Where to Start

Substrate Material

Portable coating thickness gauges measure coatings on metal substrates. For magnetic substrates such as steel, cast iron etc. gauges based on one of the magnetic principles are used. For non-ferrous metals such as aluminium, brass, etc. electronic gauges using the eddy current principle are appropriate.

The first stage in gauge selection is to determine which substrate or substrates are to be coated and classify them as either magnetic metal or non-magnetic metal.

The table below gives examples of commonly used metal substrates and their classification. If your metal substrate is not mentioned, do not despair. A simple test with a magnet will determine the type of substrate. If the coating on the metal can be measured with a magnetic gauge the magnet will be attracted to the substrate. If the magnet is not attracted to the metal then gauges suitable for non-ferrous substrates are appropriate. This test can be carried out through the coating.

Coating thickness gauge manufacturers classify magnetic substrates as ferrous (F) and non-magnetic metal substrates as non-ferrous (N or NF). It should be noted that dual F/N or FNF gauges are available with automatic sensing of the substrate type.

Coating Thickness Range

The next requirement for the coating thickness gauge to be considered is the expected coating thickness. From this information the operating range of the coating thickness gauges can be determined.

Instruments are often made with several range options so that the performance can be optimised across the range. A good rule of thumb for choosing a suitable range for a gauge to test a particular coating thickness is to make the expected thickness no more than 80% of the full range of the coating thickness gauge, e.g. if the coating is expected to be a total of 500 µm (20 mil/thou), then the range of the gauge must be at least 625 µm for satisfactory measurements.

Ensure that you take account of the total coating thickness. It is not unusual to have a multi-coat system where the individual coatings combine to produce a relatively thick total coating thickness. The application of coatings is often not very precise and in some cases, wet paint on structural steelwork for example, variations in excess of 20% around the expected coating thickness is not unusual.

Finally, it should also be noted that longer range instruments e.g. Units with 5 mm or 13 mm range, will not have the same resolution as those with 250 µm, 500 µm or 1,500 µm.

Separate or Integral Probe

Design options for coating thickness gauges vary but for the user the types of gauge can be divided into integral probe and separate probe options.

The integral probe is built into the body of the instrument such that the unit can be used in one hand at full arm's length if necessary.

The separate probe option has the probe at the end on a flexible connecting lead so that the probe can be place on awkward areas whilst the display is kept in front of the users eyes.

The benefits of these two design options can be summarised as follows: -