On a running plant, temperature is not only a question of material resistance: it also changes the speed at which ultrasound crosses the wall, and hence the value read.
- ✓ Probe and couplant rated for the actual temperature
- ✓ Dakota NDT couplants with a range from -15 to 398 °C
- ✓ Brief contacts, alternated with cooling pauses
- ✓ Automatic thermal compensation in PETP mode
Thickness inspection on plants in service almost always means working on hot walls, because shutting down and cooling a line to carry out a measurement campaign is rarely practicable. Temperature then becomes a variable of the inspection in every respect, and acts on three distinct levels: the endurance of the transducer, the endurance of the couplant and the accuracy of the value read.
The first level concerns the probe. The piezoelectric crystal and the materials enclosing it have a threshold beyond which their characteristics change permanently: an ordinary transducer left resting on a very hot surface degrades. Probes built for high temperature use materials and geometries that raise that threshold, but they remain components with a declared limit, to be respected.
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The second level concerns the couplant. An ordinary gel on a hot wall evaporates or degrades rapidly, and without coupling the measurement simply does not happen. Dedicated formulations keep their characteristics at temperature: the Dakota NDT couplant range declares a working range from -15 to 398 °C, with different products within that interval and versions suited also to application on vertical surfaces.
How the measurement campaign is organised
| Aspect | Measure to take |
|---|---|
| Wall temperature | Determined before starting: it governs probe, couplant and correction |
| Choice of transducer | A probe rated for that temperature, not a general-purpose one |
| Choice of couplant | A formulation within the declared range, from -15 to 398 °C |
| Contact time | Brief contacts with pauses, to limit heating of the probe |
| Correction of the reading | Automatic thermal compensation, or correction afterwards |
Why the reading must be corrected
A thickness gauge converts a time of flight into a thickness using the propagation velocity that has been set. That velocity depends on the temperature of the material: a calibration performed at room temperature and applied to a hot wall therefore produces a systematically shifted value, the more so the greater the thermal difference.
Instruments fitted with thermal compensation correct this drift automatically: on the CMX series this is PETP mode. Where compensation is not available, the correction must be applied afterwards knowing the actual wall temperature, and that figure must be recorded together with the measurement to make subsequent campaigns comparable.
Recommended instruments
| Product | Why it is relevant |
|---|---|
| Dakota CMX2-DL | PETP mode with automatic temperature compensation |
| Dakota CMX10-DL | Full range of modes for monitoring plants in service |
| Ultrasonic couplants | Formulations with a declared working range from -15 to 398 °C |
| Transducers for corrosion thickness gauges | A range that includes probes rated for high temperature |
Frequently asked questions
It is set by the combination of probe and couplant, not by the instrument. The declared values are stated in the documentation of the transducer and of the gel: the Dakota NDT couplant range indicates a working range from -15 to 398 °C.
Not without compensation. The propagation velocity varies with temperature, so a calibration made at room temperature leads to a shifted value. On the CMX series, PETP mode applies the correction automatically.
Because during contact the transducer heats up progressively. Brief contacts alternated with cooling pauses keep the probe within its declared limit and preserve its characteristics.
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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
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