The dry film thickness of coatings applied to cementitious substrates is usually estimated by measuring the wet film thickness as applied, calculating coverage, using Tooke Gage (a destructive in-situ technique described later) or removing debris from the dry coating. Microscopic measurements of cross-sections. If a coated sample can be removed without the substrate attached (although being able to remove such a sample may be an indicator of a problem), a micrometer can be used. There is also a relatively new technique for non-destructive measurement of dry film thickness. It involves a special instrument that uses the principle of ultrasound to measure thickness. See Figure 8. This technique is addressed in ASTM D 6132, Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Organic Coatings Using an Ultrasonic Thickness Gauge; Dry coating thickness, addressing frequency of measurements and acceptability of measurements.
The specific method for using the instrument should be followed according to the manufacturer's instructions, but the following basic steps apply:
Step 1 - Allow the probe to reach ambient temperature in the same environment as the reading by holding the probe in the air and pressing "Zero" in the main menu. This helps the gauge compensate for extreme temperatures and wear on the probe.
Step 2 - Verify the accuracy of the gage using a known reference standard. For polymer coatings, place a plastic spacer (reference standard) on the bare substrate, apply a drop of couplant on the surface of the spacer, and place a probe over the spacer through the couplant to measure the thickness of the spacer. The couplant transmits the ultrasonic signal from the probe to the coated surface (the gasket in this case). Adjust the gage to record the thickness of the shim.
Step 3 - Set the "Gate", which is the range of minimum and maximum thickness expected to be measured.
Step 4 - To measure the thickness of the coating, apply a drop of couplant to the surface of the coating and place the probe firmly on the coating through the couplant. A second reading of the same area can be taken without reapplying the couplant. But when moving to a new location, the couplant needs to be reapplied to be able to read.
