A film of air between probe and part is enough to stop the ultrasound. The couplant is the least expensive link in the measurement chain, and it is the one that most often determines its outcome.
- ✓ Without couplant the pulse does not enter the material
- ✓ Viscosity suited to the working position, including vertical
- ✓ Dakota NDT range declared from -15 to 398 °C
- ✓ Constant quantity and layer for repeatable readings
Contact ultrasonic measurement rests on an elementary physical fact: between the probe and the part there is always a layer, and if that layer is air the acoustic energy does not pass. The difference in acoustic impedance between a solid and air is so large that almost all the energy of the pulse is reflected at the contact surface and practically nothing enters the material. The couplant serves exactly this purpose: to fill that gap with a medium the ultrasound crosses without significant losses.
The practical consequence is that a great many measurements that appear to fail because of the instrument or the probe actually depend on imperfect coupling. A layer that is too thin, a gel spread unevenly, a surface with residues or flaking rust all produce the same symptom: no reading, an unstable reading, or values that change with every placement. Before questioning the configuration, it is always worth checking how the contact has been made.
Need help choosing the right product?
Not all couplants are equivalent. They differ in viscosity, behaviour with temperature, compatibility with the material of the part and ease of removal at the end of the inspection. On a vertical wall or an undercut surface a product is needed that stays in place instead of running off; on a hot wall a formulation is needed that does not evaporate. The Dakota NDT couplant range declares a service range from -15 to 398 °C, with different products within that interval.
What to consider when choosing
| Aspect | Criterion |
|---|---|
| Surface temperature | Formulation within the declared range, from -15 to 398 °C |
| Working position | Viscosity sufficient for vertical walls and overhead surfaces |
| Condition of the surface | Removal of loose rust and deposits before applying the gel |
| Quantity applied | A thin, uniform layer: excess lifts the probe off the part |
| Compatibility and cleaning | Product compatible with the material and removable at the end of the inspection |
How good coupling is recognised
The sign of good coupling is stability: the reading appears at once, stays put as long as the probe is in position and repeats identically on successive placements at the same point. On instruments fitted with A-Scan the confirmation is visual, because the backwall echo appears sharp and of constant amplitude.
A value that fluctuates, appears and disappears or changes with every press of the hand indicates partial contact. In that case it is best to clean the surface, reapply a uniform layer and try again, before attributing the problem to the calibration or to the choice of transducer.
Recommended products
| Product | Why it is relevant |
|---|---|
| Ultrasonic couplants | Range with a declared service range from -15 to 398 °C |
| Transducers for corrosion thickness gauges | Dual element probes, tolerant of imperfect surfaces |
| Dakota CMX1-DL | A-Scan to check the quality of the backwall echo visually |
| Calibration blocks | Verification of the measurement chain on a known reference thickness |
Frequently asked questions
It is not advisable. An improvised liquid may not have the necessary viscosity, may not stay in place on vertical surfaces and may not be compatible with the material of the part. Dedicated gels are formulated to ensure stable, repeatable coupling.
Only as much as is needed to eliminate the air between probe and part. A thin, uniform layer gives the most stable reading, whereas an excess keeps the probe raised and introduces an additional path that can distort the measurement.
Yes. An ordinary gel on a hot wall evaporates or degrades and the coupling is lost. Dedicated formulations exist: the Dakota NDT range declares a service range from -15 to 398 °C.
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Further information and contacts
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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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