Use of Ultrasonic Thickness Gauge

Coating Thickness Gauges using ultrasonic measurement technology are becoming more and more popular. They support or replace destructive methods of measuring coating thickness on wood and wood products.

Coatings serve multiple functions. Some are used to restore, protect, waterproof and beautify wooden structures. Others are specially formulated to seal and fill pores and provide a nice surface texture. The permeable finish is absorbed into the wood and hardens to form a strong protective barrier that won't strip

Why measure thickness?

Coatings are designed to perform their intended function better when used within the thickness range specified by the manufacturer. For example, conversion varnishes should be used less than 5ml dry thickness than other paints to prevent cracking or other finish failures. Nitrocellulose lacquer should usually be kept lower than 3 mils. A consistent MIL thickness is important to apply the base coat and crack coat to achieve the desired crack finish.

On medium density fiberboard (MDF), the thickness of the powder coating usually ranges from 3 to 9 ml. Usually the thicker the thickness, the more durable the finish. Factory specifications usually call for a stated tolerance of 1 mil. This level of quality cannot be decided just by looking at it.

Accurately measuring finish has other benefits as well. When companies fail to inspect and verify the quality of incoming materials, they waste reworked product. Make sure the coating meets the manufacturer's recommendations by checking spray operator technique. Additionally, applying too much film thickness can greatly reduce overall efficiency. Finally, regular inspections can reduce the number of internal rework and customer returns 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, the magnetic method is used. Eddy current devices are used on other metals such as copper and aluminum.

Since these instruments cannot measure the thickness of wood surfaces, alternative techniques were used:

• Optical sectioning (cutting the coated part, viewing the cut)
• Height measurement (before and after measurement with a micrometer)
• Gravimetric (mass and area calculations for measuring coating thickness)
• Wet the Film Thickness Gauge into the wet paint and use the volume percent solids to calculate the dry film thickness
• Alternative (place a steel sheet over wood and paint at the same time).

These tests are time-consuming, difficult to perform, and subject to operator interpretation and other measurement errors. Sprinklers find destructive methods impractical. Obtaining a statistically representative sample from many wood products may require scrapping as part of the destructive testing process.

With the advent of ultrasonic instruments, many Testers have switched to nondestructive testing.

Ultrasonic breakthrough

High-quality professionals are already familiar with all aspects of ultrasonic testing, in which high-frequency sound wave energy is used to perform inspections and measurements. Ultrasonic testing can detect and evaluate flaws in metals, measure dimensions, determine material properties and more.

Wall thickness measurement is perhaps the easiest and easiest method of ultrasonic testing. Precision ultrasonic wall Thickness Gauges allow rapid thickness measurement of objects without requiring two-way access. However, for coating measurements, these gages are not satisfactory. They do not have sufficient sensitivity to measure the thickness of acrylic filler plant primers, varnishes, UV topcoats, powder coatings and other materials used on wood.

Figure 1 PosiTector 200 Ultrasonic Thickness Gauge.

The first hand-held instrument designed to measure coating thickness appeared on the market 14 years ago and is now in its fourth generation. It uses a single component sensor and professional digital technology, digital echo filtering enhancement. Today's handheld ultrasonic coating Thickness Gauges are simple, affordable, and reliable to operate (Figure 1).

Acoustic Measurement Technology

Ultrasonic testing is done by sending ultrasonic vibrations through the paint using a probe (transducer) with the aid of a couplant applied to the surface.

Vibration passes through the coating until it encounters a material with different mechanical properties, usually the substrate but perhaps a different coating. Vibrations, partly reflected in this interface, return to the sensor. Simultaneously, a portion of the transmitted vibration continues to travel beyond the interface and experience reconsideration encountered at any material interface (Fig. 2).

Figure 2 Ultrasonic waves return to the surface to be tested.

Because of the large number of potential echoes that may occur, the gage design selects the largest, or highest, echo to calculate the thickness measurement. The instrument measures individual layers in multilayer applications also like loud echoes. The user simply enters the number of layers to measure, say three, and the meter measures three louder echoes. The gage ignores softness and coating defects and base layers.

measurement accuracy

The accuracy of any ultrasonic measurement corresponds directly to the velocity of sound being measured. Since ultrasonic instruments measure the transit time of an ultrasonic pulse, they need to be calibrated for the "velocity of sound" in a particular material.

From a practical point of view, the sound velocity values ​​do not vary greatly with coating materials used in the wood industry. Therefore, ultrasonic coating Thickness Gauges generally do not need to be adjusted to factory calibration settings.

Placement of coating and substrate

一个因素影响测量精度和测量重复性是这些超声波涂层界面与木质基材。图3显示了两个例子涂木。这些照片，在现场破坏性试验比大多数更高分辨率的能力，清楚显示完成和木头之间的边界。面漆涂层上看起来光滑，但厚度可能不一致。木材表面往往是颗粒状的具有不同程度的表面粗糙度和底漆渗透。这样的孔隙度和粗糙度可促进粘附反而增加获得可重复的厚度测量通过任何方式的困难。

图3不均匀的涂层/基体区两个例子。

超声仪器的设计平均小的违规行为产生有意义的结果。特别是在粗糙的表面或基板的个人阅读似乎是不可重复的，一系列比较平均的结果往往可重复性。

图4一些仪器测量多层系统中的各个层。在这个例子中，1层是1.5密耳厚。2层是1.5密耳厚，总厚度为3密耳。图形液晶displaystwo“峰”代表的是两材料界面。

确保正确的声音

超声检测对木材产业的独特优势。家具、地板、乐器制造商经常使用几层漆或类似的材料整理。一些过程需要确定各层或一系列层厚度的能力。当应用在错误的厚度，涂层，美化和保护高质量的吉他，例如，可以很容易地影响它的声音。太多的涂料可以减弱吉他的共鸣太少会有相反的效果。

图5无损检测木材漆层厚度.

乐器制造商现在使用超声波仪器的准确和非破坏性测量对他们有价值的产品，漆。由于使用这种新技术，他们不仅降低了漆的使用，但他们能够采取有意义的措施不破坏他们的生产过程。没有需要报废产品测量涂层厚度，并在整个表面的厚度可以很容易地测量确保平稳、均匀的涂层。返工最小化由于更多的控制能力的涂装工艺。

图6有些仪器提供统计分析。在这个例子中，10个被测量。18.2毫升的最后测量和显示的平均，标准偏差和所有10个读数的最大/最小值。

图7这些仪器操作简单，经济实惠和可靠的。

结束语

超声涂层厚度测量是目前公认的用于木材行业可靠的测试程序。标准测试方法ASTM d6132-04描述。“进行无损测量的标准试验方法对有机涂层干膜厚度使用超声波仪”（2004，ASTM）。为了验证量具校准，环氧涂层厚度标准可溯源到国家标准组织认证。

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

• Reduce waste in coating by controlling the thickness of the coating being applied
• Reduce rework and repairs through direct feedback to operators and improve process control
• Objects that do not need to be destroyed or repaired to destroy coating thickness measurements