Calibration and Verification of Coating Inspection Gages
Question: Could you please provide a clear explanation of the calibration and verification of coating inspection gages in the coating business? I would also appreciate if you could give an example using a coating inspection gage.
Answer: ASTM D7091 provides very good definitions of Calibration and Verification of Accuracy as they relate to use of dry film thickness (DFT) gages. SSPC-PA 2 provides additional information, again related to DFT. I have provided the definitions and additional information below, then I have explained them (relative to coating thickness measurement) in more practical terms. Also, you will find an article on KTA Univeristy from the April 2013 issue of JPCL on “Measurement of Dry Film Thickness” (based on the 2012 versions of SSPC-PA 2 and ASTM D7091). This article also describes the differences between these terms, along with a third term (adjustment), which you may find helpful (adjustment is also described below).
From ASTM D7091:
Calibration: The high-level, controlled and documented process of obtaining measurements on traceable calibration standards over the full operating range of the gage, then making the necessary gage adjustments (as required) to correct any out-of-tolerance conditions.
Verification of Accuracy: Obtaining measurements on coating thickness standards, comprising of at least one thickness value close to the expected coating thickness, prior to gage use for the purpose of determining the ability of the coating thickness gage to produce thickness results within the gage manufacturer’s stated accuracy.
From SSPC-PA 2:
Gages shall be calibrated by the equipment manufacturer, their authorized agent or by an accredited calibration laboratory. A test certificate or other documentation showing traceability to a national metrology institution is required. There is no standard time interval for re-calibration, nor is one absolutely required. Calibration intervals are usually established based upon experience and the work environment. A one-year calibration interval is a typical starting point suggested by gage manufacturers.
To guard against measuring with an inaccurate gage, gage accuracy shall be verified at a minimum of the beginning and end of each work shift according to the procedures described in ASTM D 7091. The user is advised to verify gage accuracy during measurement acquisition (e.g., hourly) when a large number of measurements are being obtained. If the gage is dropped or suspected of giving erroneous readings during the work shift, its accuracy shall be rechecked.
More simply stated:
1. Calibration: Performed by the coating inspection gages manufacturer or an accredited calibration laboratory (typically annually). This must be performed in a controlled environment using a documented process.
2. Verification of accuracy: Performed by the coating inspection gages user before and after each period of use to verify the gage is measuring properly by obtaining readings on traceable coated standards of known thickness.
3. Adjustment: Adjusting the gage as necessary to correct for substrate metallurgy, geometry/curvature, thinness and roughness. For Type 2 (electronic) gages, this is done by placing a measured shim on the prepared metal and adjusting the gage to match the shim’s measured value. This is not done in place of verification of accuracy. Verification of accuracy is always performed first to determine whether the gage is operating properly.
William D. Corbett is the Vice President and Professional Services Group Manager for KTA-Tator, Inc. He holds an A.D. in Business Administration from Robert Morris University and has been employed by KTA for over 33 years. Mr. Corbett is an SSPC Certified Protective Coatings Specialist, an SSPC Level 3 Certified Protective Coatings Inspector, an SSPC Level 2 Certified Bridge Coatings Inspector and a NACE International Level 3 Certified Coatings Inspector. He was the co-recipient of the SSPC 1992 Outstanding Publication Award, co-recipient of the 2001 JPCL Editor’s Award, received SSPC’s Coatings Education Award in 2006, and the SSPC 2011 John D. Keane Award of Merit. He is the author of the first, second and third editions of the KTA publication, Using Coatings Inspection Instruments. He also authored Chapter 8 of the SSPC Inspection of Coatings and Linings Handbook and co-authored Chapter 6, “Inspection” of the Steel Structures Painting Manual, Volume 1, Good Painting Practice. Mr. Corbett is a member of the ASTM International and SSPC: The Society for Protective Coatings, where he is Chair of the Education Committee and the Dry Film Thickness Committee.
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