Coating Thickness Gauges
Coating thickness gauges can be used to measure the coating of wet and dry coating. There are a number of different gauges for measuring dry film coating thickness with the most popular one being the Elcometer 456 Coating Thickness Gauge. This is a digital model that measures coatings on Ferrous and Non Ferrous substrates, non destructively. Other options for measuring dry film coating thickness include destructive coating thickness gauges which are used to measure coatings on non metallic substrates such as wood, plaster and plastic. Mechanical coating thickness gauges are more suited to hazardous and underwater applications where you need to measure the coating thickness. Coatings with a wet film comb or a wet film wheel are also part of the coating thickness gauge range.
Below is a comprehensive breakdown of what the different gauges are used for.
Coating Thickness Gauges are also referred to as DFT gauges or paint meters and are used to measure dry film thickness, which is where DFT comes from. Dry film thickness is one of the most important measurements in the coating industry and hence the coating thickness gauge being one of the most important gauges if you are involved in applying a coating to a substrate. A coating thickness gauge complies with a host of International Standards and ensures that you obtain vital information with regards the expected life of the substrate, the products’ fitness for purpose and it’s appearance.
Elcometer offers a wide range of coating thickness gauges to measure dry film thickness which include digital gauges as well as mechanical gauges for application where there is a hazardous application or similar, where a digital gauge is not suitable. To ensure best accuracy, Elcometer offer a wide range of probes as well as various calibration foils to make sure that most applications are covered. There are also destructive gauges for measurement on non metallic substrates.
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 Elcometer 456 – available with either integral or separate probes, the Elcometer 415 Paint and Powder Thickness Gauge, to the Elcometer 311 Automotive Paint Meter.
The Elcometer 456 Coating Thickness Gauge
Introducing the Elcometer 456
The above video clip will give you a brief summary of the various different models for the Elcometer 456 Coating Thickness Gauge, which includes the integral and separate probe models, and showing you the user friendliness of the Elcometer 456, as well as the speed with which it can take readings.
There are a number of questions that need to be asked before you select the correct coating thickness gauge for your application. After answering the following questions you should be able to narrow your choice down to one instrument.
There are a few considerations in choosing the correct Elcometer Coating Thickness Gauge / instrument :
- Is your application on a wet or dry coating? If it is a wet coating this will be covered in a future article.
- Is your coating on a metallic or non metallic substrate? If it is a non metallic substrate such as wood, plaster, concrete, plastic, rubber or similar then I will cover that in a future article.
If your application is a dry film coating thickness on a metal substrate then there are a few considerations in choosing the correct Elcometer 456 Dry Film Thickness Gauge:
1.Are you measuring on Ferrous (Steel), Non Ferrous (Aluminium) or on both?
– This would determine whether you require an F, NF or FNF unit.
2. Do you require the instrument to download to a computer and/or smart devices?
– NO – Basic unit ie Model B will suffice. (Manually write readings down & capture at a later stage)
– YES Choice between:
- Standard unit – download 1,500 readings in 1 batch.
- Top unit – download up to 150,000 readings in up to 2,500 batches.
Easy to use – ElcoMaster software comes free with the Standard and Top models.
3. Do you require an integral ie I or a separate probe ie S model?
– Integral probe is part of the unit and is the more popular model with its one handed operation and its increased robustness
– Separate probe model is better for more difficult to access areas or for smaller areas where you also have the option of Right Angle, Telescopic and Miniature probes.
– The new Elcometer scan probe with standard, auto repeat (140read/min) and scan mode is available with the Top model only.
4. What thickness are you measuring? The Scale 1 measures up to 1,500 microns, the Scale 2 up to 5mm (5,000 microns) and the Scale 3 up to 13mm. There are also probes that measure up to 30mm. The scan probe also has a Scale 2 probe which can measure to approximately 5mm.
Please visit www.elcometer456.com for a comprehensive view of the unit.
Click on the following link to access the various Catalogues : Catalogues
Within the Elcometer range, you have 2 options of mechanical coating thickness gauges that are suitable for working in hazardous high risk areas such as high temperature or flammable atmospheres such as in the Oil & Gas industry, underwater or where the risk of explosion is high and a Digital Gauge / Electronic instrument could trigger this. The mechanical coating thickness gauge provide cost-effective dry film thickness measurement for the above scenario.
The Elcometer 211 Mechanical Coating Thickness Gauge, commonly referred to as the “Banana Gauge”, is a Type I instrument designed to measure dry film thickness.
The Banana Gauge is ideal for use in environments where the use of electronic instruments is difficult, e.g. inflammable atmospheres in oil and gas production, and for underwater* dry film coating thickness inspection.
* Underwater use voids the warranty and users should replace the gauges periodically
This is one of the most popular mechanical coating thickness gauges in the world.
The Elcometer 101 Coating Thickness Gauge is the original non-destructive dry film thickness gauge, the Elcometer 101 was the world’s first portable coating thickness gauge with the original being produced in 1947. Insensitive to hot and cold surfaces – ideal for measuring coating thickness on hot sprayed metal coatings, incorporates reading hold feature.
The Elcometer 121/4 and Elcometer 141 Paint Inspection Gauges or PIG gauges are used for measuring the coating on a non-metallic substrate such as plaster, wood or plastic, or for assessing the thickness of a multi-coat paint. They are quick and easy to use but the disadvantage is that you have to make a cut through the coating to the substrate so it is a destructive test.
How does a Coating Thickness Gauge work?
Dry Film thickness measurement on metal substrates can take place in one of two ways :
Non-magnetic coatings on magnetic Ferrous surface such as steel to which a magnet will stick, using the principle of electromagnetic induction.
The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates to which a magnet will not stick. These would include aluminium and austenitic stainless steel.
Permanent Magnet Coating Thickness Gauges eg the Elcometer 211
The force required to pull a permanent magnet which is mounted on a balanced arm from the surface of the coating is a measure of the thickness of the coating. As the scale wheel is turned, the force which is applied through a helical spring attached to the balanced arm, is increased progressively until the magnet lifts from the surface. There is a pointer on the case of the instrument which translates this force into thickness units which will give you the thickness of the coating.
Electromagnetic Induction Coating Thickness Gauges
The electromagnetic induction principle is used for measuring on magnetic substrate materials. It is also referred to as Ferro Magnetic Induction principle. A three-coil probe system is used with the central coil being powered by the instrument and the other 2 coils, which are either side of the central coil, detecting the resulting magnetic field. An alternating magnetic field is established around the central coil due to the sinusoidal signal that is generated by the instrument.
The magnetic field cuts through the other 2 coils equally when there is no magnetic materials influencing the probe. A net voltage between the 2 coils is produced when the probe is brought closer to the uncoated substrate due to the field becoming unbalanced with more field cutting the nearest coil and less cutting the furthest coil. This voltage differential is a measure of the distance to the substrate ie the coating thickness
Wikipedia extract :
Electromagnetic induction is the production of an electromotive force across a conductor when it is exposed to a time varying magnetic field. It is described mathematically by Faraday’s law of induction, named after Michael Faraday, who is generally credited with the discovery of induction in 1831.
Eddy Current Coating Thickness Gauges
For the eddy current principle an alternating field is generated in the non ferrous metal under the coating, created by a single coil probe which is used with a relatively high frequency signal at several mega-hertz. Eddy currents caused by the field circulate in the substrate and they in turn have associated magnetic fields. The electrical impedance of the coil in the coating thickness probe is influenced by the fields. These changes are dependent on the coating thickness.
Wikipedia extract :
Eddy currents (also called Foucault currents) are circular electric currents induced within conductors by a changing magnetic field in the conductor, due to Faraday’s law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.
How accurate are Coating Thickness Gauges?
The accuracy of a coating thickness gauge can be determined by a number of factors. A key decision on the selection of a suitable coating thickness gauge is how accurate do you need the readings to be? Generally, the accuracy will be related to the cost of the instrument and higher cost units will usually be more accurate. The application process will also affect the accuracy and the knowledge and skill of the coating thickness gauge user will also influence the readings and the accuracy.
What does “accuracy” mean?
Accuracy is the difference between the true coating thickness that should be achieved and the reading. The accuracy with which a gauge takes readings is a critical aspect of a gauge’s performance.
What is repeatability?
Repeatability is used to assess the readings that the same person and the same gauge will be able to achieve measuring the same area.
What is reproducibility?
Reproducibility is used to compare different gauges and ultimately the accuracy of the gauge by comparing different readings by different people with the same gauge. If all the different users achieve similar results then the gauge is reproducible.
How to test a Coating Thickness Gauge for accuracy
The best way to check the accuracy of a coating thickness gauge is to use coating thickness standards that have traceability to National Standards. You then adjust the gauge to zero on an uncoated smooth substrate and use a foil standard at or near the maximum range of the gauge to set the specified thickness ie you program the gauge to measure the correct thicknesses at zero and at the maximum thickness. You then take readings on intermediate thickness standards and compare the readings you obtain to the actual thickness of the standards or foils.
The differences between the actual readings and the value of the standard are then expressed as a percentage which is used as the accuracy of the gauge.
The importance of Coating Thickness Gauge Calibration
A manufacturer during manufacture will calibrate the gauge by setting the gauge to meet the required accuracy specification. The calibration procedure usually involves stetting the coating thickness gauge to known values of thickness throughout the range of the gauge. The values at key points across the full range of the coating thickness gauge are stored as reference points in the memory of the modern electronic instruments.
Why you need to Calibrate a Coating Thickness Gauge before testing
The shape, the type of material and the surface finish of the metal substrate to be tested, all influence the accuracy of the gauge. The magnetic properties of steel alloys for example, vary depending on the make-up of the alloy. Different aluminium alloys and different non ferrous metals such as copper, austenitic stainless steel, brass etc also vary. The linearity of a coating thickness gauge will be affected by the variations. For example, a gauge that is set-up for high carbon steel will read a different value for the same thickness on mild steel. Thin or curved surfaces similarly effect the linearity as do profiled substrates such as blast cleaned steel for structural steel applications.
To maximize the accuracy of the readings and to overcome these effects, most coating thickness gauges have features that allow you to set or calibrate the gauge to the work being carried out.
Adjusting / Calibrating a Coating Thickness Gauge
Adjusting a coating thickness gauge or calibrating it, is the technique whereby you can set-up the gauge for the prevailing conditions for the work at hand. Adjusting the coating thickness gauge to prevailing conditions, greatly reduces and even eliminates the resulting errors. Prevailing conditions include material differences, shape and surface finish, elevated temperature as well as the presence of a stray magnetic field.
The surface roughness or profile which is created by deliberate profiling such as 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
Foils or Shims and pre-coated metal are the 2 basic types of coating thickness standards. For more information on coating thickness standards for coating thickness gauges click here.