How to Calibrate Ultrasonic Thickness Gauge?

Have you ever wondered why calibrating an ultrasonic thickness gauge is essential? 

To get a precise reading from an ultrasonic gauge and for appropriate analysis of the flaws detected, calibrating the system is a necessary step that must be taken periodically.

But then, realizing the importance of calibrating an ultrasonic thickness gage is one thing. Knowing how to carry out this calibration is a different ball game. Operators and inspection professionals must know how to calibrate an ultrasonic thickness gauge to ensure accountability and increased quality assurance.

This article gives you all the necessary information to perform an ultrasonic thickness gauge calibration. So, without further ado, let’s jump in. 

I. What is Ultrasonic Thickness Gauge Calibration?

An ultrasonic gauge calculates the thickness of a test sample by sending out ultrasonic signals and accurately timing the signals at both the sending and the receiving points. However, for overall accurate readings from the thickness gauge, it has to be configured to the speed of sound in the test sample. Also, this configuration is done along with all required zero offsets by the instrument, echo shape, and transducer type.

Hence, calibration, put simply, is the process of modifying and estimating the precision of ultrasonic measurement tools. Since there are different calibrations for different materials, their calibration values can be saved on the thickness gauge’s memory and retrieved anytime it is required in operation. 

II. Why Ultrasonic Thickness Gauge Calibration is Needed?

There are several reasons why calibration is needed for an ultrasonic thickness gauge. The most important of these reasons is ensuring the accuracy of the readings as per recognized and acceptable standards. However, that is just scratching the surface. 

Many parameters hang on the accuracy of the ultrasonic thickness gauge, which means if the readings are wrong, so will these parameters. These parameters, including density, angles, shapes, sizes, connections, and so on, are often considered when executing many ultrasonic tests. Additionally, transmission and reception of high-frequency sound waves through a test material by the ultrasound instrument detect corrosion and erosion of the test material. Hence, an uncalibrated ultrasonic thickness gauge might not be able to identify these material reductions. 

III. Ultrasonic Thickness Gauge Calibration

There are several methods used for calibrating an ultrasonic thickness gauge. These methods are highlighted below. 

• Probe Zero Procedure: The probe zero procedure is a process in which the thickness gauge employs the measurement of a known thickness material sample to estimate a zero-offset value. This makes up for the pulse transit time’s total, which describes other factors than the initial sound path of the test piece. Cable delays, electronic switching delays, couplant delays, and transducer delays account for the other factors included in the zero factors.

• One-Point Calibration: This regular calibration has no special effect on it. This process can be executed on a non-coated test material by applying the test probe close to a predetermined measuring position. The only reference point for this calibration is single, which is the zero value. That’s where its title comes from.

• Two-Point Calibration: This type of calibration entails two test material samples with separately known densities within the measurements thickness range. This might require a one-point calibration first, then a second test material with greater density is placed and calibrated to attain higher accuracy on the measurement.

• Dual Echo Calibration: The dual-echo calibration process also utilizes two test materials of two different thicknesses, with two separate reference points matching the various densities, i.e., the thinner and thicker measuring point values. This type of calibration often becomes relevant when measuring irregular and rugged surface materials. It is noteworthy that the calibrated materials match the material to be measured for the desired accuracy.

• Velocity Calibration: Velocity calibration entails the measurement of the speed of sound by the gauge as a reference sample for the test material, with the value then stored for use in estimating the thickness from measured time intervals. Factors that directly affect sound velocities must be considered, such as material composition, material density and elasticity, temperature, and grain structure.

IV. Ultrasonic Thickness Gauge Calibration Standard

Standards often refer to absolute references to which other measurements can be compared. This is not different for ultrasonic thickness gauges as they need to be compared to certain standards or absolutes to determine their correctness and consistency. 

Hence, Ultrasonic thickness gauge calibration standards are essential tools for carrying out calibration operations accurately. These standards come in varying shapes and sizes, and they are also dependent on the NDT application, the shape, and structure of the object of evaluation. One critical requirement is that the standard material must be the same as the material being inspected. This will help ascertain the accuracy of the calibration as well as the flaw you intend to detect including corrosion, abrasion, or erosions. Nonetheless, there are many calibration standards which all conform to a recognized authority, document or specification commonly used for general thickness gauge calibration. However, specially designed standards can also exist to serve a specific calibration need. 

V. Ultrasonic Thickness Gauge Calibration Procedure

With so much consideration and elucidation of ultrasonic thickness gauge calibrations, nothing can be achieved without the procedural steps for executing these calibrations. Hence, for accurate calibration, the following listed chain of calibration steps can be adopted:

1. The transducer is coupled to the thick reference sample
2. Employing the keypad, the “calibrate velocity” command is entered.
3. When the thickness reading is steady, then the ENTER button pressed
4. The displayed value must be adjusted to match the actual thickness of the thick reference sample using the keypad.
5. Then the transducer is coupled to the thin reference sample
6. With the keypad, the “calibrate zero” command is entered.
7. When the thickness reading is steady, press ENTER.
8. The displayed value must be adjusted to match the actual thickness of the thick reference sample using the keypad once again.
9. The MEASURE key is finally pressed to execute the measurement and complete the procedure.

➤ Related Article: How to Calibrate Ultrasonic Flaw Detector?

Conclusion

In conclusion, calibration of the measurement systems and equipment must be applied for precise and accurate measurement or reading. Not disputing whether measurement can’t still be performed without calibration, readings might be slightly inaccurate, and analysis would still be limited without calibration.

Furthermore, various calibration types such as dual-echo calibration, velocity calibration, and zero-probe calibration methods establish that calibration style or type relies on inspected materials as it determines the style of calibration to adopt, and which is appropriate. 

Finally, ultrasonic thickness gauge standards and charts are also necessary for the calibration process, with various standards applied to varying calibration measures. But the fact is this: the material of the standard to be used must match the material to be inspected.

With this wealth of information at your disposal, a calibration exercise for your ultrasonic thickness gauge shouldn’t give you any headaches.