Material thickness & wall thickness measurement information page

1. Ratio of time vs. Material thickness

Ultrasound thickness measurement is based on measuring the time it takes for the ultrasound to travel through the material. The ratio of material thickness to time is also known as the speed of sound. In order to be able to carry out precise measurements, it is essential to enter the correct speed of sound in the measuring device or to determine the speed of sound by using an integrated calibration function.

Below is a table with typical speeds of sound. The actual speed of sound can vary depending on the constellation of the particular material. Wall thickness gauges with an integrated calibration function to determine the speed of sound using a pattern of known thickness ensure increased accuracy.

2. Suitability of materials for ultrasonic wall thickness measurement

The ultrasonic wall thickness measurement is based on an ultrasonic wave that has to travel through the material. However, not all materials are equally suitable for the transmission of ultrasound.

Ultrasound is suitable for various homogeneous materials such as metals, plastics, glass, ceramics and much more. Some types of cast, composite materials and plastics can be difficult to measure, inhomogeneous materials such as wood and concrete cannot be measured.

Please contact us to discuss your application. In case of doubt you can also send us material samples for further assessment.

3. Measuring range and accuracy

In general, the possibilities of measurement, with a large variety of materials, depend on the particular nature of the material to be measured and the measurement mode used. The respective measuring range specifications of our wall thickness measuring device refer to the measurement of steel.

The actual measuring range can vary depending on the ultrasound probe used and the material being measured. The more sound the material absorbs, the smaller the possible measuring range.

The accuracy is largely determined by how constant the speed of sound is over the measured V-path and is a function of the total thickness of the material. For example, the speed in steel is typically within 0.5%, while in cast iron it can vary by 4% due to the granular structure.

4. Coupling agent

All ultrasound applications require the use of a carrier medium to transmit the sound from the test head into the material. The sound frequencies used for the thickness measurement cannot be effectively transmitted through the air. Highly viscous liquids, such as our CF-AN coupling agent, are usually used to produce the coupling.

5. Influence of temperature

The temperature has an influence on the speed of sound. The higher the temperature, the slower the sound travels through the material. High temperatures can also destroy unsuitable test heads and make the use of a regular water-based coupling agent impossible. Since temperature has an influence on the speed of sound, it is important that the calibration to a known material thickness is carried out at the same temperature as the final measurement is.

1.Pulse-echo mode:
When using the pulse-echo mode, the total thickness between the probe and the rear wall of the material is being measured. The measurements are made from the first pulse to the first back wall echo. Since only a single pulse is required, this mode is also very suitable for detecting small defects. In pulse-echo mode, the probe must be zeroed before the measurement by using the zero plate integrated in the battery compartment cover.

The pulse-echo mode is only suitable for measuring uncoated materials.

2. Echo-echo mode:
In echo-echo mode, measurements are made between two echoes. The time between two echoes is being measured, between the material back wall and material surface.

The Echo-Echo mode is suitable for the measurement of coated and uncoated materials. Since the measurement takes place between two back wall echoes, an applied layer is automatically ignored. A disadvantage is that a much stronger echo is required for the measurement between two echoes.

3. Echo-Echo-Verify mode:
The Echo-Echo-Verify mode works basically in the same way as the Echo-Echo mode with in addition a third echo also being taken into account in order to verify the previous measurement. The Echo-Echo-Verify mode thus offers additional security and is often required in shipbuilding amongst others.

4. Interface Echo Mode:
The Interface Echo Mode is used in our precision wall thickness measuring devices. In contrast to the Echo-Echo mode, this measurement takes place between the delay tip of the precision probe and the first back wall echo.

The Interface-Echo mode is only suitable for measuring uncoated materials.

There are various ways to correctly adjust the wall thickness measuring device for the pending measuring task.

If the speed of sound of the test object is already known, the speed of sound can simply be entered directly via the control panel and a measurement can be carried out.

If the exact speed of sound is unknown, the user can carry out a 1- or 2-point calibration based on a sample of known material thickness, depending on the device. For this purpose, the thickness of a sample is determined using a caliper, a measurement is carried out with the wall thickness measuring device and the actual thickness value is entered. The device automatically determines the speed of sound and shows it on the display so that it can be conveniently entered via the control panel in the future.

The calibration function is included in all devices from TI-25MX upwards. For maximum accuracy and ease of use, we generally recommend purchasing a device with a calibration function.