Coating Thickness Gauges

Coatings on metal
The thickness of a coating on metal directly affects the protection of the item when it is subjected to wear or attack
by corrosion. Generally, the more coating there is on it, the longer it lasts. But, if this layer is too thick, parts may
not fit together or the coating may break up, so the thickness of the coating must be measured.

METAL SUBSTRATES
Metals are put in two basic groups,
Magnetic metals and alloys include iron, steel, ferritic and duplex stainless,
and nickel and cobalt alloys. These are called “ferrous metals”.
The remainder of will be non-magnetic metals.
Non-magnetic metals and alloys include aluminum, brass, copper, lead, titanium as well as austenitic
stainless steel, monel and inconel. These are called “non-ferrous metals”.
Be careful using the term “non ferrous” because there are many alloys that contain iron, nickel and cobalt that do
not attract a magnet. Strangely, some modern alloys that look metallic are neither magnetic nor conductive.
Dry film thickness gauges that use a magnet, can measure on ferrous metals but not other metals.
Electronic gauges, such use an F-type probe specifically for these metals. These can also measure
on the non-magnetic metals, by using an N-type probe. If you need to
measure both ferrous (F) and non-ferrous (NF) metals an some gauges automatically switches between the two modes.
It is worth mentioning conductivity. All metals conduct electricity though some not so well, such as
lead. Only one non-metal conducts (just about): carbon.

WHICH PROBE?
Before choosing which probe to use, it is necessary to consider both the
substrate and the coating. Magnetic metal substrates require the F-type probe, which uses the
electromagnetic induction technique. Their coatings cannot be magnetic, but
that leaves many different materials, such as paint and zinc that can be
measured. The thickness limit of metal coatings is those thicker than about
1mm (40 thou or mils). Above this point, the signal from the probe becomes
absorbed in the coating and is no longer influenced by the substrate.
Non-magnetic metal substrates require the N-type probe, which uses the eddy current
technique. Their coatings cannot be other metals but carbon is allowed.
There is no problem measuring lacquer or plastic coatings on non-magnetic metals.

WHAT INFLUENCES THE READING?
Obviously, the thickness of the coating changes the gauge's reading.
There are other variables too and they must be controlled through calibration.
Substrate, Shape and Surface.
Metals have conductivity and magnetic permeability, some more than others. Sometimes these change if the metal
is heated and cooled. It is always necessary to calibrate to a similar piece of metal as the substrate of the sample.
The curvature of the sample influences the reading. A convex surface effectively bends away from the centre of the
probe. This makes the reading higher than that on a similar flat surface. In a concave, the side of the probe is
closer to the surface and the reading is reduced. That is why it is necessary to calibrate to a similar shape as the
sample. N-type probes are affected more than F-type.
Rough metal substrates increase the reading of a probe, particularly for nonmagnetic
metals. The eddy-current circulates below the valleys so the peaks of
the roughness act as an extra coating. Calibrating to a similar surface
roughness can correct for this, though other techniques can be used too.

SETTING THE GAUGE
The process of calibration adjusts the readings of the gauge so actual thickness values are displayed. This process
will be repeated each time a different sample is to be measured.
There are two setting points, one on a standard thickness of plastic and one on the bare metal. The same can be
done at two different thicknesses to improve readings over a smaller range. Intermediate thickness standards will
confirm that readings over that part of the scale.

SCATTER
Rough surfaces are generally uniform, but mechanical damage is random. If there were only one reading on a
sample, a dint in the substrate could distort it and the reading would not be representative. It is always better to
take 10 readings over an area and to calculate the average value because this result is more repeatable than a
single reading.

REPEATABILITY
Accuracy is often less important than consistency. As an example, consider a table. If all the legs were 2mm short,
the table would function perfectly. But if one was wrong by 1mm and three were right, the accuracy would be better
but the table would not be stable. Consistency is necessary especially if more than one gauge is used. Similar
calibration to the same standard and readings taken in the same way will ensure this.