Nondestructive Dry Film Thickness (DGT) Tips and Tricks:

NOTE: The following comments are the opinion of the author based on experience using the following equipment in both a laboratory and field environments. Any comments are welcome and can be forwarded to me at tswan@cci-inspection.com

Which Dry Film Thickness Gage is Best for Me?

There are two types of nondestructive DFT generally referred to as Type 1 or “Banana Gages” or Type 2. Electronic gages. Advantages and disadvantages of both types are listed below.

NOTE: Type 1 Gages also include “Pencil Pull-off gages” which are good for quick field checks but generally not used for Quality Control Purposes. They are generally rated at ±10% to ±15% accuracy when used properly.

Are There any Guidelines for Using These Meters?

The most commonly used Guideline is SSPC-PA2, “Measurement of Dry Film Coating Thickness with Magnetic Gages”.

IMPORTANT: There are significant differences between the Original PA2 (1982/91) and the Newer Version (1997, 2004). Make sure you know which version you are working from.

Other Standards that may be applicable are the following ASTM Methods:

ASTM B 499, “Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals.”

ASTM D 1186,”Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to a Ferrous Base.”

ASTM D 1400, “Nondestructive Measurement of DFT of Nonconductive Coatings Applied to a Nonferrous Metal Base.”

ASTM E 376, “Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Test Methods.”

ASTM G 12, “Nondestructive Measurement of Film Thickness of Pipeline Coatings on Steel.”

European Specifications generally refer to EN ISO 19840, “Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces”.

Understanding PA-2

Some of the following information reprinted from Elcometer’s elconews e-zine.

For Structures not exceeding 300 sq ft, take 5 spot readings per 100 sq ft.

For Structures not exceeding 1000 sq ft, Select 3 random 100 sq ft areas to test.

For Structures exceeding 1000 sq ft, Select 3 random 100 sq ft areas to test in the first 1000 sq ft and for each additional 1000 sq ft test one random 100 sq ft area.

If any area is not in compliance, the non compliant area should be determined

How do I determine the minimum number of tests required?

To determine the number of tests required if the surface area is greater than 1,000 sq ft, use the following formula to determine the minimum number of areas to test:

Example: 27,500 square feet to be coated to 12 – 15 mils of paint.

Round to 28,000

3 + [(28,000 – 1,000) / 1,000] = 30 areas to test
30 X 5 = 150 Spot Readings
150 X 3 = 450 Gage Readings

All Spot readings must be ±20& of range.

For a spot reading use roughly a 1 inch diameter circle. Within this, the probe is placed 3 times in random positions (Gage Reading). The average of these 3 gage readings is a called the ‘spot reading’

Example:

The previous example calls for a DFT range of 12 to 15 mils, the area meets SSCP PA-2 if:

The Average of all the Spot readings must fall within the 12 -15 mil range
and
• All spot reading are greater than 80% of the Specified DFT (0.8 X 12 = 9.6 mils)
and
• All spot readings are less than 120% of the Specified DFT (1.2 X 15 = 18 mils)

Therefore the PA2 Range = 9.6 to 18.0 mils for individual spot readings.

Individual Gage readings do not have a range.

Another way to look at PA2 is the thickness of the coating is acceptable if the DFT of an area fits within the curved part of the graph.

In the current PA2, it requires mapping out of areas that are out of compliance and only requires correcting those areas. The old method requires mapping out the entire project. Also, the square footage can be in any shape, not just squares.

Standardization bodies, such as ASTM, BS, CEN and DIN, hold the copyright to all their publications. Everyone wanting a Standard must therefore pay for it via the standard bodies websites,

What if ISO is specified?

EN ISO 19840, “Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces”. This standard method has special calibration techniques, (See Procedure).

Readings of the thickness of the paint film should be taken according to a sampling plan. The number of readings depends on the size of the inspection area and they are taken at random points (see table).

SAMPLING PLAN

· Most of the readings should be above the Nominal Dry Film Thickness (N) so the average ( x ) will be equal to or greater than N.

· Dry Film Thickness (N) so the average ( x ) will be equal to or greater than N.

· No more than 20% of the readings in an area can be thinner than the Nominal (N) but not thinner than 80% of nominal (0.8N).

· All readings should be less than the maximum thickness (Max).

The thickness of the coating is acceptable when readings are within the curved part of the graph. (See Procedure). There is a provision for repeating a few readings; consult the Standard for details.

ALTERNATIVE PLAN

Annex B of EN ISO 19840 is a variation of the above procedure, subject to agreement by interested parties. Say 5 thickness readings are taken in a 30mm circle and the average is calculated. The location of these little groups is according to the Sampling Plan above. The thickness of the coating is classed as acceptable when the averages are analyzed and they fit within the graph

I need an electronic Type 2 meter, what should I get?

Elcometer offers two different Electronic DFT gages. The Elcometer 456 and the Elcometer 355/365.

For most people the Elcometer 456 is the best choice. It is designed primarily for field usage, is light, compact and easy to use. It can be ordered with a built in “integral” probe or with a separate detached probe. If you are only going to measure large pieces and you are sure the model you order will meet your needs, a fixed probe is a good choice.

If you get involved with many different types of projects, need to get into small areas get involved with coating thickness from thin to thick films, the separate meter provides the flexibility to change with the job by just changing out the probe.

The 456 comes in a ferrous(F), nonferrous(NF) and a duel FNF model. If you need to measure coatings non non-magnetic surfaces, you will save money by buying the FNF model.

The last decision is memory or no memory.

If you are just going to take the reading and write it down, the Basic model is all you need. For computer downloading you need the standard model. For large projects, where batching is needed, the Top model can saves hours of work and provide you with professional, computer generated reports.

Both the Standard and Top models have Bluetooth or you can dowload via an optional cable.

FOR MORE HELP IN CHOOSING THE 456 THAT IS RIGHT FOR YOU - CLICK ON: CHOOSING AN ELCOMETER 456 UNDER IMPORTANT INFO.

Elcometer 355/365

For shop applications where SPC is being used, this meter is an excellent choice. It is portable and durable and has all the functions required for a quality control shop. It has ±1% accuracy and can be used to measure thin films of less than 1 mil accurately. Another key feature is it allows you to key in actual batch or serial numbers whereas the 456 just sequentially numbers batches. Both meters are Ferrous-Nonferrous based on the probe being used.

Is the Elcometer 456 different from other DFT Gages on the Market?

Yes. While many other manufacturers make meters with many of the same features, there are some important differences. To get at the features on most DFT meters requires remembering how to get to them. The menu system on the 456 makes it easy for first time users to reach most of the functions without even using the manual. The four “Soft Keys” make navigating the menus easier than any other meter on the market.

Quality control and durability of the 456 is second to none. The 456 takes readings twice as fast as the leading competitor. Bottom Line: there are many reasons to choose the 456 over the other available options.

How often should I calibrate the meter?

Type 1 meters should be verified each time they are used. According to PA2, they should be calibrated before and after each shift using prepared but uncoated steel, a base metal reading (BMR) should be taken. The BMR should be subtracted from each gage reading to get the proper coating thickness. PA2 specifies that the Type 1 meter should be calibrated using “Calibration Plates” not shims or foils. This may be changed in the new version and in reality calibrating with foils is just as accurate.

If the Elcometer 211 requires calibration, it is the easiest gage on the market to calibrate using the thumb wheel. After calibration, make sure the meter is accurate over the range you will be testing. The calibration is linear while the 211 scale is logarithmic.

Type 2 meters should be verified each time they are used, according to PA2 before and after each shift. In situations where many reading are being taken throughout the day, I recommend frequent verification of the calibration. If you take 500 readings over the course of the day and the calibration verification at the end of the day is off, since there is know way of knowing when it went wrong, you have to backtrack all the reading until you find the point. It is much simpler to verify the accuracy of the meter throughout the day.

Type two meters can be calibrated with foils (shims) or calibration plates. When using foils, calibrate with the shim on the prepared substrate. DFT is the actual reading taken. If the meter is calibrated to plates, then the actual DFT is the Reading – the magnetic base reading (BMR).

When using a type 2 meter on nonferrous metals, it is essential to calibrate the meter to the base metal being tested. Calibrating the ferrous part of an FNF meter does NOT calibrate the nonferrous probe.

What is a Base Metal Reading (BMR)?

This is probably one of the most misunderstood concepts in Coating Thickness Testing. There are two separate properties that effect the BMR.

1. The magnetic properties of the steel

2. The surface profile of the steel

Generally, the BMR is between 0.5 and 1 mil.

All meters are zeroed to steel with a certain magnetic property. Since many factors can affect the magnetic properties of steel during its manufacture, each metal will have its own “zero point”.

When you have a surface with peeks and valleys, there is no clear line where the “Zero Point” is. There are areas in the profile that are LESS THAN THE ZERO POINT. The meter will not register any paint as being applied to the surface until it is greater than the zero point.

In the above drawing, if the DFT gage perceives the “Zero Point” to be the dashed line, any paint below the dashed line will not be measured.

When you calibrate a DFT Gage to a flat plate, the BMR simply adjusts the Dry Film Thickness to the Zero Point.

Let’s look at the effect of BMR on a Type 1 & Type 2 meter with a 2 mil surface profile.

· with a Type 2 gage properly calibrated to the blasted substrate, if you measured every possible point from peak to valley, the average DFT reading would 2.0 mils because the meter was “zeroed” to the BMR.

· With a Type 1 gage properly calibrated to a calibration plate, if you measured every possible point from peak to valley, the average DFT would be greater than 2.0 mils. If the BMR was 0.75, the average measurement would be 2.75 mils, Since the BMR is 0.75 mils this must be subtracted from all readings to get the actual DFT.

The BMR is most important when measuring thin coats or prime coats. As coating thickness increases, not accounting for the BMR becomes less important.

Why is DFT Important?

The most obvious reason is if a contractor bids a job to give you 12 to 15 mils and you end up with 8-10 mils, you did not get what you paid for.

Since many coatings are for corrosion protection, the main way most of these coatings protect is by “Barrier Protection”. The thicker the barrier, the better the protection. There are other important reasons you do not want a coating too thin or too thick.

Many coatings today cure by catalyzation. While there is some room for error built into the coatings, some coatings require a minimum thickness to get enough catalyst to cure properly but if the coating is too thick the reaction may occur too quickly causing other problems including solvent entrapment and blistering..

What do I do if my DFT is out of Spec?

When discussing concerns on DFT, the coating manufacturers Technical Representative, should be contacted. (Nothing personnel to you sales guys out there, but unless you have confidence in your sales person, talk to the Technical Rep. and GET IT IN WRITING). Arguments over tenths of a mil above or below spec generally should be avoided as variations in the MBR, measurement errors, where readings were taken or not taken can all have a greater effect then this.

Remember PA2 allows for variation of individual points to 20% below the minimum and 20% above the maximum specified range.

Type 1 meters are generally ±5% so at 10 mils there is a possible error of 0.5 mils.

Type 2 meters are generally ± 3% so at 10 mils there is a possible error of 0.3 mils.

While many electronic meters read to hundredths (0.01) of a mil at low DFT’s, it is generally best to round to the nearest tenth (0.1) since most meter are at best ± 0.1 mil. Once you get above 2 to 3 mils, to be perfectly honest, do not worry about numbers after the decimal point. Round to the nearest .5 mil and that is more than sufficient for most applications.

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