Abstract: A hot gas turbine engine capacitive probe clearanceometer is employed to measure the clearance gap or distance between blade tips on a rotor wheel and its confining casing under operating conditions. A braze sealed tip of the probe carries a capacitor electrode which is electrically connected to an electrical inductor within the probe which is inserted into a turbine casing to position its electrode at the inner surface of the casing. Electrical power is supplied through a voltage controlled variable frequency oscillator having a tuned circuit in which the probe is a component. The oscillator signal is modulated by a change in electrical capacitance between the probe electrode and a passing blade tip surface while an automatic feedback correction circuit corrects oscillator signal drift. A change in distance between a blade tip and the probe electrode is a change in capacitance therebetween which frequency modulates the oscillator signal.

Abstract: An electrical capacitance clearanceometer system adapted clearanceometer system to measure the distance between a housing and a moving member within and closely adjacent the housing as a function of electrical capacitance therebetween, utilizes a high temperature probe which contains a high temperature electrical inductor potted in one end and an electrical capacitor electrode in a braze sealed tip at the other end and electrically connected to the inductor. The probe is inserted in a housing to have its capacitor electrode positioned adjacent the moving member. The electrode is braze attached to a ceramic insulator at the end of the probe and the ceramic insulator is brazed to a transition metal sleeve which is then brazed to the probe to fix the insulator in the probe. An air passage is formed between the insulator and the probe body to provide cooling air to the insulator and capacitor electrode.