How to use ultrasonic thickness gauge and ultrasonic sensor principle

- Jun 05, 2019-

Coating thickness gauges using ultrasonic measurement technology are becoming more and more popular. They support or replace destructive methods of measuring the thickness of wood and wood coatings.

The coating has multiple functions. Some are used to restore, protect, waterproof and beautify wood structures. Others are specifically designed to seal and fill pores and provide an aesthetically pleasing surface texture. The permeable finish is absorbed into the wood and hardens to form a strong protective barrier that does not peel off.

ultrasonic thickness gauge 1

Why measure the thickness?

The coating is designed to achieve the desired function when used within the thickness range specified by the manufacturer. For example, conversion varnish should not be dried more than 5 ml thicker than other coatings in order to prevent cracking or other failures. Nitrocellulose lacquer should usually be kept lower than 3 mils. Consistent MIL thickness is the most important application of primer and crack coatings to achieve the desired crack finish.

In medium density fiberboard (MDF), the thickness of the powder coating typically ranges from 3 to 9 milliliters. Generally, the thicker the thickness, the more durable the finish. Factory specifications usually require a tolerance of 1 mil. This level of quality cannot be determined solely by looking at it.

There are other benefits to accurately measuring the finish. When the company did not inspect and verify the quality of incoming materials, they wereted reworked products. Make sure the coating meets the manufacturer's recommendations by checking the spray operator technology. In addition, applying too much film thickness greatly reduces overall efficiency. Finally, regular inspections can reduce internal rework and the return on the number of customers due to processing defects.

How to test?

Testing coating thickness is commonplace for quality control and inspection purposes. When the base metal is carbon steel, a magnetic method is employed. The vortex device is used for other metals such as copper and aluminum.

ultrasonic thickness gauge principle

Since these instruments cannot measure the thickness of the wood surface, alternative techniques have been used:

1. Optical sectioning (cutting the coating part, seeing and cutting)
2. Height measurement (using a micrometer before and after measurement)
3. Gravimetric (measuring the quality and area calculation of coating thickness)
4. The wet film thickness is calculated from the dry paint thickness of the dry paint and the volume percentage of the solids used.

5. Replace (place a steel sheet in wood and paint at the same time).

These tests are time consuming, difficult to perform, and subject to operator interpretation and other measurement errors. It is impractical for the nozzle to find a destructive method. Getting a statistically representative sample from a lot of wood products may require scrapping part of the destructive testing process.

With the advent of ultrasonic instruments, many testers have switched to non-destructive testing.

Ultrasonic breakthrough

High quality professionals are already familiar with all aspects of ultrasonic testing, where high frequency sonic energy is used for inspection and measurement. Ultrasonic testing can detect and evaluate defects in metals, measure dimensions, determine material properties and more.

Wall thickness measurement is perhaps the simplest and easiest method in ultrasound testing. Precision ultrasonic wall thickness gauges allow for fast thickness measurement without the need for both sides to access. However, these gauges are not ideal for coating measurements. They did not measure the sensitivity of the thickness of the acrylic filler factory primer, paint, and UV topcoat. Powder coatings and other materials were used in wood.

ultrasonic sensor thickness gauge principle

The handheld is designed to measure coating thickness and it uses a single component sensor and advanced digital technology to enhance digital echo filtering. Today's handheld ultrasonic coating thickness gauges are simple, economical and reliable.

Ultrasonic measurement technology

Ultrasonic testing is aided by the application of ultrasonic vibrations in the coating using ultrasonic probes (sensors) and couplants applied to the surface.

Vibrate through the coating until a material with different mechanical properties is typically the substrate but perhaps a different coating. Vibration, partially reflected in this interface, returns to the sensor. At the same time, the part of the transmitted vibration continues to travel beyond the interface and experience re-thinking encountered at any material interface.

Since a large number of potential echoes may occur, the gauge design is chosen to maximize or the highest echo, and the thickness measurement is calculated. The instrument also measures loud echoes in the personal layer for multi-layer applications. The user only needs to input the number of layers to measure, say three, and measure the three loud echoes. The gauges ignore softness with coating defects and the base layer.

measurement accuracy

The accuracy of any ultrasonic measurement directly corresponds to the completion of the measured sound velocity. Because ultrasonic instruments measure the transit time of ultrasonic pulses, they must calibrate the "sound velocity" in a particular material.

From a practical point of view, the sound velocity values do not vary greatly and are used in coating materials in the wood industry. Therefore, ultrasonic coating thickness gauges usually do not need to be adjusted to factory calibration settings.

Where the coating and substrate are

One factor that affects measurement accuracy and measurement repeatability is the interface of these ultrasonic coatings with wood substrates. The ability to perform destructive tests in the field is greater than most of the higher resolution, clearly showing the boundary between completion and wood. The topcoat looks smooth but the thickness may be inconsistent. Wood surfaces are often granular with varying degrees of surface roughness and primer penetration. Such porosity and roughness can promote adhesion and increase the difficulty of obtaining repeatable thickness measurements by any means.

The average small irregularity of the design of the ultrasound instrument produces meaningful results. In particular, personal reading on rough surfaces or substrates seems to be non-repeatable, and a series of relatively average results are often reproducible.

Some instruments measure the various layers in a multi-layer system. In this example, the 1 layer is 1.5 mils thick. The 2 layers are 1.5 mils thick and have a total thickness of 3 mils. The graphic LCD displaystwo "peaks" represent the two material interfaces.

Ensure the correct sound

Ultrasonic testing has unique advantages for the wood industry. Furniture, flooring, and musical instrument manufacturers often use several layers of paint or similar materials. Some processes require the ability to determine the thickness of each layer or series of layers. When applied in the wrong thickness, coating, beautifying and protecting a high quality guitar, for example, can easily affect its sound. Too much paint can weaken the guitar's resonance too little and have the opposite effect.

Musical instrument manufacturers now use ultrasonic instruments to accurately and non-destructively measure their valuable products, paints. Thanks to the use of this new technology, they not only reduce the use of paint, but they are able to take meaningful measures without damaging their production process. There is no need for scrapped products to measure coating thickness, and the thickness of the entire surface can be easily measured to ensure a smooth, uniform coating. Rework minimizes the coating process due to more control capabilities.

Figure 6 shows some instruments providing statistical analysis. In this example, 10 are measured. 18.2 ml of the last measured and displayed average, standard deviation and maximum/minimum of all 10 readings.


Ultrasonic coating thickness measurement is currently recognized as a reliable test procedure for the wood industry. “Standard Test Methods for Non-Destructive Measurements Use Ultrasonics for Dry Film Thickness of Organic Coatings”. To verify gage calibration, epoxy coating thickness standards are traceable to National Standards Organization certification.

Fast, non-destructive thickness measurements can be taken from materials that previously required destructive testing or laboratory analysis. This new technology improves the consistency and throughput of finishing rooms. Potential cost reductions, including:

Reduce the thickness of the coating applied by the coating through the control coating
Reduce rework and repair through direct feedback to operators and improved process control
Objects that do not need to be damaged or repaired to destroy coating thickness measurements

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