What is a Coating Thickness Gauge?

 

 

A coating thickness gauge (also referred to as a paint meter) is used to measure dry film thickness. Dry film thickness is probably the most critical measurement in the coatings industry because of its impact on the coating process, quality and cost. Dry film thickness measurements can be used to evaluate a coating’s expected life, the product’s appearance and performance, and ensure compliance with a host of International Standards.

 

Destructive Coating Thickness Measurement
 

How to measure Dry Film Thickness

 

Dry film thickness (DFT) can be measured using two methods: destructive thickness measurement, where the coating is cut to the substrate using a cutter; and non-destructive coating thickness measurement, using techniques which do not damage the coating or the substrate such as magnetic, magnetic induction and eddy current thickness measurement methods.

 
Non-destructive Coating Thickness Measurement


The non-destructive coating thickness measurements can be taken on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium. Digital coating thickness gauges are ideal to measure coating thickness on metallic substrates. Electromagnetic induction is used for non-magnetic coatings on ferrous substrates such as steel, whilst the eddy current principle is used for non-conductive coatings on non-ferrous metal substrates.

 

 

Coating Thickness Gauges

 

Elcometer offers a wide range of coating thickness gauges to measure dry film thickness.

 

The Elcometer range of destructive coating thickness gauges includes the Elcometer 121/4 Paint Inspection Gauge. The paint inspection gauge, commonly known as the P.I.G, is a popular method for determining dry film thicknesses on non-metallic substrates.

 

The Elcometer range of non-destructive coating thickness gauges includes mechanical and digital coating thickness gauges, suitable for dry film testing, complete with a wide range of probes and calibration foils to suit your application.

 

Mechanical Coating Thickness Gauges

 

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 coating thickness gauge Elcometer 101 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.

 

Digital Coating Thickness Gauges

 

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. 

 

Elcometer offer a range of digital coating thickness gauges from the next generation Elcometer 456 - available with either integral or separate probes, the Elcometer 415 Industrial Paint & Powder Thickness Gauge, to the Elcometer 311 Automotive Paint Meter.

 

Introducing the Elcometer 456 Coating Thickness Gauge

 

Experience key attributes and benefits of the Elcometer 456 Coating Thickness Gauge. This video features the Elcometer 456 integral and separate models performing non-destructive coating thickness measurements on a variety of applications.

 

 

Introducing the Elcometer 415 Industrial Paint & Powder Thickness Gauge

 

Simple to use, with no complicated instructions - the new Elcometer 415 makes coating thickness measurements on flat or curved, smooth or thin, ferrous or non-ferrous substrates really easy. The Elcometer 415 is ideal for production line testing or simple quality checks out in the field.

 

 

Introducing the Elcometer 311 Automotive Paint Meter

 

Experience key attributes and benefits of the Elcometer 311 Automotive Paint Meter. This video features the Elcometer 311 performing non-destructive paint thickness measurements on a steel and aluminium car body panels. Detecting hidden automotive rework is now easier and faster than ever before.

 

 

 

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.

 

Electromagnetic Induction Coating Thickness Gauges

 

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 that 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.