Elcometer has, for the past 60 years, been a world leader in the field of coating inspection - and coating thickness in particular. Elcometer have been manufacturing coating thickness gages for over 60 years and have one of the widest range of paint thickness gauges on the market today.

This information page provides you with just some of the information that you will need to be aware of in choosing a coating thickness gauge.

Please select the section you wish to know more about, from the list below:

Click on the link for the product range that best suits your requirement click onto our website www.elcometer.com

Alternatively:

Click here to see the full range of elcometer coating thickness gauges.

Click here for the full range of destuctive film thickness gauges.

Or here for the range of mechanical dry film thickness gages

Why measure the coating thickness?

To answer this question you need to ask another one - Why are you painting at all? Invariably the answer is one of the following:

What ever the answer is, coating a product is expensive - especially if you get it wrong! Too much paint can cause the coating to crack as it dries, too little paint may not be sufficient to sufficiently cover the product and it will have to be re-coated.

Either way, profits will be affected - not to mention the performance of the coating - that is why paint thickness measurement is perhaps the most important measurement that is undertaken during the coatings process. It is now more than 60 years since the first Elcometer gauge for coating measurement was patented and introduced to the market. We therefore believe that our experience gives us a unique position to advise today on the choice of a suitable portable coating thickness gauge and that is why Elcometer has perhaps the widest range of coating thickness gauges in the world.

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Types of paint thickness gauges

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What does "accuracy" mean?

A key decision on the overall selection of a suitable coating thickness gauge is how accurate do the readings need to be?

There is a progression from moderately accurate to very accurate gauges, which is mirrored by the price of the gauge. 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.

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. Elcometer quote accuracy for gauges in the form:

± X% of reading or ±Y µm, whichever is the greater.

This means that a gauge with a specification of ± 1% or ±2.5 µm, whichever is the greater will give readings within 2.5 µm of the actual value from zero to 50 µm and above 50 µm the reading will be within 1% of the actual value.

Testing 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.

Misleading Accuracy Statements.

The topic of coating thickness gauge accuracy is a minefield of misleading, inappropriate and unclear statements.

In sales literature, the commendable objective of keeping things simple can result in accuracy statements that are not clear and understandable.

For example, there are gauges on the market whose accuracy performance is expressed in terms of fixed and variable tolerance. In this case these two parameters have to be added together to obtain the gauge's accuracy performance.

It is too easy for the unsuspecting reader to assume that as the variable tolerance is stated as 1% that the reading will be within 1% of the true value. In practise the variable tolerance of 1% (1 µm at 100 µm) has to be added to the fixed tolerance (2 µm over the range 50 to 1,500 µm) making at total of 3 µm or 3% at 100 µm.

Other examples of accuracy statements that can all too easily be misunderstood include:

• Accuracy is X% of full scale deflection
• Accuracy is typically Y%

In the first case this is a style of statement that goes back to the days of analogue meters that carried the scale for the gauge. An example would be a 0 to 500 µm range gauge with an accuracy of ±1% of full-scale deflection i.e. ± 5 µm across the range. At 100 µm the accuracy, in Elcometer terms would be ± 5% of reading!

In the second case the word "typically" is being used to mask the fact that not all gauges of this type achieve Y% or that Y% does not apply at all points on the full range of the gauge.

The strong message is, ask your supplier what his accuracy statement means and ask him to demonstrate it to you using measured thickness standards and where possible a sample of the substrate material for which the gauge is required.

The applications for which coating thickness gauges can be used are many and various. Here we outline some of the particular applications and their special requirements and also identifies some of the areas that create difficulties.

In general, applications fit into three main areas of activity.

1. The application of anti-corrosion coatings to structural steel is, in most cases, made over blast cleaned and profiled surfaces. The coatings are applied to protect the structure from the environment and in some cases to provide an aesthetically pleasing finish.
2. Paints are applied to automotive bodies to both protect and finish a complex engineered product. Both steel and aluminium are in common use but the substrates are smooth and thin relative to steel structures.
3. "Engineering " coatings that are applied to a variety of items for protection from wear, protection from corrosion, to provide special surface conditions and many other reasons. Both ferrous and non-ferrous substrates are coated and the coatings include plated metals and anodising.

Although coating thickness is an important parameter in all the above areas the requirements for and the use of a coating thickness gauge varies considerably.

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Coating Applications

Coatings Applied to Structural Steel

In the structural steel sector, gauges for ferrous substrates predominate and the rough surface calibration techniques are used.

Zinc and aluminium are also used as anti-corrosion coatings with application techniques such as thermal spray zinc and aluminium and hot dipping and electroplating for zinc. If the coating of these non-ferrous metals on the steel is sufficiently thick, > 50 µm, then subsequent paint layers can be measured using an eddy-current gauge calibrated over the zinc or aluminium coating.

The magnetic and eddy current coating thickness gauges are capable of measuring most commonly used anti-corrosion coating formulations but care must be taken with paints containing micacious iron oxide (MIO) pigments. Some, but not all, sources of MIO are magnetic so that when these are added to a paint formulation the resulting film can have magnetic properties depending on the distribution of the pigment in the film.

This magnetic property affects the readings of a magnetic coating thickness gauge as the slightly magnetic coating appears like a substrate to the gauge making readings significantly lower that the real film thickness. As MIO pigments are often used in coatings that are soft when applied, the thickness testing of such coatings has unique difficulties. When measuring paints containing MIO it is important to check that the MIO is not influencing the readings by cross checking a test panel coated with the material using a destructive coating thickness test method such as the Paint Inspection Gauge (P.I.G.) or the Säberg Drill.

If the MIO is from one of the many non-magnetic sources then there will be good correlation between the non-destructive and the destructive gauge readings. Regulations now make the use of thick fireproof coatings increasingly common on the steel used in buildings. Coating thickness gauges with 5 mm and 13 mm range capability are required.

Coatings Applied to Automobiles

In automotive manufacture and refinishing after damage, the highly specialised coatings are applied to smooth surfaces, often with thin substrate. Wet solvent-based sprayed paint applications have been most common. In recent years, development of high solids paints and powder paint techniques have altered the thickness measurement requirements. Like most of the automotive engineering manufacturing systems, statistical process control methods have been used to monitor processes.

The application of coatings can not be treated as a typical engineering process as the degree of control is at least one order of magnitude worse than that which would apply to machining processes.

Gauge R & R, provides information on the variance that applies to measurement systems as they are used for coatings in the automotive industry.

Engineering Coatings.

In this application area there are many considerations that will affect the measurement of coatings. Often the coatings are applied to small components, the coating thickness may be more critical to achieve satisfactory performance and is usually thin, they are often more expensive as materials due too their special properties and the component may be made of a special alloy.

All these considerations have effects on the performance of a coating thickness gauge and the adjustment of the gauge to the conditions of the measurement becomes more critical. In the case of metal coatings, Elcometer gauges can measure non-magnetic coatings on magnetic substrates. Materials such as tin, zinc, chrome, aluminium, lead etc can be measured.

The most notable exceptions are nickel and cobalt as these two metals have magnetic properties. Electro-less nickel, also known as autocatalytic nickel, has only a small magnetic effect which is further reduced by the high phosphorous content of the high wear resistance formulations. Electromagnetic induction gauges have been used successfully to measure electro-less nickel but the accuracy of measurement using a "1 - 3%" instrument is reduced to ± 10% and there are noticeable differences between the readings before and after heat treatment.

Electro-deposited nickel can also be measured but the magnetic properties do vary over the life of the plating bath so the traceability of the reading back to a master standard is poor. These gauges are used to monitor the plating thickness on the shop floor whilst the final bath control is carried out using other methods in the quality control laboratory.

The measurement of metals on non-ferrous metals is not easily achieved using eddy-current gauges unless there is a 3:1 ratio in the conductivity of the coating, relative to the substrate. The calibration of a gauge to measure a metal coating on a non-ferrous metal substrate must be carried out using samples of the materials with known thickness. Such gauges do not use the normal calibration characteristics and would have to be specially made for the specific coating and substrate.

Measuring Thickness of Non-metal Structures.

It is possible to measure materials, such as glass-reinforced plastic (GRP), sheet rubber or leather, by considering them as coatings over a metal base plate. In many cases these materials will be relatively thick compared to paint films and may therefore have a thickness in the order of several mm.

The higher range coating thickness gauges are therefore most appropriate i.e. those with ranges of 5mm or even 13mm. A suitable metal base or zero plate will be required and access to both sides of the material is essential. If the material to be measured is curved then the metal plate should be of a form that does not leave a gap, as this gap will be measured as additional material thickness.

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Measurement on Rough Surfaces

Shot or Grit Blasted Surfaces

In the field of anti-corrosion coatings applied to steel structures, it is common for the steel to be prepared for coating by blast cleaning. Angular grit and rounded shot are the most commonly used media but there are developments in the area of blast cleaning which are introducing new media such as abrasive loaded sponge and even hydroblasting using high pressure water jets. Each of these blasting methods leaves a characteristic profiled surface.

This profile has an effect on the accuracy of a coating thickness gauge. If a gauge is adjusted on a smooth surface and then used on a blasted surface significant reading errors as high as 40 - 50 µm (1.6 - 2.0 mil/thou) on a very coarse grit blast will be seen. The simplest way to account for this effect is to adjust the gauge to read correctly on a smooth surface and then correct the reading by subtracting a value which is dependant on the profile.

This effect imposes a limit on the measurement capability of magnetic and electromagnetic gauges such that coatings with a thickness at or less than the peak-to-valley height of the profile can not be determined using direct measurements. Typically this means that coatings with a thickness of 50 µm or less can not be measured on a profiled surface.

A technique recommended for these circumstances is to use a flat, smooth test panel placed over the blasted surface and coated during the application. This coating can then be measured with a coating thickness gauge when it is cured and the area covered by the test plate can be coated when the plate is removed. This method has the benefit of providing a permanent record of the coating process in that area of the job.

Smooth Surface Calibration Adjustment

The simplest and most consistent way to take measurements on a blasted or otherwise roughened surface is to adjust the gauge on a smooth surface and to correct the reading for the effect of the roughened surface.

The CEN method for measuring coating thickness on steel structures, currently at the draft stage but expected to be published in the year 2000, gives a default correction value of 25 µm when the profile is unknown and values between 10 and 40 µm for fine and very coarse profiles as defined by ISO 8503. This is applicable to all coating thickness gauges from all manufacturers but does not take account of the different probe designs.

Experiments carried out at Elcometer show that different probe designs have different characteristics on roughened surfaces and that the effect is thickness and profile dependant. Rough Surface (or Two Point) Calibration Adjustment An alternative method is described in an annex to the CEN document where the gauge is adjusted over a sample profile surface that is typical of the profile for the work in hand.

By using a thin foil value, below the expected coating thickness, to adjust the lower adjustment point and a thicker foil, above the expected coating thickness, to adjust the higher adjustment point, a gauge is set to read the thickness of the coating above the peaks of the profile.

This technique has been shown to have good agreement with the smooth surface - correction value method but it is more difficult to adjust the gauge correctly and not all instruments have the capability to be adjusted in this way. It has been shown that a statistical approach to the two-point adjustment technique is also beneficial as the gauge can be adjusted at a point that is typical of the whole surface.

Again not all gauges have this feature and mechanical gauges and the simpler electronic gauges do not have any form of reading memory. The CEN method has the rough surface adjustment technique described above included as an annex to comply with existing national standards, most notable the Swedish standard for the measurement of coatings on structural steel.

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Gauge R&R

Introduction

A Gauge Repeatability and Reproducibility (Gauge R & R) Study evaluates how a measuring system varies due to repeatability and reproducibility errors.

The assessment method used is based on statistical techniques and the study includes the effects of operators, the environment, the method of measurement as well as gauge variations when measuring items from the process in question.

There are different methods for undertaking a study depending on the depth of understanding of the measurement performance required. For example a range study provides a quick approximation of measurement variability, whereas the average and range method will determine both repeatability and reproducibility for a measurement system.

The ANOVA Method

The ANOVA method requires the data to be analysed using a computer but the resulting information not only shows the source of errors but allows their interaction to be evaluated.

Range Method

The range method uses two operators and five samples for the study. Each operator measures the coating thickness on the sample once and records the readings.

The range for each part is the absolute difference between the reading obtained by operator A and the reading obtained by operator B.

The sum of the ranges is determined and the average range is calculated. The total measurement variability is found by multiplying the average range by 4.33.

The percentage of the process variation (tolerance) that is due to measurement variation can be calculated by dividing the total measurement variability by the tolerance range and multiplying by 100.

The guideline for interpreting the gauge R &R result is that if the process variation is greater than or equal to 30% then there is need for improvement in the measurement system. This improvement may be operator training, gauge re-calibration or gauge design improvement and this method does not provide an indication of what is causing the variations.

Average and Range Method

This method allows the overall repeatability and reproducibility to be analysed as two separate components, repeatability and reproducibility but not their interaction. This in turn will help with the determination of the cause and can lead to process improvement.

For example, if repeatability is large compared to reproducibility, the reasons may be:

• The gauge needs servicing or repair.
• The gauge set-up should be redesigned to be more rigid.
• The clamp or location for the probe needs to be improved.
• There is excessive within-part variation.

If reproducibility is large compared to repeatability, then possible causes could be:

• The operators need to be better trained.
• The resolution of the gauge is not sufficient.

Help is required to allow the operator to use the gauge more consistently.

The method uses three operators and 10 parts. The parts are measured in random order by one of the operators then the other two measure the parts in the same order but without seeing the original results. It is good practise to have a fourth person record the results. The order of the parts is then changed and the measurements repeated. The data is then plotted to show the errors.

The errors are calculated as either:

• Error = Master Measurement - Observed Measurement, or
• Error = Average Measurement - Observed Measurement

The following terms are used in Gauge R & R studies:

• EV is the repeatability or equipment variation.
• AV is the reproducibility or operator variation.
• R & R is the measurement system variation.
• PV is the part variation.

The above are often quoted as a percentage of the total variation (TV) but it must be noted that due to the interaction between these values they will not total 100% when added together.

ANOVA Method

Analysis of Variance (ANOVA) is a standard statistical technique and is used to assess the error in a measurement system.

The overall variation can be separated into four areas, parts, operators, interaction between parts and operators and replication error due to the gauge.

The advantages of this method over other Gauge R & R techniques are that it can handle any experimental set-up, it can estimate variance more accurately and extract more information from the data.

The disadvantages are that, as stated earlier a computer is required to perform the calculations and a degree of knowledge and experience is required to interpret the results.

Data is collected randomly to prevent bias.

Interpretation Guidelines

The guidelines for acceptance of Gauge Repeatability and Reproducibility (% R & R) are:

• Under 10% error - The measurement system is acceptable.
• 10% - 30% error - May be acceptable depending on factors like the importance of the application, the cost of the gauge, the cost of repair to the measurement system etc.
• Over 30% error - The measurement system needs improvements.

For further information and additional background reading on the subject of Gauge R & R Studies, the reference manual Measurement Systems Analysis (MSA), jointly compiled and copyright Ford/General Motors/Chrysler, is recommended.

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Thickness Standards and Traceability

Types of Thickness Standard

There are two basic types of coating thickness standard, foils and pre-coated metal. The following sections describe the two types in more detail and discuss their relative merits and disadvantages so that you can make a choice appropriate to the condition under which you will be working.

Foil Thickness Standards

The lowest cost and most convenient thickness standards are made from plastic foil material and have been supplied over the years as measured and unmeasured foils or shims. The current range of thickness extends from 12.5 µm (0.5 mil/thou) to 20 mm (787.4 mil/thou 0.787 inch).

The unmeasured foils are 25 x 50 mm strips of non-compressible plastic in the range 25 to 500 µm.

Measured foils use the same non-compressible plastic that is designed for use as gasket material but these are measured using a linear displacement transducer (LVDT) system so that the measurement is independent of the coating thickness gauges for which they provide the thickness standard. In this lies one of the major advantages of the foil thickness standards.

The foils can easily be measured from either side and, therefore, the transfer standards used to calibrate the LVDT can themselves be calibrated and certified to National Standards. Stainless steel slip gauges are used and these are sent annually to a UKAS (Formally NAMAS) Measurement Laboratory for independent certification.

The LVDT system is also independently calibrated and certified by a UKAS Laboratory on an annual basis. The slip gauges are used regularly through the working day to ensure that the system stays within calibration and the system is located in a designated area with restricted access to further maintain its integrity.

A further service is offered where a set of foils is supplied with a calibration certificate. Up to 8 foils can be listed on an individual certificate where they are identified with a serial number that traces the operator who performed the measurement and the date it was carried out.

The certificate also gives details of the certificates that apply to the measuring system and the slip gauges used to calibrate it. The foil measurement and the associated calibration techniques are recorded within the ISO 9002 Quality Management procedures and therefore fall within the scope of Elcometer's ISO 9002 accreditation.

The Correct Use of Foils.

The point at which thickness measurements are taken on a foil is at the intersection of two imaginary lines drawn from corner to corner across the foil. In practise, three measurements are taken at this point and these measurements must be within 1 µm of each other for the foil to be acceptable as a calibration standard.

The final reading is recorded and printed on the label in both metric and imperial (English) units. When using the foil it should be placed flat on the surface and the coating thickness gauge probe placed at the point on the foil where the two imaginary lines intersect.

Care must be taken to ensure that no dirt or dust is trapped under the foil between the foil and the substrate, as this will affect the reading taken. Foils must be considered as consumable items and replaced regularly.

The probe is brought into contact each time the foil is used and wear will occur with use particularly in the case of the thinner values, 12.5, 25 and 50 µm (0.5, 1.0 and 2.0 mil/thou) and where blast cleaned, roughened surfaces are used.

The thinner foils can be used on curved surfaces, both concave and convex. Foils of 250 µm (10 mil/thou) and above are rigid and are not capable of adopting the shape of a curved surface.

Typical Foil Thickness Standard Pre-Coated Thickness Standards.

The original coated thickness standards available from National Standards bodies were the, so-called, NBS standards. These originated with the National Bureau of Standards in the USA. This body has now changed its name to NIST. (The National Institute of Standards and Testing.)

Although NIST has been operating for some years these coated standards are often referred to as NBS Standards. The standards were made using steel plates plated with copper to build the thickness required and finished with a layer of chrome plating to provide a hard wearing surface. As these films are adhered to the substrate, a non-destructive coating thickness method is used to measure the final coating thickness.

The technique chosen is beta-backscatter where the coating is illuminated with beta radiation from a calibrated source and the absorbed energy is a measure of the coating thickness. The back-scattered energy is detected using a Geiger counter and the system is calibrated using coatings of known thickness.

The fundamental accuracy of the count is ± 5% so the accuracy of measurement on NIST standards is no better than this limitation. When these standards were used exclusively for the mechanical pull-off gauges, originally designed to a ± 10% accuracy specification, ± 5% accuracy was sufficient.

These days even pull-off type gauges have ± 5% accuracy specifications so the NIST standards are no longer sufficient for checking these gauges. Electronic gauges are capable of much better accuracy and NIST standards are not relevant to these gauges.

n recent years, pre-coated standards, using hard wearing epoxy coatings on both steel and aluminium plates, have become available. With these standards the substrate is carefully prepared and the individual plates are serialised and characterised for thickness using a linear displacement transducer system. The coating is then applied, lapped and polished back to the required thickness before the total thickness of plate and coating is re-measured.

The coating thickness is the difference between the total thickness and the thickness of the plate alone. The accuracy of this process is typically ± 2% but with care and over restricted thickness ranges a ± 1% accuracy, traceable to National Standards using the LVDT slip gauges previously described, can be obtained.

Sets of pre-coated epoxy coating thickness standards are supplied in binders with a zero plate and two or three thickness values together with a calibration certificate. The certificate can be renewed by re-measuring the total thickness with reference to the original substrate data traceable through the serial number of the set.

Advantages and Disadvantages of Foils and Pre-Coated Standards.

As with many engineering solutions there are advantages and disadvantages with both types of the thickness standards in current use. These are listed in the following table.