Eddy-Current DFT Gauges Instruments based on the eddy-current

Environmental Instrument Test Lab Data (continued)

Chapter 10: Advanced Nondestructive Test Instruments

10.10 Eddy-Current DFT Gauges Instruments based on the eddy-current

prin-ciple are used to measure the Dry Film Thickness (DFT) of non-conductive films applied to conductive substrates such as alu-minum, copper, brass, and stainless steel.

The instrument may look exactly like the electromagnetic gauge, but it induces an eddy current in the substrate using a high frequency alternating current fed to the probe. Many manufacturers refer to eddy-current DFT gauges as N (non-ferrous) gauges (Figure 10.11).

Some instruments operate using both elec-tromagnetic induction and eddy current.

Many manufacturers refer to the equipment used to measure non-conductive coatings on

Range - Wood 1 14% - 30% (% Moisture Content)

Range - Wood 2 15% - 30% (% Moisture Content)

Range - Plaster 8% - 20% (% Moisture Content)

Range - Concrete 5% - 14% (% Moisture Content)

Range – Linear Reference 0 - 10

Instrument Dimensions 43 x 91 x 146 mm

Resolution 1% (Not Linear Scale)

Power 1 x 9V MN1604 PP3 Battery

Carry Case Dimensions 60 x 155 x 165mm

Instrument Weight 230g (0.5lb)

ferrous (F) substrates, using eddy-current principles (N), as either FN or FNF gauges (Ferrous/Non-Ferrous). FNF gauges typi-cally have single probe (either separate or integral). Some gauges, however, use a dif-ferent probe for each principle.

10.10.1 Proper Use

There are a wide variety of electronic gauges available; always follow the manufacturers’

instructions to ensure accurate measure-ments are made. Although eddy-current gauges can be used to take measurements on any non-ferrous metal, the shape and size of the probe, the conductivity and surface fin-ish of the metal substrate are significant.

Electromagnetic probes (F) cannot measure a coating over a non-ferrous (N) substrate.

The eddy-current technique can give a false reading on ferrous substrates.

FNF gauges, such as the Elcometer^†1 456, come with automatic substrate recognition (sometimes referred to as dual probes).

These gauges first check for a magnetic field and, if it is not found, automatically switches to the eddy-current mode. These gauges generally work well; however, some com-pound-type metals may have just enough magnetic properties to make the probe regis-ter the metal as ferrous when in fact it is not.

If readings are suspect on low grade, com-posite stainless steels, or nickel alloys, change the gauge to nonferrous mode to force the meter to measure in eddy-current mode. Note: linearity, and hence accuracy, on intermediate thickness values (those between the calibration points) are affected

by the low conductivity of some non-ferrous metal substrates.

Figure 10.11 Eddy-Current DFT Gauges

If a user wants to measure the DFT of a material such as aluminum-pigmented mas-tic over a substrate like copper, do not rely on results obtained using either electromag-netic or eddy-current instruments. Instead, estimate the DFT from the WFT of the coat-ing as applied, or, alternatively, use a paint inspection gauge (PIG) or Tooke gauge.

Standard methods for the application and performance of DFT tests using eddy-cur-rent gauges are available in ASTM B 244, ASTM D 7091-05, and ISO 2360.

Users should always study and comply with the specific instructions and recommenda-tions of the instrument manufacturer.

10.10.2 Calibration

Regular calibration over the life of the gauge is a requirement of quality management pro-cedures, i.e., ISO 9000 and other similar standards. Certification by independent labs and some method of verification in the field is also necessary. Verify the gauge calibra-tion on the actual substrate or on a substrate similar to that of the particular surface.

Users can make calibration verification checks using thickness standards with cur-rent and traceable calibration certificates.

Ensure the standards are available on the jobsite and are used to verify calibration and make day-to-day calibration adjustments.

The field calibration verification procedure is:

• Use a plastic shim of known thickness on the uncoated substrate to ensure the gauge is set up for the substrate to be measured.

Choose a shim with a thickness value slightly higher than the maximum reading expected.

• Different gauges may require a minimum substrate thickness. Typically, a substrate should be a minimum of 70 mils thick.

• Make calibration verification on the pre-pared, uncoated surface (with the profile).

• Set instruments with multiple scales to the appropriate measuring scale.

• Gauge calibration verification procedures vary between manufactures. Verify and adjust the gauge per the manufacturer’s instructions.

• For guidance purposes only, verification on smooth surfaces can be done using a shim thickness value slightly above the

expected maximum DFT value of the uncoated base. Verifying on a profiled surface may require a two-point (or rough surface), so two shims are used — one with a thickness above the maximum expected DFT and the second with a thickness below the target DFT value.

• For maximum accuracy, a two-point veri-fication should be done every time the meter is used.

Once the verification and any adjustments are made, measurements should be reason-ably accurate across the scale; that is, at intermediate points between the calibration values used.

To achieve accurate results, test measure-ments may have to be repeated until mea-surements stabilize. Older instruments, in particular, may require a sequence of “zero/

high/zero/high. . .” adjustments until consis-tent results are achieved.

10.10.3 Operating Parameters

It is the user’s responsibility to know and understand the proper use of the DFT gauge.

For detailed instructions, always refer to the manufacturer and model-specific operating instructions; however, there are a few basic operations that are common among the dif-ferent instruments.

The accuracy and precision of the DFT gauge differs between manufacturers and models. Most manufacturers’ guidelines state the degree of accuracy and the preci-sion (resolution) of the specific instrument.

In general, the gauges could have a measur-ing range up to 500 mils (13 mm). The most commonly used gauge has a range from 1.5 mm to 60 mil (0 to 1,500 μm) with an accu-racy of ±1-3% or ±0.1mil (±1-3% or

±2.5μm). This accuracy statement applies to

the 1.5 mm to 60 mil (0 - 1,500 μm) range;

however, the accuracy of gauges can be affected by many factors.

The following factors affect the accuracy of eddy-current gauge measurements:

• Magnetic and conductive properties of the substrate. Linearity and accuracy on inter-mediate thickness values (those between the calibration points) are affected by the low conductivity of some non-ferrous metal substrates.

• Substrate thickness. Depending on the specific instrument, the required mini-mum substrate thickness varies. Some instruments work over substrates as thin as a few mils.

• Edges. Generally, measurements will not be accurate when made closer than 1 in.

(25 mm) to any edge. Some manufactur-ers have special probes to use if with a measurement requirement closer than 1 inch (25mm) to the edge.

• Curved surfaces. If this type gauge is used to measure DFT on a curved surface, hold the probe held at right angles to the sur-face and, if possible, make the calibration on a similar curved surface.

• Conductivity of coatings. Measurement of DFT of conductive coatings, such as alu-minum pigmented coatings, almost always have problems; therefore, consult with the manufacturer for their recom-mendations.

The repeatability of the instrument depends on each individual instrument’s manufac-turer; therefore, review the manufacturer’s instructions. Question the readings anytime the highs and lows are outside known parameters.

Errors that can cause inaccurate readings include:

• Failure to calibrate the gauge prior to use

• Moving the probe too quickly

• Debris on the end of the probe

• Touching the probe to a surface that is too hot

• Use of a dual gauge, but not switching to non-ferrous mode

• Damage to the probe tip, causing probe wear

• Not taking a measurement perpendicular to the surface

10.11 Advanced Data Collection Methods

Many of the advanced electronic testing instruments have the ability to store data for future use. This stored data can be trans-ferred to a computer and other devices using various methods.

10.11.1 Equipment Connectivity Depending on the manufacturer and model of the instrument, there are various ways to transfer stored data:

• USB – Many of the data collections devices can connect to a computer via a high speed data transfer cable. The infor-mation downloads from the device to the computer and stores for future use or, can connect directly to a printer.

• IR - Some models can print information immediately via a portable infrared (IR) printer.

• Bluetooth – Some devices are Bluetooth, which allow remote monitoring and recording. Information can be down-loaded for review on mobile devices.

10.11.2 Software Systems

Some manufacturers have software available to manage stored data. The software

trans-fers data from the instrument to a computer or printer (Figure 10.12).

Some features available, depending on soft-ware manufacturer, may include the ability to: