Conventional ultrasonic transducers cannot withstand high temperatures for two main reasons: the piezoelectric elements within them depolarize, and differential thermal expansion of the different materials within a transducer causes them to fail.  The use of thin and long waveguides of the rectangular cross-section allows large temperature gradients to be sustained over short distances without the need for additional cooling equipment. The main problems that had to be addressed were the transmission and reception of ultrasonic waves into and from the testpiece that the waveguides are coupled to, and optimization of the wave propagation along the waveguide itself. It was found that anti-plane shear loading performs best at transmitting and receiving from the surface of a component that is to be inspected.

The Challenge

Although thickness gauging is performed at normal environmental temperatures, there are many situations where it is necessary to test a material that is hot. This most commonly happens in process industries, where hot metal pipes or tanks must be tested without shutting them down for cooling, but also includes manufacturing situations involving hot materials, such as extruded plastic pipe or thermally molded plastic immediately after fabrication, or testing of metal ingots or castings before they have fully cooled. Conventional ultrasonic transducers will tolerate temperatures up to approximately 50 °C. At higher temperatures, they will eventually suffer permanent damage due to internal disbonding caused by thermal expansion. If the material being tested is hotter than approximately 50 °C , then high-temperature transducers and special test techniques should be employed.

The Solution

The measurement of remaining metal thickness of hot pipes and tanks with instruments such as the 38DL PLUS® or 45MG thickness gauges. Most of the transducers that are designed for use with Olympus corrosion gauges are suitable for high temperature use. The commonly used D790 series transducers can be used on surfaces as hot as 500 °C or 930 °F.  As with all ultrasonic tests, the best transducer for a given application will be determined by specific test requirements, including the material, the thickness range and the temperature. The following factors should always be taken into consideration in establishing a test procedure for any high temperature application:

  1. Coupling of Transducer The best technique to be to apply a drop of couplant to the face of the transducer and then press the transducer firmly to the test surface, without twisting or grinding it (which can cause transducer wear). Any dried couplant residue should be removed from the transducer tip between measurements.
  2. Selection of Couplant –Ultrasonic testing at high temperatures requires specially formulated couplants that will remain in a stable liquid or paste form without boiling off, burning, or releasing toxic fumes. It is important to be aware of the specified temperature range for their use, and use them only within that range.
  3. Gain Adjustment:The 38DL PLUS and 45MG gauges have user adjustable gain functions. Because of the higher attenuation levels associated with high-temperature measurements, it is often useful to increase gain before making measurements.
  4. Velocity Variation:Sound velocity in all materials changes with temperature, slowing down as the material heats up. Accurate thickness gauging of hot materials always requires velocity re-calibration. In steel, this velocity change is approximately 1% per 55 °C or 100 °F change in temperature. (The exact value varies depending on the alloy).
  5. Zero Re-calibration: When performing thickness gauging with dual element transducers, remember that the zero offset value for a given transducer will change as it heats up due to changes in transit time through the delay line.                    


  • Due to high temperature of heated material, there is reduction in velocity but re-calibration of the system aided in measurement easily without reduction of  sound velocity.
  • Increasing the Gain helped us to manage higher attenuation levels associated with high-temperature.
  • Periodic re-zeroing is necessary to maintain measurement accuracy. With 45MG and 38DL Plus corrosion gauges, this can be quickly and easily done through the gauge’s auto-zero function; also referred to as Do-ZERO.                   

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