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Method For Preventing Measurement Error Of Ultrasonic Thickness Gauge
Nov 15, 2018

Method for preventing measurement error of ultrasonic thickness gauge

The following small series talk about the prevention method of ultrasonic thickness gauge measurement error

Ultra-thin material

With any ultrasonic thickness gauge, when the thickness of the material being tested falls below the probe's lower limit of use, measurement errors will result, and if necessary, the minimum limit thickness can be measured by the test block comparison method.

When measuring ultra-thin materials, sometimes an erroneous result called "double refraction" occurs. The result is that the display reading is twice the actual thickness. Another error result is called "pulse envelope, loop jump." The result is that the measured value is greater than the actual thickness. To prevent such errors, the measurement check should be repeated when measuring the critical thin material.

2. Rust spots, corrosion pits, etc.

The rust pits on the other surface of the material to be tested will cause the reading to change irregularly. In extreme cases, there is no reading, and small rust spots are sometimes difficult to find. When pits are found or suspected, the measurement of this area is very careful, and the probe crosstalk can be selected at different angles for multiple tests.

3. Material identification error

When the instrument is calibrated with one material and another material is tested, an erroneous result will occur and care should be taken to select the correct speed of sound.

4. Probe wear

The surface of the probe is made of acrylic resin. The long-term use will increase the roughness, resulting in a decrease in sensitivity. If the user can determine the error caused by this cause, the surface of the probe can be polished with a small amount of sandpaper or whetstone to smooth and ensure parallelism. If it is still unstable, you need to replace the probe.

5. Laminated materials, composite materials

It is impossible to measure the uncoupled laminate material because the ultrasonic waves cannot penetrate the uncoupled space. Since ultrasonic waves cannot propagate at a uniform velocity in a composite material, instruments that measure thickness using the ultrasonic reflection principle are not suitable for measuring laminated materials and composite materials.

6. Effect of metal surface oxide layer

Some metals can produce a dense oxide layer on the surface, such as aluminum, which has a tight bond between the oxide layer and the substrate without a distinct interface.

However, the propagation speed of ultrasonic waves in these two substances is different, so it will cause errors, and the thickness of the oxide layer will be different. Please pay attention to the use of the same batch of materials to be tested. Micrometers or calipers measure the sample and calibrate the instrument.

7. Abnormal thickness readings

The operator should have the ability to discern abnormal readings, usually rust spots, corrosion pits, and internal defects in the material being tested will cause anomalous readings. The solution can be found in Chapters 5 and 6.

8. Use and selection of coupling agent

The couplant is used as a high frequency ultrasonic energy transfer between the probe and the material being tested. If the type is selected or the method is not used properly, it may cause errors or the coupling mark will flash and cannot be measured. The coupling agent should be used in an appropriate amount and evenly coated.

It is important to select a suitable type of couplant. When used on a smooth material surface, low viscosity couplants (such as randomly configured couplants, light motor oils, etc.) are suitable. When used on rough surfaces, or vertical and top surfaces, higher viscosity couplants (such as glycerin cream, butter, grease, etc.) can be used.