How to Calibrate Coating Thickness Gauge?

Calibration is the most important aspect of maintaining your coating thickness gauge. 

This is often to ensure there is continuous accuracy in measurement and precision in analysis. Calibrating your coating thickness gauge guarantees proper procedural inspection and accuracy as industry standards require. You will find the various methods and techniques of calibrating your coating thickness gauge in this post, with reasons why calibrating your coating thickness gauge is paramount. Also, in this post, you will find how you can execute this calibration as well.

Follow this article closely as we dissect all you need to know about calibrating your coating thickness gauge.

I. What is Coating Thickness Gauge Calibration?

Calibration is simply setting the coating thickness gauge already established standard references. This is usually in two stages vis-a-vis manufacturers’ calibration and user calibration. The calibration of the coating thickness gauge executed by the equipment’s manufacturer is done before releasing the device to the user. The quality of the equipment is ascertained and certified to meet the required standards.

When calibrating by the user for operational purposes, meeting manufacturers’ specified accuracy is the goal here. This operation usually involves the coating thickness gauge set to known thickness values, then observed on intermediate thickness values. 

II. Why Coating Thickness Gauge Calibration is Necessary?

Calibration assists producers of the coating thickness gauge certify that the gauge corresponds with standard accuracy specifications. Therefore, it is essential to calibrate a coating thickness gauge to protect the type, shape, and the surface of the material to be tested from giving out non-conforming results of the test object. 

For instance, the magnetic attributes of steel alloys differ, and the conductivity of distinct aluminum alloys and different non-ferrous metals also vary. These attributes can alter the dimension of a coating thickness gauge. It means that a gauge tuned on mild steel can have an unusual value for a similar thickness coating on high carbon steel.

III. Calibration Methods for Coating Thickness Gauge Calibration

As introduced earlier, the various calibration methods that reduce the margin for measurement errors are listed below.

• Zero Calibration/ Single Point Calibration:

The zero method, also referred to as the single-point method, is the simplest calibration method. This is because the featured point of the probe is specific to one reference point alone and that is the zero-value reference point. 

The gauge automatically regulates the calibration when it is confirmed to have complete contact with the uncoated surface to be measured, and with that, coating thickness measurement begins.

• Smooth Calibration:

Smooth Calibration is a more precise calibration technique for the single point or zero calibration. It uses the calibration foil in assisting the gauge to be calibrated as the calibration foil is a suitable calibrating standard on the surface finish or shape. Therefore, to ensure the best accuracy from your coating thickness gauge, smooth calibration is an excellent choice of calibration.

• Two-Point Calibration:

For rough metal surfaces, blasted profile surfaces, or substrates with varying electrical or magnetic dissimilarities, this calibration method is the best way of calibrating the coating thickness gauge. Calibrating your coating thickness gauge requires that the two-foil values are set differently, with one with a value above film density and the other below film thickness.

• Two-Foil Calibration:

The two-foil calibration method employs the use of two calibration foils that ensure accurate and precise measurement of blasted plastic objects. Perfect for measuring feeble magnetic elemental materials. Calibrations such as this could be measured either by the eddy-current method or according to the magnetic-inductive method.

• Zero Offset Calibration:

This is a method of film thickness measurement compliant with measuring standards ISO 19840 as it is a patented calibration method. It employs a zero offset or correction value that is enforceable to every reading to analyze the effect of irregular surfaces. The original surface details determine its offset value and as required by the ISO 19840 standards. It also employs the use of the smooth surface technique.

IV. What are the Coating Thickness Gauge Calibration Standards?

• SSPC PA2: This is a coating thickness gauge calibration standard that represents a procedural practice that determines the field or shop conformity to standards of a specified coating dry film thickness measurement for either ferrous or non-ferrous materials via the use of non-destructive coating thickness gauges, i.e., eddy-current, or magnetic.

• ASTM D1186: This test technique encompasses a non-destructive examination of nonmagnetic coatings’ dry film thickness (DFT). It is administered on a ferrous-based material employing test equipment commercially available. Deformable coatings do not apply to this type of test as probes are directly connected to the coatings being measured. And this standard does not address all safety issues associated with the test materials.
• ISO 19840: This is an international standard that specifies the procedural technique for confirmation of dry film thickness as opposed to dry film thickness on irregular or rough surfaces, including sampling plants, the definition of inspected parts, measurement techniques, and acceptance/rejection requirements, and adjustment of equipment deployed.

V. How to Calibrate Coating Thickness Gauge? 

Calibration is executed under three major procedures: Zero Calibration, One-Point Calibration, and Two-Point Calibration.

The Zero calibration – This requires no coating, and the calibration steps include:

• Press the Zero button so that the mean mark displays and the zero-mark flickers to represent the selection of the zero mark.
• Measurement can be done once or multiple times from the zero mark.
• Then, press the zero mark again to complete the zero-calibration process. When calibration is deemed a success, the mean mark disappears, and the zero point is displayed on the screen.

The One-Point calibration – This requires the calibration foil’s thickness to be close to the test material, and then the following procedures are executed to attain a one-point calibration.

• Press the CAL button to input the calibration mode, and then the mean mark is visible on the screen, and the CAL signal is displayed.
• Set calibration foil to zero on the test object, measure either once or multiple times, then press the upward arrow or the downward to ensure the reading on the screen is a standard set value.
• Press the CAL button one more time to complete calibration and set it to a one-point calibration mode as the mean mark disappears and the CAL mark appears on display. Any prior calibration values will hinder the lightness of the gauge.

The Two-point calibration – This requires two calibration foils with two separate densities. The thickness at both the higher and the lower limits must be considered for the highest and ideal conditions. The two-point calibration includes:

• Conclude the on-point procedure first. Then the CAL mark is visible on the screen.
• Conclude another one-point calibration process with another foil. Then the 2CAL mark is visible on the screen as well.

VI. Do’s and Don’ts to Increase the Accuracy of Your Calibration

For every method, some rules and regulations apply to its excellent performance. To increase the accuracy of calibrations, the following dos and don’ts also apply as several routines can increase accuracy in the calibration of your coating thickness gauges.

• Ensure that the uncoated surface material is rust-free and clean, as this will greatly increase the accuracy of your calibrations.
• Stacking calibration foils to increase foil density must be carefully done, and stacks must be placed on non-labeled surfaces.
• During measurements, the foil labels should be avoided, and a probe should be placed at the core of the foil.
• Unimpaired foils should be applied. Therefore, it’ll be helpful to have spare foils.
• Place the probe down gently, right-angled to the surface of the test object.
• For multiple readings, the probe and gauge must be held systematically, and the heel of your hands can be used as support.
• Multiple readings must be taken on each foil to achieve certified value foil. Then the gauge averages the reading. Thereby increasing the adjustment accuracy and then using the up/down keys, the average readings can be synchronized.

VII. When you should Get Your Coating Thickness Gauge Recalibrated?

A general understanding of your coating thickness gauge affords you knowing when to recalibrate your coating thickness gauge. 

Typically, equipment should be recalibrated when evaluation standards also change. But as far as a coating thickness gauge is concerned, recalibration can occur when the test materials’ surface configuration changes. Also, you should recalibrate when the design of the test material changes. Such changes may include changing from a flat surface to a curved surface.

Recalibration might be necessary at these varying points to consistently achieve accuracy and precision in measurements, especially when the method or technique of measuring is also a factor for consideration. For every change occurrence, recalibration is of utmost importance.

Conclusion

In conclusion, it is a fact that calibration of coating thickness gauge ensures consistent measurement accuracy, and certification of machinery safety is assured. It has also been noted that equipment manufacturers usually calibrate to attain industry standards benchmarks for safety.

Furthermore, different calibration methods, such as zero calibration or single-point calibration, two-point calibration, and foil calibration, have their respective working principles and areas where they are applicable.

And lastly, there are industry standards that you need to meet when calibrating with their steps and procedures of calibration. Additionally, some dos and don’ts of calibration procedures also apply to increase accuracy and then to recalibrate your coating thickness gauges.