Coating thickness gauges (also referred to as a paint meter) are used to measure dry film thickness. Dry film thickness is probably the most critical measurement in the coatings industry. A coating thickness gage provides vital information as to the expected life of the substrate, the product's fitness for purpose, its appearance and ensures compliance with a host of International Standards.
Elcometer offers a wide range of coating thickness gauges to measure dry film thickness. Also known as a paint gauge or a mil gage, it includes mechanical and digital coating thickness gauges suitable for destructive or non-destructive testing, complete with a wide range of probes and calibration foils to suit your application.
The Elcometer range of dry film thickness gauges (DFT gauge), provide reliable and accurate coating thickness measurements on almost any metal substrate, whether ferrous or non-ferrous.
Elcometer offer a range of easy to use mechanical coating thickness gauges suitable for areas where no electrical instruments are permitted or high temperatures prevail.
Primarily used in multi-coat procedures and non-metallic substrates, Elcometer offer a complete range of paint inspection gauges (PIG gauge) designed to measure the thickness of single or multiple layers of a coating.
The Elcometer range of digital coating thickness gauges has been specifically designed to provide highly accurate, reliable and repeatable coating thickness measurements on almost any substrate, whether ferrous or non-ferrous.
Dry Film Thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium using a digital coating thickness gauge. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates.
Elcometer offer a range of digital coating thickness gauges from the new Elcometer 456 - available with either integral or separate probes, the Elcometer 415 Paint and Powder Thickness Gauge, to the Elcometer 311 Automotive Refinishing Gauge.
The Elcometer range of mechanical coating thickness gauges provides cost-effective dry film thickness measurement. Mechanical Coating Thickness Gauges are suitable for working in high risk areas such as high temperature or flammable atmospheres, underwater or where the risk of explosion is high and could be triggered by the use of an electronic instrument.
From the simplest pre-calibrated pull-off coating thickness gauge Elcometer 157 which will provide you with quick and immediate results to the more accurate coating thickness gauge Elcometer 211, also called the "banana gauge" which is ideal for cold and underwater surfaces.
Designed for measuring on a non-metallic substrate or assessing the thickness of a multi-coat paint, Elcometer offer a range of destructive coating thickness gauges such as the Elcometer 121/4 and Elcometer 141 that are portable and easy to use.
How does a Coating Thickness Gauge work?
Dry film thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium using a digital coating thickness gauge. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates.
Permanent Magnet Coating Thickness Gauges
A permanent magnet is mounted on a balanced arm and the force required to pull this magnet from the surface of the coating is a measure of the thickness of the coating. Force is applied through a helical spring attached to the balanced arm at one end and to a scale wheel at the other. As the scale wheel is turned the force is progressively increased until the magnet lifts from the surface. The scale is drawn in thickness units rather than force and the thickness of the coating can be read against a pointer on the case of the instrument.
Electronic coating thickness gauges for measuring on magnetic substrate materials use the electromagnetic induction principle. A three-coil probe system is used where the central coil is powered by the instrument and the other two coils, either side of the central coil, detect the resulting magnetic field. The signal generated by the instrument is sinusoidal and therefore an alternating magnetic field is established round the central coil.
When there is no magnetic materials influencing the probe then the magnetic field cuts through the other two coils equally. As the probe is brought closer to the uncoated substrate the field becomes unbalanced with more field cutting the nearest coil and less cutting the furthest coil. This produces a net voltage between the two coils which is a measure of the distance to the substrate (the coating thickness).
Eddy Current Coating Thickness Gauges
In the case of the eddy current principle, a single coil probe is used with a relatively high frequency signal, several mega-hertz, to generate an alternating field in the non-ferrous metal under the coating. The field causes eddy currents to circulate in the substrate which in turn have associated magnetic fields. These fields influence the coating thickness probe and cause changes to the electrical impedance of the coil. These changes are dependant on the coating thickness.
How accurate are Coating Thickness Gauges?
A key decision on the overall selection of a suitable coating thickness gauge is how accurate do the readings need to be? Within the range of gauge types available, there is a progression from moderately accurate to very accurate gauges, this is mirrored by the prices of the coating thickness gauges, the more accurate, the higher the cost. In addition the coating application process and other factors affect the variability of the coating thickness on a particular surface and the skill and knowledge of the coating thickness gauge operator also influences results.
What does "accuracy" mean?
The basic measure of a coating thickness gauge's performance is the accuracy with which the gauge takes readings. That is the difference between the reading and the true coating thickness.
How to test a Coating Thickness Gauge for accuracy
In order to test the accuracy of a particular gauge it is important to have traceable coating thickness standards. With the gauge adjusted to zero on an uncoated smooth substrate and set to a known thickness standard at or near to the maximum thickness, intermediate thickness standards are measured and the readings compared to the actual thickness of the standard. The errors are the differences between the values of the reading and the value of the standard. These are most conveniently expressed as a percentage of the reading.
The importance of Coating Thickness Gauge Calibration
Calibration is the process whereby manufacturers of a coating thickness gauge sets-up, during manufacture, to ensure that the gauge meets the required accuracy specification. The procedure normally requires the coating thickness gauge to be set to known values of thickness and checked on intermediate thickness values. In modern electronic instruments the values at key points across the coating thickness range are stored as reference points in the memory of the gauge.
Why you need to Calibrate a Coating Thickness Gauge before testing
Calibration of coating thickness gauges will be affected by the type of material, the shape and the surface finish of the metal substrate to be tested. For example the magnetic properties of steel alloys vary and the conductivity of different aluminium alloys and different non-ferrous metals, copper, brass, stainless steel etc. also vary. These variations can affect the linearity of a coating thickness gauge. This means that a gauge set-up on mild steel for example will read a different value for the same thickness coating on high carbon steel. Similar linearity effects are seen on thin or curved substrates and particularly on profiled substrates such as blast cleaned steel used for structural steelworks.
To overcome these effects most coating thickness gauges have features that allow you to set the gauge to the work being carried out, thus maximising the accuracy of the readings.
Adjusting a Coating Thickness Gauge
Adjustment is the technique whereby you can set-up the coating thickness gauge for the conditions prevailing for the work in hand. In addition to material differences, shape and surface finish the adjustment may be carried out at an elevated temperature or in the presence of a stray magnetic field. By adjusting the coating thickness gauge to these prevailing conditions the resulting errors are greatly reduced and even eliminated.
The effect of surface roughness, particularly those produced by deliberately profiling the substrate by blast cleaning with either grit or shot or by mechanical cleaning, is quite significant, to find out more click here.
Using Coating Thickness Standard to Calibrate a Coating Thickness Gauge
There are two basic types of coating thickness standard, foils and pre-coated metal. For more information on coating thickness standards for coating thickness gauges click here.
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The Digital Coating Thickness Gauges Elcometer 456 Models B, S and T are all compatible with the powerful, fast and easy to use Elcomaster™ 2.0 Data Management Software.
Elcomaster™ 2.0 gives you the power to review your coating inspection data and create custom professional reports quickly and easily.
With Elcomaster™ for Android you will be able to instantly monitor and analyse the data collected from the field without having to turn on a computer.
For more information about Elcomaster™ 2.0 Data Management Software, please click here.
