Adjustment Using Nonmagnetic Shims

16.3.4.1 Adjustment Using Nonmagnetic Shims

Adjust Type 2 gauges to account for the pro- file of the substrate; this allows the gauge to read the coating thickness directly. Use a portion of the substrate, after blast cleaning but prior to coating, to adjust the gauge.

Another method is to use an uncoated test panel (blasted at the time the structure was blast cleaned) with a profile representative

Figure 16.9 Two common types of DFT Reference Standards

of the structure, to adjust the gauge. Note:

16-8 Dry Film Thickness Measurement Instrumentation

ensure the test panel is of material with mag- netic and geometric properties similar to the substrate to be measured. If this is not avail- able, apply a correction value to a smooth- surface adjustment, such as that described below.

It is customary to adjust Type 2 gauges for substrate roughness by using plastic shims. Verify shim thickness with a micrometer. Please note: gauge measurements are less accurate when shims are used.

The following are four adjustment examples (some gauges have limited adjustment options):

1. Single Point Adjustment Use a single shim value which is at or close to the upper thickness to be measured. Mak- ing an adjustment affects the range of thick- ness in which accurate measurement is possible. This range varies with gauge design.

If the coating thickness to be measured is 4.0 mil (100 µm), then use a shim of approxi- mately 4.0 to 5.0 mil (100 to 125 µm) to adjust the gauge. Place the shim on an area of the substrate that has been blast cleaned to the required standard, or on a blasted test coupon with a similar shape and surface pro- file.

Take at least 10 measurements on the shim, then average the results. This is a sufficient number of measurements to allow for statis- tical variation in the blast profile. Avoid excessive pressure that could bend, indent, or crush the peaks of the shim.

This method ensures that the gauge reads the thickness of the coating over the peaks of the profile.

2. Two Point Adjustment

This example uses two shim values: one above and one below the expected film thickness range to be measured.

If the coating thickness to be measured is 4.0 mil (100 µm), then shims of 10.0 mil (250 µm) and 2.0 mil (50 µm) are appropriate to use to set the upper and lower values on the gauge scale. Because protective coatings are normally applied to blast cleaned metal sur- faces, a statistical approach must be used to obtain a typical value for the adjustment. Ten measurements on each shim are suffi- cient to establish a reliable average value for the roughened surface. Follow the manufac- turer’s instructions for adjusting the gauge to match each shim’s thickness value.

This method ensures that the gauge reads the thickness of the coating over the peaks of the profile.

3. Smooth Surface Adjustment Sometimes the bare blasted substrate is not available for measurement. Place the shims on a bare steel plate that has at least 3 x 3 x 0.125 in (7.6 x 7.6 x 0.32 cm) free of mill scale and rust. If the surface of the plate does not have a profile and the coated surface does, estimate an offset value to allow for the profile. This is typically about 0.5 to 0.8 mils (12 to 20 µm) to subtract from the mea- surements made (as noted in ISO 19840).

4. Reference Sample Another procedure sometimes used is to cre- ate a reference sample. Use a small steel sample approximately 6 x 4 in (15 x 10 cm), then blast clean it according to the specifica- tion at the start of a project. This panel can then act as the reference panel to check sur- face profile and DFT gauge measurements. Ensure the panel is made of similar material

Dry Film Thickness Measurement Instrumentation 16-9

(i.e., steel alloy) with a similar shape and

16.3.6 Equipment Connectivity

thickness to the structure being coated. Depending on the manufacturer and model of Type 2 DFT gauges, there are numerous