Detection Principle and Classification of Thickness Gauges

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Up to now, the nondestructive testing technology of thickness gauges on the market has become an indispensable means for the processing industry to inspect the quality of finished products for users and ensure that the products meet high-quality standards. There are basically three types of thickness gauges as follows: three types of thickness gauges using magnetic measurement, eddy current measurement and ultrasonic measurement.

The principles and methods commonly used in classification of gauges generally include:

Magnetic measurement

It is suitable for measuring the thickness of non-permeable layer on permeability magnetic materials, which are: steel, iron, silver, and nickel. This measurement has high accuracy.

Eddy current measurement

It is suitable for measuring the thickness of nonconductive layer on conductive metals. This eddy current coating thickness measurement has lower accuracy than that of magnetic measurement.

Ultrasonic measurement

It is suitable for accurate measurement of various plates and processed parts, and can also monitor the degree of thinning of various pipes and pressure vessels in production equipment after being corroded during use.

The classification of gauges can be divided into two types: the thickness gauge meter of magnetic attraction principle and the thickness gauge meter of magnetic induction principle. The thickness gauge meter of eddy current measurement principle has only one kind, which is eddy current thickness gauge meter.

The thickness gauge of magnetic attraction principle is to measure the thickness of the coating by using the suction force between the permanent magnet probe and the magnetically conductive steel to be proportional to the distance between the two. This distance is the thickness of the coating. So as long as the difference between the magnetic permeability of the coating and the substrate is large enough, it can be measured.

The magnetic thickness measurement induction principle uses the magnetic flux of the measuring head to flow into the iron substrate through the nonferromagnetic coating to measure the thickness of the coating. The thicker the coating is, the smaller the magnetic flux will be. When the probe with a coil on the soft iron core is placed on the test object, the instrument automatically outputs the test current. The magnitude of the magnetic flux affects the magnitude of the induced electromotive force. The instrument amplifies the signal and then indicates the thickness of the coating.

The eddy current thickness measurement uses high-frequency alternating current to generate an electromagnetic field in the coil as the measuring head. When the probe is close to a conductive metal body, an eddy current is formed in the metal material. This eddy current increases as the distance from the metal body decreases, and will affect the magnetic flux of the probe coil. This feedback effect is a measurement of the distance between the probe and the base metal.

The eddy current probe is used to measure the thickness of the coating on a non-ferromagnetic metal substrate, so we usually call the probe a nonmagnetic probe. Compared with the magnetic measurement principle, their electrical principle is basically the same. The main difference is that the probe is different, the frequency of the test current is different, and the signal size and scale relationship are different. In the thickness gauges of the past two years, through continuous improvement of the probe structure and microcomputer technology, by automatically identifying different probes, calling different control programs, outputting different test current and changing scale transformation software, two different types of probes are finally connected to the same thickness gauge meter. Based on the same idea, the thickness gauge which can be connected with up to 10 kinds of probes has also emerged. (You may also be interested in magnetic coating thickness gauge, and metal thickness meter)

The ultrasonic thickness gauge is based on the principle of ultrasonic pulse reflection to measure the thickness. When the ultrasonic pulse emitted by the probe reaches the material interface through the measured object, the pulse is reflected back to the probe, and the thickness of the measured material is determined by accurately measuring the ultrasonic propagation time in the material.

Although there are many differences in the selection of measuring points and standard materials in the calibration of several thickness gauges, there are some common points to be noted in the operation. For example, each thickness gauge has a lower limit on the surface curvature and minimum thickness of the substrate, and the substrate with reasonable size should be selected for operation in the actual calibration; The orientation and pressure of the probe will also affect the measurement results. It is necessary to keep the probe perpendicular to the substrate and keep the pressure constant and as small as possible; In addition, attention should be paid to the interference of external magnetic field and matrix remanence when the coating thickness gauge is calibrated, and the influence of temperature change and coupling agent viscosity when the ultrasonic thickness gauge is calibrated.

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Measuring thickness standards requires special application gages, such as this gage block.

Thickness is one of the most important dimensions to measure, after inside diameters (IDs) and outside diameters (ODs). As with any dimensional check, there are as many different gaging approaches as there are thicknesses, and the best solution depends on the specified requirements.

Four different gage types for measuring thickness stand out, ranging from the simplest to those that can measure to a millionth of an inch. When paired with the right task, each can successfully simplify and speed up the inspection process.

What are four types of gages for measuring thickness?

The two most common portable hand tools have been around for a hundred or more years: the dial indicator-based handheld thickness gage and the handheld micrometer.

A simple handheld indicator thickness gage is best for fast, general-purpose thickness checks. Typical applications include in-process verification checks of items undergoing manufacturing, such as films, sheet metal or even small parts like washers. This type of gage “calipers” the film between its flat reference and sensitive anvils, providing a continuous and direct reading of part size compared to the reference anvil. A lifting lever provides large retraction so that the gage is easily positioned on the part and provides wide-use application.

Micrometers can provide a similar form of thickness check, though there are some differences. Micrometers have flat contacts, but operators conduct the retraction and gaging by rotating the thimble. Thus, retraction and gaging takes longer and going to various sizes can take some time. There is also the issue of the rotating sensitive spindle. As the user rotates the sensitive contact down onto the part — a film, for example — the rotating anvil can grab and twist the material. However, recent micrometer designs can prevent this twisting and increase the speed of retraction and measuring. These new micrometers include a sliding measuring spindle and quicker threading to the thimble. This provides retraction speed five times faster than most micrometers. Though not as fast as an indicating thickness gage with its lifting lever, this type of thickness gage can provide better performance.

While some films are durable, there are parts and materials where touching them might be prohibitive. For example, silicon wafers and precision components may require non-contact measuring. Due to the delicacy of the parts, operators can use two opposed air jets to carry out this non-contact measurement. Because of the differential characteristics of measuring thickness without regard to position, this air-gaging solution offers the protection and the performances needed for this thickness requirement.

Inspectors must also consider compressibility when measuring thickness. Imagine a number of sister plants making the same compressible film, all using different gages with separate gaging forces and contact shapes. It is unlikely there would be any correlation between these diverse gaging operations. However, the industry has established a set of standards for thickness gages that define gaging force and contact/anvil shapes and sizes. By having all the plants use the same standard for their gages, all parts have identical inspections and the chances of correlation between plants increases.

One can imagine that gage blocks are also thickness standards. As standards, the blocks must be measured in very precise thickness gages, following research and standards to find the proper and most exact ways to verify them. Being millionth-class measuring instruments, these thickness gages must take into account and squeeze out any potential errors. Therefore, the thickness gages must be bench-mounted, robust, used in environments consistent with millionth measurements, using differential gaging for point-to-point measurement and factoring in contact radius and gaging pressure as part of the process.

While thickness gages can range from portable, pocket-carried tools to precision lab systems, they are still used for one of the most common checks required today. It all comes down to matching the measuring tool to the measurement requirements — something that requires consideration in any measurement task.

 

 

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