Volume 2, Number 2 April 2003
TNT Tips:
Applying Couplant in Restricted Areas
When taking ultrasonic thickness measurements in areas with restricted access and application of ultrasonic couplant to the exact location is a problem, this approach can be helpful. Load couplant into an inexpensive large syringe available from chemical supply catalogues or many plant chemical labs. Using a syringe allows couplant application to measurement locations before transducer placement. In addition to ensuring transducer coupling, if couplant of a contrasting color is used, e.g., light colored couplant gels on dark surfaces and dark couplants on light surfaces, predetermined thickness measurement locations are very distinct.Bill Perrin
Fredericton, New Brunswick
Canada
D-Meter Resolution with Analog Display
If you need to measure wall thickness to 0.1 mm on a thick-walled specimen and have only an analog ultrasonic set, you can still measure with that degree of accuracy using this set. Calibrate the timebase to 10 mm using a calibrated step wedge, then simply delay off the backwall echoes - not forgetting to count them as you wind them off. This way the timebase progresses 0-10 mm, 10-20 mm, 20-30 mm, etc. and each subdivision represents 0.2 mm. Now read the timebase as if it's 0-10 mm, e.g., 4.6 mm. Interpolation between gives 0.1 mm, making 4.7 mm, add on the thickness that you have delayed off, 20 mm. Now you are ready to give that measurement as 24.7 mm.John M. Armstrong
Dorset, England
Couplant Degradation in Summer Heat
We use dry powder ultrasonic couplants and recently have experienced some problems during the summer heat. If all mixed couplant is not used, our inspectors save the leftover mixed couplant for use on the next job. In summer heat, mixed couplant sometimes loses consistency and reverts to a watery state. After investigation, it was discovered that simple emulsifier might aid in correcting this problem.We mixed a fresh batch of dry powder couplant and placed equal amounts of couplant into identical bottles. To one of the bottles, we added a small amount of lipophilic emulsifier, just enough to change color of the couplant to a very light pink. The other bottle was left as plain couplant. Both were placed in the trunk of a car for about ten days. At the end of the trial period, the emulsified couplant was still gelatinous while the untreated couplant had reverted to a liquid state. The emulsifier worked. In addition, we have not experienced any sound transmission problems, nor any reaction with materials under test.
If you have experienced this problem, try adding a small amount of lipophilic emulsifier to your couplant. Certain codes or specifications may not allow this mixture of couplant and emulsifier, but for general testing applications it works for us.
Richard L. Tiefenauer
St. Louis, Missouri
[ The NDT Technician ]
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