Standards landscape
Coating thickness measurement is governed by three overlapping standard systems: ASTM (instrument operation), SSPC/AMPP (acceptance procedure) and ISO (measurement method and acceptance). Understanding which standard applies to your project is essential for passing inspection.
US standards
ASTM D7091-22
The operating procedure for magnetic and eddy-current coating thickness gauges. Covers calibration, verification, adjustment and measurement technique. All Dongru gauges comply with D7091 Type 2 requirements.
SSPC-PA 2-2022
The coating thickness acceptance procedure. Specifies spot count by area, reading requirements per spot, and the 80/20 pass/fail rule. Must be used together with ASTM D7091.
ISO standards
| Standard | Year | Scope |
|---|---|---|
| ISO 2178 | 2016 | Magnetic method on magnetic substrates |
| ISO 2360 | 2017 | Eddy current on non-magnetic conductive substrates |
| ISO 2808 | 2019 | All film thickness determination methods |
| ISO 19840 | 2012 | Acceptance for rough surfaces |
| ISO 12944 | 2018 | Corrosion protection paint systems (9 parts) |
The 80/20 acceptance rule
Both SSPC-PA 2 and ISO 19840 use the 80/20 rule:
- Area mean of all readings ≥ nominal thickness
- Fewer than 20% of readings between 80% and 100% of nominal
- No single reading below 80% of nominal
Calibration traceability
US projects typically require NIST-traceable calibration certificates. The certificate must show traceability to national standards, measurement uncertainty and the calibration lab’s accreditation status. Both normal form and long form certificates are available depending on project requirements.