Guide · Paint-thickness gauges
How to calibrate, verify and zero a paint gauge
Calibration, verification and zeroing are three different jobs. A two-minute check before every inspection, and what shims, foils and certificates prove.
Short answer
You do not calibrate a paint gauge at home; you verify it and, if needed, adjust it. DeFelsko separates the three: calibration is a documented lab process against traceable standards, verification is your own check on known references, and adjustment aligns readings to a known sample, with zeroing as the simplest form.[1] Before each inspection, check zero on bare metal, then check the shims.[2]
Three words, three jobs
| Term | Who does it | What it is |
|---|---|---|
| Calibration | The maker or a qualified lab | A controlled, documented process of measuring traceable standards and checking results are within the gauge's stated accuracy[1] |
| Verification | You | An accuracy check on known reference standards covering the thickness range you expect[1] |
| Adjustment | You | Aligning readings to a known sample for a specific surface or part of the range; zeroing on an uncoated sample is the simplest form[1] |
ASTM D7091, the industrial practice for magnetic and eddy-current gauges, describes the same three steps for accurate measurement.[3]
Watch the vocabulary. Elcometer calls its zero, smooth, rough or two-point, and zero-offset routines on the 456 "calibration" methods, and says the gauge is factory calibrated but should be adjusted for the application.[4] In DeFelsko's terms those routines are adjustments. Same idea, different word. Pressing "zero" or "calibrate" on a gauge does not make it lab-calibrated.
The check before every inspection
DeFelsko's detailing guide gives a simple routine: measure an uncoated part (a zero check), then measure the included plastic shims laid on the bare surface.[2]
- Clean the probe tip and the reference. Dirt counts as coating.
- Zero check. Place the probe on bare metal of the same type you will measure: steel for Fe mode, aluminum for NFe mode. Many kits include zero plates for this; Linshang's LS221 kit, for example, includes Fe and NFe zero plates and one standard film.[5]
- Shim check. Lay a shim on the bare metal and measure through it. Pick shims near the thickness you expect to see, since DeFelsko says verification standards should cover the expected range.[1]
- Compare. The reading should agree with the shim's value within the gauge's stated accuracy plus the shim's own tolerance.
- Adjust only if needed, following your gauge's manual, then check again.
DeFelsko says cheaper instruments drift and should be checked before use each day.[2] It is a two-minute job.
When a check fails
DeFelsko says that if readings disagree with the reference, every measurement since the last good check is suspect.[1] Re-zero, re-check, and measure the car again. If the gauge still will not agree with its references, stop relying on it and contact the maker.
What can push a gauge out of adjustment
DeFelsko lists substrate composition and magnetic properties, shape, roughness and edge effects, coating properties, and ambient and surface temperature as reasons a gauge may need adjusting.[1] Its PosiTector 6000 FAQ adds mass, metallurgy and curvature, and advises checking zero (or a shim) on the uncoated part before measuring.[6] See paint gauge readings that mislead.
Shims, foils and certified standards
Not every reference is equal.
- Plastic shims are the films bundled with most gauges. DeFelsko says a plastic shim is less accurate than a coating thickness standard, so its tolerance has to be added to the gauge's tolerance when you use it for adjustment.[1]
- Calibration foils are sold separately. Elcometer states its 990 foils at ±1%, each with a unique serial number for traceability, in thicknesses from 12.5 µm to 20 mm.[7]
- Coating thickness standards are coated metal blocks. DeFelsko's are smooth steel or aluminum with an epoxy coating of known thickness, supplied with a certificate traceable to NIST.[1]
A film in the box is not a certified standard unless the maker supplies a certificate for it. Traceability means an unbroken chain of comparisons back to a national or international standard;[1] a "traceable" label without paperwork does not show that chain.
Lab calibration and how often
DeFelsko puts no interval on its calibration certificates. It says customers should set their own, with one year as a typical starting point.[1] Its PosiTector 6000 ships with a certificate traceable to NIST or PTB, and the FAQ says that certificate has no expiry date.[6]
So one year is a starting point, not an expiry. For occasional used-car checks, daily verification on shims matters more than a certificate. A professional inspector who reports readings to others has more reason to keep a calibration record.
ASTM D7091 also sets out recommended frequencies for verifying and adjusting gauges.[3] The full text is a paid standard; we have read only its public summary.
Next
Questions
Is zeroing a paint gauge the same as calibrating it?
No. DeFelsko defines zeroing on an uncoated sample as the simplest form of adjustment; calibration is a documented lab process against traceable standards.[1]
How often should a paint gauge be calibrated?
DeFelsko puts no interval on its certificates and says users should set their own, with one year as a typical starting point. Check it yourself on shims before each use.[1]
Is the plastic shim in the box a calibration standard?
Not by itself. DeFelsko says a plastic shim is less accurate than a coating thickness standard, so its tolerance adds to the gauge's tolerance.[1]
What if my gauge reads a shim wrong?
DeFelsko says that if readings disagree with the reference, all measurements since the last check are suspect. Re-zero, re-check and measure again; if it still disagrees, contact the maker.[1]
How often should I check a cheap gauge?
DeFelsko says cheaper instruments drift and should be checked before use each day, with a zero check on bare metal and the included shims.[2]