Thickness is one of the most frequently measured dimensions, and one that’s very easy to understand. So, you might think that someone would come up with a one-style-fits-all measurement approach that’s good for just about every kind of thickness application. But it just isn’t so.

There are many approaches to measuring thickness, depending on the requirements of the part. Some of the most common include micrometers, thickness gauges, air thickness gauges, and motorized gauges with many variations of each. They range in price and complexity from a few hundred dollars for a standard handheld tool to a few thousand dollars for ones that are custom-built for the application. One important consideration that makes each of these solutions different from the others is the way the gauge makes contact with the part.

Whether it’s the thickness of a piece of sheet metal, a silicon wafer, a latex glove, or photographic film, we’re usually talking about measuring the distance between two parallel surfaces. Accurately determining the length of a line that is perpendicular to two parallel surfaces has everything to do with how the gauge makes contact with the part.

Micrometer: A handheld micrometer is a simple low-cost method for measuring the thickness of a piece of sheet metal, for example, which is relatively stiff and thick. It provides a lot of measurement range. With flat and parallel contacts, and constant gauging force applied with the friction or ratchet drive, the micrometer can self-align to the part for a fast and accurate reading. A potential problem with the self-aligning flat contacts of the micrometer is that they can bridge across and “average out” minute variations in thickness. If higher resolution is required, look for a different approach.

Thickness gauge: The portable thickness gauge raises the ante on resolution by combining a flat, anvil-type reference surface on the bottom with a radiused (ball) measurement contact on top. Making a single-point measurement eliminates the possibility of gauge error that could be caused by faulty parallelism of the contacts. This would be a good gauge for a narrow strip of photographic film where a spot check of thickness is required.

Air thickness gauge: In some cases, the part may be so susceptible to scratching or marring that any amount of gauging contact could destroy the part. An air thickness gauging system directs thin, precision-aligned, opposing streams to each side of the part, which is held perpendicular to the air streams on a ground base. The gauge measures back pressure on each of the air streams. Because the back pressure is directly proportional to the distance between the contact point and the nozzle, it’s easy for the gauge to automatically calculate thickness. This noncontact technique provides a means of sliding the part around for measuring thickness variation. The air provides enough cushion to help float the part as it’s repositioned. The use of differential probing provides fast, accurate measurement regardless of where the part is positioned. Most important, the part isn’t damaged by the gauge contacts.

Custom thickness gauge: Many soft, compressible parts have rigid, standardized specifications for how they are to be measured. In such cases, the design of custom gauges will frequently take into consideration the size and shape of the contact along with the gauging pressure applied to the part. Usually, the gauging pressure is defined as a dead load weight to ensure constant force over the full range of measurement.

However, with some compressible materials, such as paper or latex rubber, the amount of time that the load is applied to the part will also affect the reading from the gauge. In these cases, the gauge might incorporate a motorized measuring contact along with the cam-actuated device to retract the contact after a specific period of time. This uniform load-and-dwell measurement prevents the parts from deforming and setting.

As you can see, the method of probe contact is a very helpful way to consider how to set up thickness gauging systems across a wide range of applications. Choosing the right approach can dramatically decrease the chance of inadvertently making a bad measurement.

Published Jan. 22, 2026, in the “Mahr’s Gaging Tips” blog.