# Basic principles of Ultrasonic devices operation

Non-Destructive Testing includes many inspection techniques, among which Ultrasonic inspection has a special distinct place. Ultrasonic waves are familiar to the human civilization for a long time – if you heard how a whale sounds, you are somehow familiar with it as well. A whale listens to echoes from its own sound, and sees how far everything is, this is called echo-location. Let us dive deeper into how it all works, so that Ultrasonic testing for beginners sounds rather easy.

Just like in nature, a whale emits a wave of sound that is reflected back, allowing it to see clear, an ultrasound device’s probe creates ultrasound waves that are to enter a certain testing object. Just like in nature, a device receives an echoed ultrasonic pulse back. Then, it calculates how long it took to propagate the sound until the wave reaches any bottom, and is reflected back. Based on this, many fascinating techniques of measurement are developed.

Let us imagine a swimming pool and a swimmer. Let us simplify and imagine that the swimming speed is always the same. A swimmer starts the clock, dives into water, gets to the other side of the pool, bumps back and swims to the starting side. Once the dive is complete, the clock is stopped. With a tape-measure, the length of the pool is easily known. Based on it, we can calculate the swimmer’s velocity. With the known velocity of the swimmer, we will then easily know the length of any pool, even without the tape-measure.

It is based on a simple formula: distance = velocity * time.

You can manipulate this equation responsively to what value you need to find out.

This is exactly how ultrasonic thickness measurement works! The device emits a wave; it gets back from the bottom. For acoustic contact between the probe and the sample, a coupling liquid needs to be applied between the two.

The device remembers the time it took to get back and forth, subtracts the delay of the device, and divides the result basically by two. This is the time it took for the wave to reach a certain depth in a certain material. Based on this, the ultrasound propagation velocity is calculated. This procedure is overall called “calibration” – establishment of the procedure for further operation. Calibration is done on samples that are suitable for a certain application and convenient to the user. Samples can be of many forms and materials, intended for any possible task.

If the device is calibrated (depth-for-time ratio is known), then the device can measure the thickness of the same material with precision close to 100%.

Based on usual thickness measurement techniques, more advanced devices are developed – these are Flaw Detectors for example. When we add more hardware and software to the electronic unit of the device, we make the device “smarter” and thus can expand the array of information we receive. Thickness Gauges and Flaw Detectors can show you scans of the object you are testing – and with knowledge, you can see different flaws in objects, determine where they are and what is their form, whether the flaw is detrimental or not, and many more!

There are many types of probes, made for a vast array of applications – there are various techniques used (UTG, UFD, EMAT, PAUT intended probes), as well as measurement diapasons, application angles and many more to your choice. All the probes have their own characteristics, and these need to be taken into account upon calibration. It is up to your understanding of your task – and NOVOTEST are always ready to help you find a solution!

The main things to remember while measuring with ultrasonic equipment:

• The correct probe should be chosen for your use; For example, angle-type UT probes have another type of ultrasound waves with different velocities.
• The device always needs to be calibrated before measurement; The characteristics of the probe taken into account.
• It is advisable to always calibrate the device on the same metal that needs to be tested, in the place where one can measure thickness manually, so that the results of measurement are surely the same as intended. This is so, because different materials have different physical properties; For example, metals can be too grainy or not, plastics can have one layer or many, being compound etc.;
• Some materials are too hard or impossible to measure with ultrasound; It is advisable to firstly search this possibility; For standardized metals, one can look it up on the Internet – ultrasound velocity in different materials can be easily found. For special materials, we always advise to calibrate the device on the same metal.

There is much more to know about Ultrasonic testing, and the more knowledgeable the user is, the more successfully and quickly any job is done; NDT sphere rewards those who know how it all works, and practice makes perfect.

There are many advanced techniques in Ultrasonic Thickness Gauges (aka UTG) and Ultrasonic Flaw Detectors (aka UFD). There are also Electromagnetic Acoustic Thickness gauges (aka EMAT) and Phased Array Flaw Detectors (aka PAUT), and more tech that will be discussed later. Each of the device types is more economically and practically useful for certain tasks. For example, if you need to measure only thickness, then just a Thickness Gauge device is needed. In case there are potential uses for detecting invisible flaws inside a sample – then a Flaw Detector will come in handy, because it can measure thickness and detect flaws as well.

Even a non-professional user can learn how to use our standard UT-1M to measure correctly, then with knowledge and practice one can use more advanced devices. For example, UT-2A with A-scan requires understanding of the nature of signals, their analysis and correct conclusions. Thus, A-scan requires some education, and more for Phased Array and so on. The more you know about your device – the more you can do with it. NOVOTEST in our turn, are always ready to help you see the details of your technical tasks, to ask the right questions, and to develop understanding of Ultrasonic inspection!