Guide · Paint-thickness gauges
Paint gauge accuracy vs resolution
A 0.1 µm display is not 0.1 µm accuracy. How to read a paint gauge's accuracy formula, with six makers' published figures side by side.
Short answer
Resolution is the smallest step the display shows. Accuracy is how far a reading may be from the true thickness. They are different numbers: UNI-T lists the UT343E's resolution as 0.1 µm below 100 µm but its accuracy as ±(2%H + 2 µm), which works out to ±4 µm at 100 µm.[1] Read the accuracy line, and the conditions under it, before the resolution.
Two numbers that look alike
A display that shows 0.1 µm steps can still be several microns off. At 100 µm, UNI-T's own formula for the UT343E gives ±4 µm, forty times the display step.[1] DeFelsko likewise quotes resolution and accuracy as separate figures for its instruments.[2]
- Resolution: how finely the reading is displayed.
- Accuracy: the band, stated by the maker, within which a properly adjusted gauge should read on a known standard.
- Repeatability: whether the same spot gives the same number twice. None of the spec lines below states it; you can see it for yourself by measuring one spot several times.
How to read an accuracy formula
Most makers write accuracy as a percentage of the reading plus a fixed amount, or as "whichever is greater."
- %H means a percentage of the reading (H for the measured thickness). At 100 µm, 2%H is 2 µm.
- + 2 µm is a fixed allowance added at every thickness, so it matters most on thin readings.
- "Whichever is greater" means the wider of two limits applies. On a thin coating the fixed figure usually wins.
What the makers publish
| Gauge | Range | Stated accuracy | Resolution | At 100 µm (our arithmetic) |
|---|---|---|---|---|
| DeFelsko PosiTector 6000 (popular probes) | 0-1500 µm | ±(1 µm + 1%) up to 50 µm, ±(2 µm + 1%) above[3] | Not stated in our source | ±3 µm |
| Elcometer 456 integral (Scale 1) | 0-1500 µm | ±1-3% or ±2.5 µm, whichever is greater[4] | 0.1 µm to 100 µm, 1 µm above | ±2.5 to 3 µm |
| DeFelsko PosiTest DFT | up to 1,000 µm | ±3%[5] | 2 µm | ±3 µm |
| UNI-T UT343E | 0-2000 µm | ±(2%H + 2 µm)[1] | 0.1 µm below 100 µm | ±4 µm |
| Linshang LS221 | 0-5000 µm | ≤±(3%H + 2 µm)[6] | 0.1 µm below 100 µm | ±5 µm |
| Elcometer 311 | 0-500 µm | ±5% or ±20 µm, whichever is greater[7] | 10 µm | ±20 µm |
The last column applies each maker's formula at 100 µm, roughly a thin factory finish. It is arithmetic, not a measurement. UNI-T prints its formula once, beside its first range row, without a separate figure for thicker readings, so treat it as UNI-T's stated accuracy rather than a promise for any particular part of the range.[1]
The conditions under the headline
An accuracy figure holds only under the maker's conditions. Two examples:
- Elcometer 311: the stated accuracy needs at least 800 µm of steel or 300 µm of aluminum under the paint.[7]
- Linshang LS221: Linshang lists a minimum curvature (convex 5 mm, concave 25 mm), a minimum measuring area of 25 mm diameter and a minimum substrate of 0.2 mm (Fe) or 0.05 mm (NFe).[6]
On a car, curved panels, edges and thin sheet metal can all fall outside those conditions. See paint gauge readings that mislead.
A spec sheet is not an independent check
Maker accuracy figures describe a gauge that has been adjusted and verified. Independent results are rare. In AUTO BILD's 2021 test of German-market gauges, one NexPTG Professional sample deviated by up to 118% from the reference device on the test panels.[8] That is one unit of one model in one test, but it shows why the spec sheet is a starting point. Check your own gauge on shims or standards before each inspection: see how to calibrate and verify a paint gauge.
Microns and mils
US spec sheets sometimes use mils. One mil is one thousandth of an inch, or 25.4 µm; a micron is one thousandth of a millimeter.[5] So 5 mils is 127 µm.
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Questions
Does 0.1 µm resolution mean the gauge is accurate to 0.1 µm?
No. UNI-T lists the UT343E's resolution as 0.1 µm below 100 µm but its accuracy as ±(2%H + 2 µm), which is ±4 µm at 100 µm.[1]
What does ±(2%H + 2 µm) mean?
H is the reading. The allowed error is 2% of the reading plus a fixed 2 µm, so at 100 µm it is 2 + 2 = 4 µm either way.[1]
Why does the Elcometer 311 list such a wide accuracy?
Elcometer states ±5% or ±20 µm, whichever is greater, with 10 µm resolution, and says the figure needs at least 800 µm of steel or 300 µm of aluminum under the paint. It is an automotive paint meter, not a laboratory gauge.[7]
Has anyone checked paint gauges against their specs independently?
AUTO BILD compared ten gauges in 2021 on German-market models; one NexPTG Professional sample deviated by up to 118% from its reference device. It is one sample in one older test, so verify your own gauge on shims or standards.[8]
How do I convert mils to microns?
Multiply by 25.4. One mil is a thousandth of an inch, 25.4 µm, so 5 mils is 127 µm.[5]