Buyer's guide

How to choose a coating thickness gauge

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This page is an unofficial buyer's guide. It is not affiliated with or endorsed by Amazon, and it is not a review written for hire. It is not affiliated with or endorsed by AWS, API, or AMPP. No logo of any of those organizations is used. Some links below are paid Amazon affiliate links to example products. Your specification decides what you need.

This page is for someone who has to measure dry film on real work and is about to buy the gauge that will do it. It will not name a best model. A gauge is fit for a job when its principle matches the metal, its range covers the film, and you can check it on the day you use it. Price, on the day you look, is whatever the seller lists. A higher price is not proof the gauge meets your specification.

The project specification and the gauge maker's instructions govern the work. This page is general field practice. It does not copy a test method, a frequency table, or an acceptance limit from any standard.

What you are actually buying

You are buying a way to turn a film on metal into a number you can defend. The number is only as good as three choices: the right physical principle for that substrate, a range that can see the thinnest and the thickest film you inspect, and a check, that day, against shims or foils the maker and the specification accept.

A display that shows a tenth of a unit is not the same fact as accuracy to a tenth. Accuracy is whatever the maker states on the data sheet, after you have done the check the method requires. Read the data sheet before you believe the screen.

Types you will have to tell apart

Magnetic pull-off. A mechanical gauge, often a small scale you watch while a magnet lets go. No battery. Slow. Still a serious check on carbon steel. A good one is a backup when the electronic gauge is in a calibration lab, or a second opinion when a number smells wrong. It will not read coating on aluminum.

Electronic magnetic induction. A probe uses a magnetic field to infer the gap between the probe and ferrous metal. That gap is the coating, if the coating is not itself magnetic. This is the everyday gauge on carbon steel. It is fast enough to take the several readings a spot needs, which a slow mechanical gauge tempts people to skip.

Eddy current. A different probe principle, for non-ferrous metal substrates such as aluminum. It does not do the ferrous job, and a ferrous-only gauge does not do this one. Some stainless steels are magnetic enough to confuse a casual magnet test, and some are not. If you do not know what the substrate is, do not buy on a guess and do not record a number until you do.

Combination bodies. One instrument, ferrous and non-ferrous, sometimes with the probe deciding and sometimes with you selecting. Worth the money when your week really does mix steel and aluminum. Not worth it as a slogan if every job you have is carbon steel. Know which principle just fired before you write the number down.

Match the range to the film, not to the brochure

Thin films, the kind you meet on some fabricated items and on touch-up, need a low end and a resolution that can see them. Heavy films, linings and high-build liquid coatings, need a high end that is still inside the maker's stated range when the film is at its thickest. A gauge that tops out below the film you inspect is the wrong gauge, even if the brand is familiar.

This page will not print a universal thickness. Your specifications name the films. Write down the thinnest and the thickest you are actually asked to measure, in the units you report, and compare those two numbers to the maker's range before you buy.

Probe shape is the geometry of your work

An integral probe is quick on flat panels and on large easy surfaces. A separate probe on a cable reaches into pipe, behind stiffeners, and onto pieces you cannot lay the body on. A small probe is for fasteners, edges, and tight geometry, when the specification wants those places measured. Buy the probe for the shapes you actually inspect. A beautiful gauge that cannot touch the weld you are paid to measure is a gauge for a different job.

Rough blasted steel makes single touches jump. You want an instrument you can place several times quickly, because a spot is several readings, not one. The gauge does not know your specification's rule for how those readings become a spot. You do.

What to check on the data sheet

• Substrate principle: ferrous, non-ferrous, or both, and how you can tell which one is active.
• Range, and the accuracy the maker claims inside that range.
• Probe style, cable length, and the smallest convex or concave surface the maker says the probe can handle.
• How the check is done: which shims or foils, and whether they want that check on a surface like the work.
• Surface temperature limit of the probe. Hot steel after a cure check can be outside it.
• Units you can switch, and whether the unit is obvious on the screen you will photograph or write from.
• Data logging, only if you will actually download it and tie each batch to a location. A logged number with no place is still a weak record.
• Batteries, a case, and whether a spare probe is a sane purchase for remote work.
• Whether a calibration certificate from a lab is a separate purchase. Many projects want one on a schedule. The gauge and the certificate are not the same line item. This page does not sell or arrange a certificate.

Mistakes that waste the purchase

Buying a ferrous gauge for an aluminum job. Buying a range that cannot see the thick film. Trusting a new screen without a shim check that morning. Treating one reading as the area. Assuming a familiar name on a data sheet means the specification allows that model. Any of those will pass a shopping cart and fail a job.

What this page will not do

It will not pick a model number. It will not say a gauge meets a specification. It will not quote a price, because the price on a shelf changes. Brand names and linked products are examples of the category, not a statement that a model meets your specification.

Follow the project specification and the maker's instructions. If they disagree with this page, they win.