Application: Nondestructive measurement of the thickness of silicone coatings on porcelain high-voltage insulators.

Problem: Porcelain insulators are universally used in high-voltage power transmission systems. Over time, a buildup of atmospheric contaminants such as salt spray or chemical dust can create a conductive path that causes a flashover. Silicone coatings such as Dow Corning's Sylgard® have been developed to prevent contaminant buildup. These coatings must be applied in some minimum thickness (typically 0.020 in. or 0.5 mm) to be effective. Because of the fairly high-cost of the coating material, most users will want to apply no more than the minimum required thickness. Mechanical measurement of coating thickness is not possible without cutting into it. However, a properly set up ultrasonic gage can measure the coating thickness quickly and nondestructively.

Theory of Operation: High-frequency sound waves will reflect from boundaries between materials of differing acoustic impedance. (Acoustic impedance is a physical property defined as density multiplied by the speed of sound in the material). Because silicone coatings have an acoustic impedance much lower than ceramics, a pulse of sound energy will reflect back from a silicone/porcelain boundary. All ultrasonic thickness gaging is based on a precise measurement of the time interval that represents the transit time of an ultrasonic pulse through the test material. Usually this interval will be measured as a round-trip transit time that must be divided by two to obtain the one-way transit time. Ultrasonic gaging also requires knowledge of the speed of sound in the test material, which is normally determined with the aid of calibration standards. Using this known sound velocity and the measured sound transit time, the instrument computes material thickness using the simple formula:

Thickness = Velocity x One-Way Transit Time

The accuracy of measurement will be proportional to the accuracy with which the time interval is measured and the accuracy to which the sound velocity is known. For silicone coating measurements, accuracies of ± 5% can usually be achieved.

Equipment: The recommended instrument for silicone coating measurement is a specially set up Panametrics-NDT Model 35, 35DL, or 38DL PLUS portable thickness gage with an M2055 handle-style 20 MHz delay line transducer. Measurements can be made over a thickness range of approximately 0.005 in. (0.12 mm) to greater than 0.050 in. (1.25 mm). Note that gage setups for measurement of silicone over porcelain are dedicated to this application and should not be used for other types of testing without first consulting us.

Procedure: Using a properly set up gage, couple the transducer to the measurement point using firm and steady pressure. Normally, it will be necessary to use a drop of liquid couplant (glycerin, propylene glycol, oil, or water) between the transducer tip and the test surface to insure good coupling of sound energy. The gage will display the thickness of the silicone coating at the point of measurement; with 35DL or 38DL PLUS gages this thickness reading may be stored in the internal datalogger. Gage calibration should be periodically verified using a reference standard consisting of a known thickness of silicone bonded to a ceramic base. For acoustic reasons it is essential that the reference standard exactly duplicate measurement conditions; unbonded silicone may not be used. We can supply further gage calibration information on request.