Wall Thickness Meter / Wall Thickness Gauge

In many areas of industry, wall thicknesses or wall strength are checked during product inspections. Often, a wall thickness gauge is used to take measurements on blanks in goods receipt and on products in the course of production. It is also important to know the wall thicknesses of existing structural elements during repairs or extensions on machines or structures. In the case of wear parts, the wall thickness gauge can be used to determine the thickness that is still available. 

Before deciding on a particular wall thickness gauge or method of thickness measurement, it is important to clarify which measurement conditions are specified by the application. Many measurement methods work quickly and non-destructively, but require either access to both wall surfaces, complex technology, or are limited to specific materials.

The following questions will help narrow down the search for a suitable instrument:

- Should the total thickness or also the thickness of partial layers be determined?

- Are both or only one of the wall-surfaces accessible?

- Should the measurement be carried out stationary or at changing locations?

- Should the measurement be performed during the ongoing process and should  the latter be influenced?

Measuring templates can simply be held against the material cross-section with the appropriate perforation, indentation or marking and then read off. However, conventional templates, calipers, measuring clips or micrometers can only be used as wall thickness gauges if both wall boundary surfaces are accessible to them. In the case of glass thickness templates for flat glass, one accessible surface is sufficient. The glass thickness can then be determined on the basis of the control line mirrored at both glass edges. These devices can be used regardless of the type of material and ambient conditions. Profiled cross sections and remaining wall thicknesses at grooves or blind holes can also be determined using special measuring clips and measuring forceps. 

Microscopes can be used as wall thickness gauges for very small wall thicknesses. Microscopic measurement is also suitable as a comparison method for all other methods. The wall cross-section is viewed and measured with a reflected light microscope. If individual layers are to be distinguished, a finely ground surface is required. The method of optical measurement on the cross-sectional surface is regulated in DIN EN ISO 1463. It is suitable for thicknesses from three micrometers. For thinner layers, bevel grinds or the calotte grind can be used. The enlarged measuring surfaces can be better recorded and then converted into wall or layer thicknesses according to their grinding angle. 

A wall thickness gauge with ultrasonic sensors determines the wall thickness over the time period with which the sound passes through the material. In many instrument models, the ultrasonic transmitter and the ultrasonic receiver are located in the same component. This sensor is placed on the component to be measured. The contact to the component surface can be improved by coupling gel. The sound transmitted by the wall thickness gauge passes through the material of the component and is partially reflected at the opposite interface. 

The time for outward and return travel is determined by the reflections hitting the receiver. From this time and the specific sound velocity of the material, the distance traveled can be calculated in the instrument. For the correct determination of the wall thickness with the ultrasonic method, the sound velocity of the material must be known and it must be possible to set it in the wall thickness gauge. With some instrument models, the speed of sound of a material can be determined on a sample part with a known thickness and then stored in the instrument for measurement.

The magnetic induction and eddy current methods only work in the case of metallic substrates. The wall thickness gauge for measuring the thickness of materials on metallic substrates usually has combination sensors with which both magnetic induction and eddy current-based measurements can be performed. For the measurement, the sensor must be placed on the measuring point. Between the distance from the sensor tip to the magnetic or electrically conductive material and the strength of the received signal, there is a direct relationship which is displayed as distance or thickness.

In the magneto-inductive method, low-frequency current is used to generate a magnetic flux to the ferromagnetic base material. The probe then detects the higher magnetic resistance due to the intervening non-ferromagnetic material. The thickness of the applied material can be determined from this measured value. The magnetic inductive wall thickness gauge can determine the thickness of the non-magnetic materials if there is sufficient magnetic material on the side facing away from the sensor. This makes testing of the thickness of non-magnetic materials simple and non-destructive. Wall thicknesses of hollow bodies, pipes or housings can also be measured if a suitable piece of iron can be placed on the inside as a counterpart for the measurement.

The wall thickness gauge based on eddy current causes eddy currents in conductive materials because of high-frequency current. If there is non-conductive material between the probe and the electrically conductive base material, the resulting magnetic field overlaps the magnetic field of the sensor to a lesser extent. From this, the thickness of the non-metallic material can be determined. This method can also be used to determine the wall thicknesses of hollow bodies, tubes or housings if a suitable part made of non-magnetic metal can be applied on the opposite side.

The laser wall thickness gauge functions as a range finder. It can be used when separate measurements of the material substrate and the material surface are possible. In manufacturing processes, for example, the thickness of the material on a work table or conveyor line can be determined. The laser wall thickness gauge records the measured values very quickly and without contact. For this purpose, the reflection of the emitted laser light is evaluated. For continuous production monitoring, several laser range finders can be used in series or in parallel. With adapted laser light, the method is also suitable for glowing, transparent or organic material. 

X-ray fluorescence analysis allows qualitative and quantitative determination of very thin materials. It is applicable for thicknesses from less than 0.2 to well over 50 micrometres. In this process, the material under examination is excited by X-rays and emits a material-specific fluorescence radiation that can be evaluated by the wall thickness gauge. The material thickness can be determined based on the intensity of the fluorescence radiation. Advantages of the measurement method with X-ray fluorescence are the fast and non-destructive measurement of single and multiple layers and the high accuracy. In addition, the method is independent of the electrical and magnetic properties of the material.