8.2 Dry Film Thickness Gauges and Calibration

Quick Answer: Dry film thickness (DFT) is measured on the cured coating using magnetic or electronic gauges governed by SSPC-PA 2 (the measurement procedure) and ASTM D7091 (the gauge practice). Type 1 magnetic pull-off gauges are mechanical and ferrous-only; Type 2 electronic gauges use magnetic induction on ferrous substrates and eddy current on non-ferrous substrates. Every gauge is zeroed on a bare metal reference (BMR) before use and verified with certified thickness shims.

Governing Standards and Type 1 Magnetic Pull-Off Gauges

SSPC-PA 2 is the SSPC procedure that defines how DFT measurements are organized — how many readings constitute a spot and an area, and how those readings are judged against the specification. ASTM D7091 is the practice standard for the gauges themselves: it covers calibration, verification, measurement technique, and the two gauge types. The inspector must know both standards because PA 2 tells you where and how many readings to take, while D7091 tells you how the instrument behaves and how to keep it honest.

Type 1 magnetic pull-off gauges are mechanical instruments that work only on ferrous substrates. A permanent magnet inside the gauge is balanced against a calibrated spring; when the gauge is placed on the coated steel, the substrate pulls the magnet toward the surface with a force proportional to the distance (the coating thickness). The inspector rotates a dial to pull the magnet away from the surface, and the dial reading at the moment the magnet breaks free is the coating thickness. Thicker coatings weaken the magnetic attraction, so a larger dial rotation is required and the reading is higher. Because the mechanism is mechanical, Type 1 gauges are rugged, need no batteries, and tolerate field abuse, but they are slower than electronic gauges and less precise on very thin films where the magnetic force is large and changes little with thickness.

The ferrous-only limitation is an exam staple: a magnetic pull-off gauge cannot read aluminum, stainless, or any non-magnetic substrate because there is no magnetic attraction to balance. On non-ferrous substrates the inspector must switch to a Type 2 electronic gauge in eddy-current mode.

Type 2 Electronic Gauges, BMR Zero, and Certified Shims

Type 2 electronic gauges measure coating thickness by magnetic induction on ferrous substrates and by eddy current on non-ferrous substrates. In magnetic induction mode, the gauge's electromagnetic coil generates a magnetic field that the ferrous substrate distorts; the degree of distortion depends on the distance (coating thickness), and the instrument converts that signal to a mils reading. In eddy-current mode, the coil induces eddy currents in a conductive non-ferrous substrate such as aluminum, copper, or stainless; the coating thickness alters the impedance of those currents, and the gauge translates the change to thickness. A single Type 2 gauge often has both modes and auto-detects or is manually set for the substrate.

Bare Metal Reference (BMR) zero calibration is the first step in every DFT measurement session. The inspector places the gauge on the same uncoated steel (or non-ferrous metal) that will be coated, on a spot cleaned to bare metal, and zeroes the instrument. This establishes the baseline so that all subsequent readings measure only the coating, not any residual offset in the electronics or any magnetic property of the substrate. BMR zeroing must be repeated whenever the substrate composition, geometry, or temperature changes, because those factors shift the magnetic or eddy-current baseline.

After zeroing, the inspector verifies the gauge with certified thickness standards — also called calibration shims or reference standards. A shim is a thin, precisely machined non-magnetic plastic or metal foil of known thickness (for example 5.0 mils). The inspector places the shim on the bare metal reference, takes a reading, and confirms the gauge reads within the shim tolerance (typically within the manufacturer-stated accuracy). If the reading is out of tolerance, the gauge must be returned for factory calibration or adjusted per the manufacturer procedure; field-adjusted gauges that cannot be verified are taken out of service.

The full calibration procedure is:

1. Inspect the gauge for physical damage.
2. Power on and allow the electronics to stabilize.
3. Perform BMR zero on a representative bare-metal surface.
4. Measure a certified shim of a thickness near the expected coating range and confirm the reading is within tolerance.
5. Measure a second shim near the low end of the range to confirm linearity.
6. Document the zero and verification on the daily inspection report.

If the gauge fails any verification step, it is tagged out of service and a backup gauge is used. Common exam traps: confusing BMR zero (on bare metal, sets the substrate baseline) with shim verification (confirms the gauge reads a known thickness correctly); assuming a Type 1 magnetic gauge can read non-ferrous substrates; and skipping re-zeroing when moving from one substrate composition or temperature to another.