Coating Thickness Calibration Standards
What is the purpose of Standards?
Certified thickness Standards are used to verify the accuracy and operation of coating thickness gages and are an important component in fulfilling both ISO/QS-9000 and in-house quality control requirements. Contracts often specify that coating thickness measurements be taken by gages whose measurement accuracy is traceable to a National Metrology Institute such as NIST or PTB.
Standards are typically purchased as an accessory to DeFelsko coating thickness gages. Many customers find it more practical to verify the accuracy of their own gages in-house, rather than utilize DeFelsko’s calibration services. This is particularly true when many gages are in use and/or when accuracy verification is performed often.
What are my Choices?
There are 5 types of coating thickness Standards available from DeFelsko:
- certified coated metal plates
- certified polystyrene blocks
- certified plastic shims
- non-certified plastic shims
- zero plates
Why should I use certified coating thickness STANDARDS to verify the accuracy of my gage when I have plastic shims?
Your electronic PosiTector and PosiTest gages come with NON-CERTIFIED PLASTIC SHIMS for use as a quick reference out in the field or on the floor. But these non-certified plastic shims are not ideal for verifying gage accuracy for the following reasons:
- they are not certified or traceable to any national body.
- their accuracy is far less than most of the gages being verified.
- they cannot be used with Type 1 mechanical gages, such as our PosiTest FM or PosiPen.
- their thickness may be inconsistent due to pressure from probe tip
- they may suffer distortion due to environmental conditions such as temperature
We include non-certified shims with our electronic gages to provide a quick verification of gage operation and allow the user to perform practice measurements when placed over metal.
Can Certified Coating Thickness Standards be recertified?
In most instances, it will be more economical to trade-in Standards for a new set. Trade-in reduces the price of new Standards by 20%.
What are other purposes of CERTIFIED and NON-CERTIFIED plastic shims?
- Probe protection: Shims can be placed over rough or hot surfaces to protect the probe from physical damage. Coating thickness is obtained by subtracting the shim thickness from the total measurement of the shim and the coating.
- Force distribution: Shims can be placed over soft/tacky coatings to prevent the gage probe from depressing the coating film. Again, coating thickness is obtained by subtracting the shim thickness from the total measurement of the shim and the coating.
- Rough surface calibration adjustment: Since gages are verified for accuracy using smooth-surfaced Standards (or using a smooth zero plate), a compensation value may be required if the substrate to be coated is different from the Standard or roughened from the manufacturing process (for example, castings) or from abrasive blast cleaning. This is known as a Base Metal Reading or BMR. The BMR is the effect of substrate (for example, surface profile) on a coating thickness gage. The user obtains several readings on the prepared, uncoated substrate. The arithmetic mean of these values becomes the Base Metal Reading. The BMR is deducted from the coating thickness values in order to report the thickness of the coating layer(s) over the surface profile.
- One-point calibration adjustment: A one-point adjustment involves fixing the gage’s calibration curve at one point after taking several readings on a single coating thickness Standard, shim or reference sample. Adjusting to zero on an uncoated sample of the test specimen is the simplest form of a one-point adjustment (see rough surface calibration adjustment above). If the user elects to perform a one-point adjustment procedure to a known thickness, a reference sample or shim representing the target range of gage use should be selected and a measurement taken. If the value displayed by the gage is within the combined accuracy of the shim and the stated gage accuracy, then no adjustment is necessary. If the gage reading is outside the combined accuracy of the shim and the stated gage accuracy, then the user should carefully follow the gage's instructions for proper adjustment.
- Two-point calibration adjustment: A two-point adjustment fixes the instrument's calibration curve at two known thicknesses. Coated reference samples or shims placed over the uncoated substrate or over an uncoated reference sample may be used. The two thicknesses selected must be on either side of the expected coating thickness. It is recommended that the gage be verified for accuracy during the measurement procedure and after the coating thickness measurements have been acquired.