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ORGANIC FINISHING: CASE STUDY Calibration Terms for Thickness Gauges Learning the language of coating measurement is critical. By David Beamish, General Manager, DeFelsko Corp., Ogdensburg, N.Y. T he following article provides definitions, expla- nations, limitations and practical examples of metrology terminology as it relates to DeFelsko coating thickness measurement gauges. While many of these terms and definitions also apply to other instruments, consult the manufacturer for details spe- cific to your individual instrument. TYPE 1: PULL-OFF GAUGES In Type 1 pull-off gauges a permanent magnet is brought into direct contact with the coated surface. The force necessary to pull the magnet from the sur- face is measured and interpreted as the coating thick- ness value on a scale or display on the gauge. The magnetic force holding the magnet to the surface varies inversely as a non-linear function of the dis- tance between magnet and steel, i.e., the thickness of the dry coating. Less force is required to remove the magnet from a thick coating (Figure 1). TYPE 2: ELECTRONIC GAUGES A Type 2 electronic gauge uses electronic circuitry to convert a reference signal into coating thickness (Figure 2). • Electronic ferrous gauges operate on one of two dif- ferent magnetic principles. (Some ferrous gauges use the principle of magnetic induction.) When a permanent magnet is brought near steel, the mag- netic flux density at the pole face of the magnet increases. Coating thickness is determined by meas- uring this change in flux density, which varies inversely to the distance between the magnet and the steel substrate. Hall elements and magnet resistance elements positioned at the pole face are the most common ways this change in magnetic flux density is measured. • Other ferrous electronic gauges operate on the princi- ple of electromagnetic induction. A coil containing a soft iron rod is energized with an AC current, thereby producing a changing magnetic field at the probe. As with a permanent magnet, the magnetic flux density within the rod increases when the probe is brought near the steel substrate. A second coil detects this change. The output of the second coil is related to the coating thickness. Many gauges also need tempera- ture compensation due to the temperature depend- ence of the coil parameters, although some gauges utilize a balanced secondary coil configuration to help improve temperature independence. • Non-ferrous electronic gauges also operate on a principle of electromagnetic induction. A coil con- ducting an alternating current sets up an alternat- ing magnetic field at the surface of the probe. As the probe is brought near a conductive surface, the mag- netic field creates eddy currents on the surface. A second coil, the output of which is related to the coating thickness, detects these eddy currents. Some gauges utilize only one coil to both set up the mag- netic field and to detect the opposing magnetic field produced by the eddy currents. REFERENCE STANDARDS A reference standard is a sample of known thickness against which users may verify the accuracy of their gauge. Reference standards are typically coating thickness standards or shims. They may or may not be traceable to a national or international registry. If agreed to by the con- tracting parties, a sample part of known (or accept- able) thickness may be used as a reference stan- dard for a particular job. COATING THICKNESS STANDARDS For most instruments, a coating thickness stan- dard is typically a smooth, metallic sub- 42 www.metalfinishing.com Figure 1: Pull off gauge. Figure 2: Electronic gauges.
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Page 1: ORGANIC FINISHING: CASE STUDY Calibration Terms · PDF fileORGANIC FINISHING: CASE STUDY Calibration Terms for Thickness Gauges ... and ultrasonic gauges ... metal, or other material

ORGANIC FINISHING: CASE STUDY

Calibration Terms for Thickness GaugesLearning the language of coating measurement is critical.By David Beamish, General Manager, DeFelsko Corp., Ogdensburg, N.Y.

The following article provides definitions, expla-nations, limitations and practical examples ofmetrology terminology as it relates to DeFelsko

coating thickness measurement gauges. While manyof these terms and definitions also apply to otherinstruments, consult the manufacturer for details spe-cific to your individual instrument.

TYPE 1: PULL-OFF GAUGESIn Type 1 pull-off gauges a permanent magnet isbrought into direct contact with the coated surface.The force necessary to pull the magnet from the sur-face is measured and interpreted as the coating thick-ness value on a scale or display on the gauge. Themagnetic force holding the magnet to the surfacevaries inversely as a non-linear function of the dis-tance between magnet and steel, i.e., the thickness ofthe dry coating. Less force is required to remove themagnet from a thick coating (Figure 1).

TYPE 2: ELECTRONIC GAUGESA Type 2 electronic gauge uses electronic circuitry toconvert a reference signal into coating thickness(Figure 2).• Electronic ferrous gauges operate on one of two dif-

ferent magnetic principles. (Some ferrous gaugesuse the principle of magnetic induction.) When apermanent magnet is brought near steel, the mag-netic flux density at the pole face of the magnetincreases. Coating thickness is determined by meas-uring this change in flux density, which variesinversely to the distance between the magnet andthe steel substrate. Hall elements and magnetresistance elements positioned at the pole face are

the most common ways this change in magnetic fluxdensity is measured.

• Other ferrous electronic gauges operate on the princi-ple of electromagnetic induction. A coil containing asoft iron rod is energized with an AC current, therebyproducing a changing magnetic field at the probe. Aswith a permanent magnet, the magnetic flux densitywithin the rod increases when the probe is broughtnear the steel substrate. A second coil detects thischange. The output of the second coil is related to thecoating thickness. Many gauges also need tempera-ture compensation due to the temperature depend-ence of the coil parameters, although some gaugesutilize a balanced secondary coil configuration to helpimprove temperature independence.

• Non-ferrous electronic gauges also operate on aprinciple of electromagnetic induction. A coil con-ducting an alternating current sets up an alternat-ing magnetic field at the surface of the probe. As theprobe is brought near a conductive surface, the mag-netic field creates eddy currents on the surface. Asecond coil, the output of which is related to thecoating thickness, detects these eddy currents. Somegauges utilize only one coil to both set up the mag-netic field and to detect the opposing magnetic fieldproduced by the eddy currents.

REFERENCE STANDARDSA reference standard is a sample of known thicknessagainst which users may verify the accuracy of theirgauge. Reference standards are typically coatingthickness standards or shims. They may or may notbe traceable to a nationalor international registry.If agreed to by the con-tracting parties, a samplepart of known (or accept-able) thickness may beused as a reference stan-dard for a particular job.

COATING THICKNESSSTANDARDSFor most instruments, acoating thickness stan-dard is typically asmooth, metallic sub-

42 www.metalfinishing.com

Figure 1: Pull off gauge.Figure 2: Electronic gauges.

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strate with a nonmagnetic(epoxy) coating of knownthickness that is traceableto national standards(NIST). The substrate isferrous (steel) for magneticgauges or non-ferrous (alu-minum) for eddy currentand ultrasonic gauges High-accuracy coatingthickness standards areused to calibrate gauges aspart of the manufacturingprocess. The same standards are available for pur-chase by customers to be used as calibration stan-dards in a calibration lab or as check standards in thefield or on the factory floor.

Coating thickness standards to be used with ultra-sonic gauges can also be solid plastic (polystyrene)blocks that have been machined to a flat smooth sur-face. In addition to a known thickness traceable tonational standards, these standards also have a knownsound velocity.

Calibration Standards are purchased as acces-sories to help meet the increasing customer need tofulfill ISO/QS-9000 and in-house Quality Controlrequirements. Many customers find it more practicalto verify accuracy of their own gauges in-house,rather than utilize vendor services. To assist thesecustomers, sets of calibration standards are avail-able with nominal values selected to cover the rangeof each gauge. Standards usually come with aCalibration Certificate showing traceability to NIST.In addition, some vendors make calibration proce-dures available to their customers.

SHIMSA shim is a thin strip of non-magnetic plastic, metal,or other material of known uniform thickness used toverify the operation and make adjustments to dry filmthickness gauges (Figure 3). While a plastic shim isable to take the form of most substrates to be meas-ured, the accuracy of the shim is more limited thancoating thickness standards. Therefore, when using ashim to make adjustments with Type 2 (electronic)gauges, it is important to combine the tolerance of theshim with the tolerance of the gauge before determin-ing the accuracy of measurements.

Plastic shims are often used to adjust a gauge in theintended range of use over the surface of the repre-sentative substrate material.

We do not recommend shims be used with Type 1(mechanical pull-off) gauges. Shims are usually fairlyrigid and curved and do not lie perfectly flat, even on a

smooth steel test surface. Near the pull-off point of themeasurement with a mechanical gauge, the shim fre-quently springs back from the steel surface, raising themagnet too soon and causing an erroneous reading.

THREE STEPS TO ENSURE BEST ACCURACYWhen using a coating thickness gauge, there are

three basic procedures to ensure measurement accu-racy: calibration, verification, and adjustment.Following is a breakdown in detail:

CalibrationCalibration is the high-level, controlled, and documentedprocess of measuring traceable calibration standardsover the full operating range of the gauge and verifyingthat the results are within the stated accuracy of thegauge. If necessary, gauge adjustments are made to cor-rect any out-of-tolerance conditions.

Calibrations are typically performed by the gaugemanufacturer or by a qualified laboratory in a con-trolled environment using a documented process. Thecoating thickness standards used in the calibrationmust be such that the combined uncertainties of theresultant measurement are less than the stated accu-racy of the gauge. Typically, a 4:1 ratio between theaccuracy of the standard and the accuracy of thegauge is sufficient. The outcome of the calibrationprocess is to restore/realign the gauge to meet/exceedthe manufacturer’s stated accuracy.

Calibration Interval: A calibration interval is theestablished period between recalibrations (recertifica-tions) of an instrument. As per the requirements ofISO 17025, some vendors do not include calibrationintervals as part of Calibration Certificates issuedwith their coating thickness gauges.

For customers seeking assistance in developingtheir own calibration intervals, we share the followingexperience: Non-age related factors have shown to bemore critical in determining calibration intervals.These factors are primarily the frequency of use, theapplication in question, as well as the level of caretaken during use, handling and storage. For example,a customer that uses the gauge frequently, measureson abrasive surfaces, or uses the gauge roughly (i.e.drops the gauge, fails to replace the cover on the probetip for storage, or routinely tosses the gauge into a toolbox for storage) may require a relatively shorter cali-bration interval.

Note: We recommend that customers establishgauge calibration intervals based upon their ownexperience and work environment. Customer feed-back suggests one year as a typical starting point.Furthermore, our experience suggests that customerspurchasing a new instrument can safely utilize the

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January 2007 43

Figure 3: Shims.

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44 www.metalfinishing.com

instrument purchase date as the beginning of theirfirst calibration interval. The limited effect of age min-imizes the importance of the actual calibration certifi-cate date.

Calibration Certificate: A Calibration Certificateis a document that records actual measurementresults and all other relevant information to a suc-cessful instrument calibration. CalibrationCertificates clearly showing the traceability to anational standard are included with most new recali-brated or repaired instruments. Note: Some manufac-turers charge extra for this document.

Traceability: Traceability is the ability to followthe result of a measurement through an unbrokenchain of comparisons, all the way back to a fixed inter-national or national standard that is commonlyaccepted as correct. The chain typically consists ofseveral appropriate measurement standards, thevalue of each having greater accuracy and less uncer-tainty than its subsequent standards.

Recalibration (Recertification): Recalibration,also referred to as recertification, is the process of per-forming a calibration on a used instrument.Recalibrations are periodically required throughoutthe life cycle of an instrument since some probe sur-faces are subject to wear that may affect the linearityof measurements.

Verification of AccuracyVerification is an accuracy check performed by theinstrument user on known reference standards priorto gauge use for the purpose of determining the abil-ity of the coating thickness gauge to produce reliablevalues compared to the combined gauge manufac-turer’s stated accuracy and the stated accuracy ofthe reference standards. The process is intended toverify that the gauge is still functioning as expected.

Verification is typically performed to guardagainst measuring with an inaccurate gauge at thestart or end of a shift, before taking critical meas-urements, when an instrument has been dropped ordamaged— or whenever erroneous readings are sus-pected. If deemed appropriate by the contractingparties, initial agreement can be reached on thedetails and frequency of verifying gauge accuracy.

If readings do not agree with the reference standard,all measurements made since the last accuracy checkare suspect. In the event of physical damage, wear, highusage, or after an established calibration interval, thegauge should be removed from service and returned tothe manufacturer for repair or calibration.

The use of a checking measurement standard isnot intended as a substitute for regular calibration

and confirmation of the instrument, but its use mayprevent the use of an instrument, which, within theinterval between two formal confirmations, ceases toconform to specification.

Adjustment (Optimization, Calibration Adjustment)Adjustment is the physical act of aligning a gauge’sthickness readings (removal of bias) to match those ofa known sample in order to improve the accuracy ofthe gauge on a specific surface or within a specific por-tion of its measurement range.

In most instances it should only be necessary tocheck zero on an uncoated substrate and begin meas-uring. However, the effects of properties such as sub-strate (composition, magnetic properties, shape,roughness, edge effects) and coating (composition,mass, surface roughness), as well as ambient and sur-face temperatures, may require adjustments to bemade to the instrument.

Most Type 2 gauges can be adjusted on known ref-erence standards, such as coated parts or shims.However, Type 1 “pull-off ” gauges have nonlinearscales. Since their adjustment features are linear noadjustments should be made. Instead, the user shouldtake a base metal reading (BMR) and subtract thatvalue from the coating thickness reading.

With a Type 2 gauge ,where an adjustment methodhas not been specified, a 1-pt adjustment is typicallymade first. If inaccuracies are encountered then a 2-PtAdjustment should be made.

1-Pt Adjustment (Offset, Correction Value): 1-ptadjustments involve fixing the instrument’s calibra-tion curve at one point after taking several readingson a known sample or reference standard. If required,a shim can be placed over the bare substrate to estab-lish such a thickness. This adjustment point can beanywhere within the instrument’s measurementrange, though for best results should be selected nearthe expected thickness to be measured.

Zeroing is the simplest form of 1-pt adjustment. Itinvolves the measurement of an uncoated sample orplate. In a simple zero adjustment, a single measure-ment is taken and then the reading is adjusted to readzero. In an average zero adjustment, multiple meas-urements are taken, then the gauge calculates anaverage reading and automatically adjusts that valueto zero.

2-Pt Adjustment: This method is preferred for veryunusual substrate materials, shapes, or conditions. Itprovides greater accuracy within a limited, definedrange. Two-pt adjustments are similar to 1-pt exceptthe instrument’s calibration curve is fixed at twoknown points after taking several readings on known

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ORGANIC FINISHING: CASE STUDY

samples or reference standards. The two thicknessesmust be within the instrument’s measurement range.Typically, points are selected on either side of theexpected coating thickness (Figure 4).

BASE METAL READINGThe base metal reading (BMR) is the measured effectof substrate roughness on a coating thickness gaugethat is caused by the manufacturing process (forexample, castings) or surface profile (roughness)—producing operations (for example, power tool clean-ing, abrasive blast cleaning, etc.). Non-compensationfor the base metal effect can result in an overstate-ment of the true thickness of the coating. The basemetal reading is measured, recorded, and deductedfrom the thickness of each coat in order to correctlystate the thickness of the coating over the surfaceroughness.

A BMR is a zeroing technique typically used withType 1 (mechanical pull-off) gauges on rough surfaces.Adjustments to a Type 1 gauge are linear in nature;however, the scale of the gauge is nonlinear. Therefore,it is important not to adjust the gauge to read zero onthe bare substrate.

The BMR is calculated as a representative value(average) of several measurements taken from sever-al locations across a bare substrate.

REPEATABILITYCoating thickness gauges are necessarily sensitive tovery small irregularities of the coating surface or ofthe steel surface directly below the probe center.Repeated gauge readings on a rough surface, even atpoints very close together, frequently differ consider-ably, particularly for thin films over a rough surfacewith a high profile.

ROUGHNESSIf a steel surface is smooth and even, its surface plane isthe effective magnetic surface. If the steel is roughened,

as by blast cleaning, the “apparent” or effective magnet-ic surface that the gauge senses is an imaginary planelocated between the peaks and valleys of the surfaceprofile (Figure 5). Gauges read thickness above theimaginary magnetic plane. If a Type 1 gauge is used, thecoating thickness above the peaks is obtained by sub-tracting the BMR. With a correctly adjusted Type 2gauge, the reading obtained directly indicates the coat-ing thickness.

REFERENCES1. ASTM 7091-05 “Standard Practice for

Nondestructive Measurement of Dry FilmThickness of Nonmagnetic Coatings Applied toFerrous Metals and Nonmagnetic, NonconductiveCoatings Applied to Nonferrous Metals,” WestConshohocken, Pa., ASTM; 2005.

2. SSPC-PA2, “Measurement of Dry CoatingThickness with Magnetic Gauges,” Pittsburgh, Pa.;2004.

3. ISO 2808:1977, “Paints and varnishes—Determination of film thickness,” Geneva,Switzerland; ISO 2004.

David Beamish is general manager of DeFelsko Corp.,a New York-based manufacturer of hand-held coatingtest instruments. He has a degree in Civil Engineeringand has more than 17 years of experience in the design,manufacture, and marketing of these testing instru-ments in a variety of international industries, includ-ing industrial painting, quality inspection, and manu-facturing. He conducts training seminars and is anactive member of various organizations, includingNACE, SSPC, ASTM, and ISO. For more information,please contact him at [email protected]. mf

Figure 4: Example of 2-point adjustment.

Figure 5:


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