DFT gauge calibration and verification
Every DFT acceptance decision inherits the credibility of the gage it was measured with. The vocabulary matters because specifications use it precisely: calibration is done by the manufacturer or an accredited lab against traceable standards; verification is the user's field check of accuracy against reference standards; adjustment aligns the gage to a known thickness.
As commonly applied under SSPC-PA 2, gage accuracy is verified before each work shift at minimum, and the verification is recorded.
A defensible field routine
Verify against certified shims or coated thickness standards spanning the range you expect to measure, ideally on the same base metal and curvature as the work. Record the shim values, the readings, the gage serial number, and the time.
On blasted steel, measure a base metal reading set on the prepared, uncoated substrate first. The profile raises the apparent zero; the base metal reading is how that offset is handled so coating thickness is not overstated. The standard's procedure for applying it governs.
- Verify before each shift and after any drop or battery change
- Use standards on the same base metal and curvature where possible
- Record every verification with instrument serial and time
- Re-verify at the end of the shift when results are contested
When verification fails
If the gage reads outside the tolerance on the reference standard, readings taken since the last good verification are suspect. That is why the verification log matters: it bounds how much work has to be re-measured.
Keep the calibration certificate current and its due date visible. CoatSpec tracks the instrument identity and calibration due date per project and flags an expired date on the report.
Frequently asked questions
What is the difference between calibration and verification?
Calibration is performed by the manufacturer or a qualified lab against traceable standards, usually documented with a certificate. Verification is the user's routine field check of gage accuracy against reference standards such as certified shims. Field inspectors verify; labs calibrate.
How often should a DFT gage be verified?
As commonly applied, at least at the start of each work shift, plus after any event that could disturb accuracy - a drop, extreme temperature change, or battery swap. Many inspectors also verify at shift end so the day's readings are bracketed by good checks.
Why take base metal readings on blasted steel?
Surface profile raises an electronic gage's apparent zero on bare steel. Measuring the prepared, uncoated substrate establishes that offset so coating thickness is not overstated. Follow the procedure in the measurement standard your project cites.