to mount the probe in proximity to the turbine blades of the gas turbine engine in a position at which the electrodes extend into the turbine area sufficiently close to the turbine blades such that an electrical field applied to the electrodes will be altered by the passage of the turbine blades.