We use a paint thickness gauge to measure the vehicle’s surface paint. Touch the probe to the car panel surface, and it will take the reading. It’s not only the main object to measure with this paint meter, it also measures other coatings such as the pipe coating, etc.
The base of the paint/coating may be ferrous or non-ferrous (plastic). Either is challenging. In this post, we only talk about the magnetic paint thickness device that measures the thickness of a non-magnetic coating on ferrous substrates.
It’s also worth noting that the best practice for paint thickness gauge calibration is to rely on the manufacturers, authorized agents of the manufactured, and certified calibration laboratory. The purpose of these calibrators is to restore the gauge accuracy to the original one as stated by the product manufacturer.
The Importance of Paint Thickness Gauge Calibration
As people sell and buy used cars, the car’s paint quality can be a determining factor to decide the car price rate. In this case, the paint thickness gauge becomes a piece of critical equipment for the inspection, verification, and calibration. For auto paint shops, this tool also plays a significant role in monitoring the car paint thickness.
It seems that there is no alternative to measure the paint thickness unless this meter. You cannot clamp the paint with a micrometer or caliper. It’s impossible. This meter only requires a single side of thickness to measure.
As essential equipment, what if your paint thickness meter misleads you by giving wrong readings. The worst scenario is you will eventually lose money. So, how to know that your paint thickness gauge is correct or correctly said as accurate? The answer is calibration.
In many cases, when you buy this instrument, the product will come with a set of calibration kits which is a nice purchase. You can immediately check whether your meter is actually giving you the correct reading or not.
All in all, the calibration of a paint thickness gauge is ever so important because its determining factor and the frequency of its usage are very high, leading to possible deviations in its reading.
Calibration Standard for Thickness Gauges
When it comes to calibrating the paint thickness gauge, the gauge (better say it as instrument) will measure the accepted standard. The standard is made with a fixed thickness. It should not be a single standard; you have to have several standards with different thicknesses (microns or mils).
But, how do you know that your standard is accurate, too?
The quickest to find the reliable standards is by buying the Standard Reference Materials (SRMs) that are traceable to your national metrological institutes. If you are living in the United States, you can purchase the SRMs that are traceable to NIST. But, you should not because it’s too expensive and overkills. If your job is to inspect the car paint, these SRMs are not required.
However, those SRMs are critical in case to calibrate the primary calibration standard. Then the primary standard becomes the standard to calibrate the secondary standards. Finally, the secondary standard becomes the standard to calibrate the working standards. The working standard is the calibration kits that come with your paint thickness gauge when you purchase it.
So, if your working standards have been calibrated to SRMs, it means that your standard is traceable. Surely, it’s reliable to use for calibration. But, don’t expect the same accuracy as the SRMs. Your kit may have larger tolerance accuracy.
It is important to mention here that an instrument or standard is always less accurate than the standard that calibrates it. This is why uncertainty increases as one moves down the chain of standards. This is the reason why a Test Uncertainty Ratio (TUR) of 4:1 is applied, meaning that if a coating thickness gauge is calibrated against a standard with uncertainty of ±0.05 mils, then the tolerance accuracy would be ±0.2mils.
Various Standards to Use
If you want to search further about the authority that set the best practices of how to calibrate the paint thickness gauge, you may want to check the following standards.
- SSPC PA 2 (widely used)
- ASTM D1186
- ISO 19840
Step By Step
These are general steps for calibration. You need to read the manual of your gauge to access the calibration function. Maybe, you can directly push the button available for calibration. Once you have accessed to that function, you can get into the steps below.
Basically, the calibration requires these equipments: zero plate, different thickness of plastic shims. These plastic shims are typically available with the instrument along with the zero plate (uncoated ferrous surface). The platic shims will be put on the zero plate. You can run the calibration by choosing the desired plastic shims. But it would be better to test all the plastic shims.
It is recommended to calibrate the gauge on-site with the change of nature of dry film thickness to be measured.
- Place the plastic shim on the zero plate. Make these equipments on an even surface. Also, make sure the surface is free from dirt, dust, and water.
- Lead the gauge probe onto the top of the plastic shim. And then take the measurement.
- If the thickness value shown on your gauge is deviating from the thickness of the plastic shim, then use the associated button to adjust the reading value.
- After measuring the plastic shim, go head to check the zero reading. Place the gauge on top of the zero plate and take the reading.
- If the gauge does not show any zero reading, then simply press the “zero” button (if any). Or simply, you can also set the reading to a “0” value. This is to ensure that the gauge is reset to its zero value when measuring nothing.
- Repeat the process with the plastic shim to take several readings and ensure calibration.
- Users must adhere to this practice strictly to obtain the highest level of calibration and ensure accurate working.
Verification of A Calibrated Gauge
A calibrated gauge with a certificate does not certainly signify a trusted calibration. However, oftentimes, reputable brand have their own lab. You need to search more about the reputation of the calibration lab that calibrates your gauges. Their standards need to be traceable. If the lab is accredited to ISO 17025, it’s good news.
Factors Affecting The Reading
The thickness gauge does not measure the thickness of a coating directly. Instead, they measure the magnetic property of the substrate. These magnetic properties become weak as the probe moves away from the substrate. The following factors affect the magnetic properties of the substrate:
- Surface roughness
- Composition
- Mass
- Geometry
How to Maintain the Standard
The ASTM D1186 standard recommends the following practices to maintain the standards:
- Avoid creases in the plastic shims by avoiding their folding.
- Place the shim perfectly on the zero plate and avoid any air entrapment. The shim should lie flat.
- Perform calibration in a controlled environment at a controlled temperature.
- Be mindful of the pressure of the probe on the shim as it may cause shim thickness inconsistency leading to inaccurate calibration.
- Using of shims for calibration is recommended against the use of plated or painted metallic standards.
- Irrespective of the standards used for calibration, they must be verified periodically. Even coated metal standards may experience wear and tear such that their readings may be affected. It is recommended to perform inspection before any crucial use of the instrument, depending upon the user’s personal preference.
Conclusion
The car paint thickness gauge is an important device for car paint inspection because it doesn’t require you to clamp the paint to measure the thickness. It’s also helpful for measuring coatings on ferrous bases.
The pain thickness calibration must use trusted standards. Due to price and accuracy requirements, you can use the standard kit that comes with your purchase. An inspection of the instrument before every use is great to do to ensure the coating thickness meter accuracy. The calibration to an authoritative calibration lab periodically (maybe once a year) can be critical to note the error that may arise. Herewith, you know when to purchase the new one.
