Apply a small amount of couplant to the area that is being measured. Use water for smooth coatings and glycol gel for rougher surfaces.
Place the probe of your measuring tool flat on the plastic's surface.
Press and hold the probe firmly against the surface to generate a measurement.
Lift the probe from the plastic's surface when a double beep is heard. After the green indicator light stops blinking, you can check your measurement on the probe's LCD.
When finished, make sure to clean the probe and the plastic's surface with a soft cloth.