Measurements at elevated temperatures (higher than approximately 125 °F or 50 °C) represent a special category. Many dual element transducers used in corrosion applications are designed to withstand high temperatures, in some cases up to 930°F or 500 °C in brief contact. However standard contact transducers will be damaged or destroyed by exposure to temperatures higher than approximately 125 °F or 50 °C because of the varying thermal expansion coefficients of the
materials used to construct them, which will cause disbonding at elevated temperatures. Contact transducers should never be used on a surface that is too hot to comfortably touch with bare fingers. High temperature measurements with the Model 35 will always be done in Mode 2 or Mode 3 with either a delay line transducer (using an appropriate high temperature delay line) or an immersion transducer. Consult Olympus NDT for further information on specific transducer selection.
Sound velocity in all materials changes with temperature, normally increasing as the material gets colder and decreasing as it gets hotter, with abrupt changes at freezing or melting points. This effect is much greater in plastics and rubber than it is in metals or ceramics. For maximum accuracy, the gage sound velocity setting should be calibrated at the same temperature at which measurements will be made. Measurement of hot materials with a gage set to room temperature sound velocity will often lead to significant error. Finally, at temperatures greater than approximately 200 °F or 100 °C, special high temperature couplants are recommended.