Ultrasonic Thickness Measuring
Ultrasonic thickness meters use ultra sound for measuring - mechanical vibrating of material environment similar to audible sound. But unlike the audible sound is the frequency of ultrasound beyond audibility. The thickness measuring by ultrasound is enabled only due to the fact that ultrasound easily spreads through solid materials.
Ultrasoud is created in the probe, which is then applied on the surface of the tested material. Ultrasound is not sent out to the material permanently but in short impulses. The impulse spreads through the material, echoes from the opposite surface, or alternatively the internal flaw, and returns back to the probe, which then transfers the ultrasound into electric signal and which is then further processed in the device.
Ultrasound is being echoed entirely on the boundary of solid material - gas. Therefore it is necessary to place a suitable liquid environment between the probe and the tested surface - the so called bonding gel.
The principle of the US thickness method.
The ultrasonic method of thickness measuring is not actually measuring the thickness of the material, but the time of passing through the tested material there and back. Simply it can be said that the thickness gauge is precise stopwatch, which measures the time between the input and output impulse. If the time and one of the quantities thickness/speed, the remaining quantinty can be then calculated using the known pattern. Keep in mind the passing time - it is doubled (there and back).
Speed of spreading of ultrasound through the material
The speed of ultrasound spreading in the material depends on the physical property of the material and to a certain point on its temperature too.
|Material||Speed of longitudial waves spreading in m/s|
|Cast iron||3500 – 5800|
|Copper||4660 - 4700|
|Brass||3830 – 4400|
|Ferritic steel||5850 – 5920|
|Austenitic steel||5200 – 5800|
|Ceramics||3000 – 6500|
|Hard rubber||ca 2300|
|Plexiglas||2650 – 2730|
|Polyamid||1800 – 2600|
|Window glass||ca 5700|
|Fused quartz||ca 5570|
For accurate measuring it is not sufficient to set the spreading speed from the chart. The accuracy can be reached only after calibration on the level gauge (of known thickness) on the same material of the similar thickness. Example: if we measure steel ca 24mm, it can be calibrated on the stell level gauge 25mm thick.
Use of ultrasound thickess meters
Thickness meters are used for measuring of the materials on places where the opposite side is unreachable by usual measuring devices. As an example can serve:
- Material Abrasion – steam pipes, disc brakes
- Material corosion – water pipes, constructions, street lightning poles
- Control of the material quality – metal plate delivery for pipes in production and distribution
- Special application – measuring of some plastics, glas
Measuring over coating Echo - Echo
The basic thickness meters usually are not equipped by the Echo - Echo function - measuring over the coating. If such thickness meter is used over the coating, a measuring error occurs, as it is unknown how thick the coating is and what is the spreading speed within the coating. The measuring error can occur even in couple milimeters. The devices with the echo-echo function ignore the coating thickness - they measure only the clean material under the coating.
Thickness meter with the B-scan
This function can be found on the more advanced thickness meters with graphic display. With the B-scan you move the probe ver the material surface and the display graphically shows the opposite side (thickness histogramm).
Precise US thickness meters
These are special thickness meters for very accurate measurements with the accuracy for thousandths of milimeters. These devices use the single element probes and are able to measure even extremely thin materials.