On a hot wall the limit is not the instrument but what touches it: the crystal and the gel. Both have a threshold beyond which they are damaged, and the measurement must be organised accordingly.
- ✓ Probes built to withstand contact with hot surfaces
- ✓ Dedicated couplants, with rated range up to 398 °C
- ✓ Brief contacts to limit heating of the transducer
- ✓ Reading correction for the change in acoustic velocity
Measuring on a plant in service often means measuring on a hot wall, because stopping and cooling the line to carry out an inspection is hardly ever an option. The resulting problem concerns the two elements in contact with that wall.
The first is the transducer. The piezoelectric crystal and the materials enclosing it have a temperature beyond which their characteristics change permanently: an ordinary probe left too long on a very hot surface is damaged. High-temperature probes are built with materials and geometries that push that threshold higher.
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The second is the couplant. An ordinary gel on a hot wall evaporates or degrades, and without coupling there is no measurement. Couplants formulated for these conditions exist: the Dakota NDT range is rated from -15 to 398 °C, with formulations also suitable for application on vertical surfaces.
How to organise the measurement
| Aspect | Precaution |
|---|---|
| Probe choice | Transducer rated for the actual wall temperature |
| Couplant choice | High-temperature formulation, within the rated range |
| Contact duration | Brief placements, with pauses to let the probe cool |
| Reading correction | Acoustic velocity varies with temperature |
| Operator safety | Protection suited to work on hot surfaces |
Not just the transducer: the reading too
Protecting the probe is not enough: temperature also changes the velocity at which ultrasound crosses the material, so a reading taken on a hot wall with a calibration made at room temperature is shifted.
Instruments with temperature compensation correct this drift automatically: on the CMX series it is the PETP mode. Without compensation the correction must be applied afterwards, knowing the wall temperature.
Probes, couplants and instruments
| Element | Use |
|---|---|
| Dual element transducers | Range that includes high-temperature probes |
| Couplants | Standard and high-temperature gels, range -15 / 398 °C |
| Dakota CMX1-DL | Automatic temperature compensation in PETP mode |
| Oil & Gas and Petrochemical | Typical application context for measurement on hot walls |
Frequently asked questions
It depends on the combination of probe and couplant used, not on the instrument. The rated values are given in the documentation of the transducer and the gel: the Dakota couplant range indicates a range from -15 to 398 °C.
Because the probe heats up while placed: brief contacts alternating with pauses limit the heating of the transducer and preserve its characteristics.
Yes, because the propagation velocity varies with temperature. Instruments with temperature compensation do this automatically; otherwise the correction must be applied knowing the wall temperature.
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Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
The information contained on this web page is provided for information purposes only. Although it has been compiled with the utmost care, it does not constitute a contractual offer or a binding commitment to supply. It may contain transcription, translation or typographical errors. For precise and up-to-date information, please contact our company directly.
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