Thickness Gauge

For the non-destructive measurement of material thickness, an electronic material thickness gauge is now very commonly used. There is a wide range of electronic devices available, which differ in measuring principle and functionality. Due to the measuring principle, each type of device has certain requirements for the measuring conditions and the measurement only works if these conditions are met.

Before searching for a suitable device, it should be determined for which measuring tasks and under which operating conditions the material thickness gauge is to be used. For some applications different measuring principles and many instrument types can be used, for others there is only a limited choice of instruments. In most cases, the following questions about the intended application help in narrowing down to suitable devices.

Where and how is the material thickness to be measured?

Stationary or mobile measurement
Should the material thickness gauge be used to test new materials or components in the incoming goods department or in the production department of a company, or the parts that have already been in use for a long time, such as steel structures?

Single measurement or continuous measurement
Should individual areas or points be inspected at a time, or should complete web widths of rollers or presses, or complete coating surfaces be monitored continuously?

What is the shape of the materials or components whose thickness must be determined?
Should the material thickness gauge be used to determine:

• the sheet thickness of flat plane or curved components,
• the thickness of components of any shape at specific points,
• the web thickness of flexible materials,
• the wall thickness of flat or curved components with cavities or
• the layer thickness of single-layer or multi-layer coatings on one of the aforementioned substrates?

What are the specific properties of the substance to be measured or the coating substrate?
The material properties important for the measurement include:

• the homogeneity and the specific sound velocity of the material for ultrasound-based measurements,
• the electrical conductivity of the substrate for measurements with eddy current,
• the magnetic effect of the subsurface for measurements using electromagnetic induction,
• the thermal conductivity of the material for area-based measurements by means of a thermal imaging camera.

Which measurement range must be covered?
For coatings as well as for material and component thicknesses, the desired measuring range can be

• a few micrometres or
• several decimetres.

For even higher measuring ranges, length or distance measuring devices are more commonly used.

In the material thickness gauge with a wide measuring range, the measured values are usually no longer displayed with a resolution of 0.1 µm. There, where both very thin and considerably thicker materials have to be measured, several devices matched to the respective measuring range should be used.

To ensure that the measurement results are correct and precise enough, the material thickness gauge should be calibrated regularly. It is recommended to use only devices with adjustment function. These can be adapted by adjusting if it is determined that the measurement accuracy is insufficient.

Calibration of the magnetic induction or eddy current material thickness gauges
The material thickness gauge for measuring coating thicknesses on metallic materials is tested for its measurement accuracy using calibrated reference foils. These foil calibration standards with thicknesses of, for example, 191 µm; 483 µm; 1,016 µm; 1,951 µm; 2,794 µm or 3,446 µm, when used for ISO or DAKKS calibrations, are themselves regularly checked for accuracy. For the magnetic inductive measurement, a ferromagnetic base plate is required for calibration, for the measurement with eddy current – a non-magnetic but electrically conductive base plate.

Calibration of the ultrasonic material thickness gauge
Also, with the ultrasonic material thickness gauge, standards with the known and confirmed thickness are used for the calibration. It must be ensured that not only the thicknesses of the gauge blocks used as calibration standards, but also their materials are selected in such a way so that they cover the range of application for which the material thickness gauge is to be calibrated as well as possible. The wider the range of application of the gauge, the more reference points should be checked. The sound speed for the material of the respective gauge block must be set correctly in the material thickness gauge before the measurement. Since the speed of the sound is temperature-dependent, the appropriate temperature for the environment, the gauge and the calibration standards must also be ensured.