⦁ To get as much of the ultrasonic energy from the transducer into the test part
⦁ To get as much of the ultrasonic energy that is reflected by the flaw of back test surface back into the transducer
1. What parts are being inspecting?
A part with a rough surface texture means a couplant has to do more work to create a reliable “bridge” for the ultrasonic energy to travel across. A thicker couplant will fill in the gaps and ridges on a rough-textured part better than a thin-watery couplant.
However, on smooth test surfaces, you don’t need a thick viscosity couplant. Here, a thin couplant is ideal for helping a transducer glide easily over the smooth or machined surface.
Type of metal / material
Unlike some non-spec approved materials commonly used as a couplant (such as high-temp grease, Vaseline, motor oil, hair gel, etc.), an NDT-couplant should have independent testing to guarantee it is safe to use on the material the part is made of.
Also, a couplant with good wetting characteristics will help ensure uninterrupted transmission during an UT inspection over a large test area.
2. What are the test environment / conditions?
3. What type of UT inspection?
Flaw detection, defect sizing
This type of ultrasonic testing often involves high-temperature pipes or may require a couplant that is resistant to salt. This is because some of the thickeners used to control a couplant’s viscosity will degrade very quickly if used in the presence of salt (like on some bridges or boilers).
4. What approvals / specifications / conformances are required?
Make sure to consider all the different parts you are responsible for testing and make sure to select a couplant, or couplants, which meet all specifications.
5. Other requirements / factors to consider?
If you are inspecting a lot of parts, it is more important to select a couplant that is easy to apply and remove than if you are only inspecting a few parts since application and removal can impact the overall time spent on a UT inspection.
For expensive, critical or precision components it is essential to use an NDT-approved couplant from a trusted manufacturer.
Final Thoughts to Consider
A couple more questions you may want to ask yourself:
⦁ What product are you currently using?
⦁ What do you like about the current UT couplant?
⦁ What don’t you like about the current UT couplant?
⦁ Why are you looking to make a change?