The major specifications regarding dry film thickness measurement, such as SSPC-PA 2, incorporate ASTM D7091 requirements regarding gage calibration & certification, verification of gage accuracy and gage adjustment procedures. Gage calibration is typically carried out by an accredited calibration laboratory under controlled conditions. Often times the original equipment manufacturer is accredited and performs the initial calibration, but it is not required to have the original manufacturer perform the calibrations.
The most widely accepted labs are those meeting ISO 17025 requirements, as these are the most stringent requirements and internationally accepted. A certificate documenting traceability to a national metrology institute is required to be provided with every ISO 17025 calibration. Most large coatings contracts require that the inspector use calibrated and certified gauges.
Gauges should be regularly recalibrated and recertified, as for various reasons the accuracy of most instruments degrades with use. There purposely is no ISO 17025 defined frequency requirement, as the need for recalibration and recertification varies with amount of use, environmental conditions, contamination, and other factors. A yearly recalibration & recertification interval is the most common. But having a valid calibration certificate is not a guarantee of accuracy for the reasons cited above. Therefore most standards require that accuracy is verified on a routine basis. A good rule of thumb is to verify accuracy before using the gauge every day, as well as when it may have become damaged, such as after dropping the gauge.
Verification of gauge accuracy is accomplished by measuring a traceable certified standard with a known thickness, ensuring that the DFT provides the same readings as the standard, within the defined tolerances. Traceable means that the result can be traced back in an unbroken chain to a measurement performed on an international standard that is commonly accepted as correct, such as NIST or BAM.These Certified Film Thickness Shims are available from Gardco as well as other suppliers. Most ISO 17025 calibration laboratories can also certify shims and thickness standards, including BYK-Gardner.
DFT recalibration and recertification is important in any quality management system and is an easy way to ensure that you not only comply with contract requirements, but that your end result is of the highest quality, year after year.
Applicable standards include:
- ASTM B499, ASTM D1186, ASTM D1400, ASTM D7091
- ISO 2808, ISO 2178, ISO 19840
- DIN 50981, DIN 50984
- AS 3894.3
- BS 3900-C5, BS 5411-3, BS 5411-11
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