Ultrasound – reveals what’s hidden
Metal frameworks form the backbone of our modern cities. Metal sub-structures hold houses and skyscrapers together. Steel-and-concrete bridges connect metropolises. And metal aqueducts and pipelines are like the blood vessels of this giant organism. If any of these parts fail, it puts the whole system at risk.
In order for these vital "organs" to continue to serve reliably for decades and to withstand the seasons, they must be maintained and regularly checked. Fields as diverse as medicine and heavy corrosion protection rely on the same gentle method: ultrasonic testing.
A wide span of applications …
"Ultrasound refers to acoustic waves with frequencies between 20 kHz and 1 GHz, which puts it above the range that humans can hear. Ultrasound was used technically for the first time over 100 years ago: Shortly after the sinking of the Titanic, attempts were made to use it for locating icebergs. Today, the ultrasound method has conquered a wide range of industries, from medical imaging to non-destructive materials testing.
How well the waves propagate in a medium depends primarily on the nature of the material: The stiffer the substance – that is, the higher the impedance –, the faster the wave propagates. For this reason, air and other gases transmit sound far less effectively than a solid matter like steel.
It is precisely these differences that form the basis for Fischer's UMP series, which measures wall thickness non-destructively. At the transitions between materials, for example between the steel wall of a tank and the liquid inside, the impedance changes. This leads to the ultrasound being partially reflected at this interface – an echo. Using the speed of sound in the material, the UMP calculates the wall thickness based on the time elapsed between the emission of the signal and the return of its echo.
In addition to impedance, there are several other factors that affect the propagation of sound: temperature, for example. A deviation of just a few degrees can already distort the measurement result. Therefore, the device must always be calibrated under actual measuring conditions.
… from light bulbs …
Ultrasonic testing is very flexible: From ultra-fine measurements on glass to analyzing heavily corroded steel tanks, a wide variety of applications are possible. In order to meet the many requirements, the units of the UMP series use two fundamentally different types of transducers: single- and dual-element.
The UMP 150 uses single-element transducers. Here, the piezoelectric element that generates the ultrasonic pulse is simultaneously the receiver of the echo. This allows for very fine thickness determination with a resolution of just 1 micron (0.0001 ″). These devices are most frequently used in quality assurance, in production or in incoming goods inspection. For example, in automotive manufacturing, the engine blocks can be inspected ultrasonically. Especially in applications such as cylinder walls, where a precise wall thickness is critical for part fit.
But with the UMP 150, one can test not only metals but nearly any kind of processed solid, including plastic and glass. For very thin samples, such as light bulbs, the interval between signal and echo is very short. However, the probe needs some time to "recover" after transmitting the signal before it can "hear" the echo. In such cases, one can use a delay line. This attachement increases the physical distance between the piezo element and the sample, which delays the echo just long enough that the probe is ready to receive it by the time it comes back.
… to shipping containers
The single-element transducers need smooth, parallel surfaces in order to measure accurately. When the back wall is uneven, the sound waves that arrive vertically are scattered in all directions, making for a noisy signal. This happens especially when the inner walls of tanks and pipes are corroded. To address this problem, Fischer recommends and offers dual-element transducers for rough surfaces.
On a dual-element transducer, there are two piezo elements. The transmitter and the receiver are tilted towards each other, forming an angle to the test surface. Due to the inclination of the transmitter, the sound waves tend to be concentrated and are therefore better reflected back to the receiver.
Such transducers are used by the UMP series 20 to 100, which was designed especially for testing corrosion protection. These units make it possible to determine the thickness of just the walls – even on severely pitted, rusty and uneven surfaces – and to ensure that steel girders, bridges, tanks and pipelines have not lost any of their stability.
Some applications, such as painted steel structures or tanks, require that one ignore surface coatings when determining the thickness of a metal wall. Since the speed of sound in paint is much slower than it is in steel, even a relatively thin layer can seriously distort the measurement result.
To measure only the thickness of the metal and ignore the finish, the units of the UMP series are equipped with an echo-to-echo mode. As with classical thickness measurement, the transducer emits an ultrasonic pulse that is reflected – but in this case, multiple times. The first echo arises at the transition between the paint and the metal. The second echo is then generated at the interface between the back of the metal wall and whatever lies behind it. In echo-to-echo mode, the UMP registers both reflections so it can subtract the thickness of the coating from the overall thickness.
The A-Scan displays all returning echoes. Selecting empty times (blank) allows the irrelevant echoes to be filtered out
Always the right choice
Besides these examples, the UMP series offers the right solution for many applications, as well as for every budget. The smallest full-featured device on the market, it is still extremely easy to use. Intuitive menu guidance and acoustic alarms make it easy even for non-technical personnel to get the right results. And if your measurement tasks change over time, you can easily upgrade the instruments. Many software options and measurement modes can be activated directly via the device’s keyboard. Thus, Fischer’s UMP series gauges offer all the professional powerful thickness measurement capabilities in an easy to use, compact package.
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