Reference standards and calibration samples

Reference standards and calibration samples for coating thickness gauges, surface profile instruments and accuracy verification devices are essential accessories for ensuring that measuring instruments work correctly and that the measurements taken are metrologically traceable. They include certified shims of known thickness (in plastic or non-magnetic metal), reference substrates (steel, aluminum, zinc), calibrated roughness comparators, dollies for pull-off testing, and V1/V2 blocks for UT. They are available in the accuracy classes the sector standards require, with ISO 17025 calibration certificates issued by accredited laboratories.

Types of standard

For DFT coating thickness gauges the following are available: non-magnetic shims (plastic, PET) in various thicknesses (50, 100, 250, 500, 1000, 2000 μm) for calibrating magnetic instruments; conductive shims for eddy current instruments; and certified substrates of carbon steel, aluminum, zinc, copper and brass for verifying that the material is correctly identified. For the blasting profile: visual-tactile comparators with G (angular grit) and S (shot) references, and sets of replica tape gauges. For UT: V1 and V2 blocks and step gauges, for calibrating linearity and accuracy.

Metrological characteristics

Shims for DFT calibration are characterized by: nominal thickness and certified tolerance (typically ±2% of the nominal value for the standard class), flatness of the surface (to avoid measuring errors), dimensional stability over time, and individual identification (serial number). Reference substrates are plates of certified metal with known chemical composition, controlled surface finish and standardized geometry. All samples come with an ISO 17025 calibration certificate, traceable to the national primary standards of SIT/Accredia (in Italy) or their international equivalents.

Calibration procedures

The typical calibration of a DFT gauge involves: thorough cleaning of the probe and of the reference substrate; zeroing the instrument on the bare substrate; verification with a low-thickness shim (for example 100 μm); verification with intermediate and maximum thickness shims; and calculation of the error (the difference between the reading and the certified value), with correction of the calibration factor if necessary. Periodic calibration is required by acceptance specifications (typically before every measurement campaign, or daily for intensive inspection).

Storage and management

Calibration samples must be kept in a controlled environment (temperature typically 23±2°C, relative humidity 50±10%), protected from contamination, impact and chemical agents. Regular cleaning with non-aggressive solvents (isopropyl alcohol) preserves the surface. Replacement is recommended in the event of visible damage or when a sample falls outside tolerance at periodic verification. A company metrology management program (with a register of samples, calibration expiry dates and replacements) ensures conformity with the requirements of ISO 9001 and ISO 17025.

Reference standards

The requirements for calibration standards are governed by the standards for the respective instruments: ISO 2178 and ISO 2360 for coating thickness gauges; ISO 8503-1 for blasting comparators; EN 12668 and EN 27963 for UT blocks; ISO 4624 for pull-off dollies. Metrological traceability to the SI (International System of Units) is documented in the sample’s calibration certificate, issued by laboratories accredited to ISO/IEC 17025.

Selection criteria

The choice of calibration samples depends on the instrument to be verified, on the typical working range, on the accuracy class required, on the calibration frequency, and on reporting requirements (some specifications call for separately certified samples for each campaign). For multi-instrument laboratories, complete kits are available covering several measurement classes. Supplying samples with an ISO 17025 certificate from an accredited laboratory is recommended for formal acceptance and certification applications.