Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

2.2 High Temperature Couplants
Most common ultrasonic couplants such as propylene glycol, glycerin, and
ultrasonic gels will quickly vaporize if used on surfaces hotter than
approximately 100° C or 200° F. Thus, 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. Poor acoustic performance and/or safety hazards may
result from using high temperature couplants beyond their intended range.
At very high temperatures, even specialized high temperature couplants must
be used quickly since they will tend to dry out or solidify and no longer
transmit ultrasonic energy. Dried couplant residue should be removed from
the test surface and the transducer before the next measurement.