Ultrasonic Pyrometer for Industrial Applications

Abstract

An ultrasonic pyrometer has been developed to measure the temperatures in various gaseous environments above 1000°C and in liquid metals. It is based on the temperature dependence of the velocity of sound in solid materials. A resonance technique is used to measure the frequency of a sensor and hence the sound velocity. The pyrometer can be described in three parts; the probe with the sensor at the end, the material and the electronics. An integral line probe of circular cross-section was first developed, The sensor is machined on the end of the line, is small and, being integral with the line, does not require a high temperature joint. This probe is suitable for a gaseous environment. A strip line probe, similar to the integral probe, has also been developed for use in liquid metals. The damping due to acoustic radiation losses is much less for this geometry and thus makes the application possible. In addition it is better suited for ceramic sensors which have poor thermal shock-resistance, User tests on a nuclear reactor, a jet engine and in molten lead established the practicality of these probes, A literature study of melting point, elastic constant, internal friction and thermal shock-resistance, the most significant parameters for this application, showed the refractory metals rhenium, molybdenum, iridium and platinum and the oxide ceramics alumina, magnesia, thoria and zirconia to be the most promising. Among these materials molybdenum, platinum, alumina and sapphire, which is the single crystal form of alumina, were investigated for their calibration stability and thermal hysteresis. Internal friction sets an upper limit in all cases, and this was 1400°C for molybdenum and platinum and 1900°C for sapphire, Alumina, being polycrystalline was found to have a very poor performance, An electronic system has been developed to automate the frequency measurement, It operates by comparing the sensor decrement, which is always at the resonator frequency, with a reference signal. The temperature is found from the calibration of the particular probe.