In any ultrasound thickness measurement application, the correct choice of the gauge is as important as the choice of the transducer, which is why in this article we are going to provide a series of tips aimed at the successful purchase of both products.
The vast majority of commercially available gauges are capable of both corrosion assessment and accurate thickness measurement on a wide range of materials. What generally varies from one technique to another is the type of transducer used. Many manufacturers offer kits that basically contain the meter, the coupling gel, and at least one pair of transducers, as we see in the figure below. This guide titled “What to consider when buying an ultrasound thickness gauge?” is primarily intended for those who purchase instruments separately and / or wish to add other transducers to their meter.
The Most Wanted and Most Recommended Features to Consider When Choosing the Thickness Gauge properly said are:
Versatility: the meter should offer a wide range of measurement possibilities in a portable, lightweight and easy-to-handle structure. This means that regardless of the use (unless it is very specific) it is important to choose a meter that allows, at least, measurements in the mode pulse-echo Y eco-eco (mode on painting).
Functionality: features such as good resolution, calibration options, high-contrast color screen, graphical representation, automatic data logging, data storage memory, touch keyboard, light and / or audible indicators, good autonomy and even various capture modes Data are all important, and we must consider them according to our needs and the budget we have.
Sturdiness: An ultrasonic thickness gauge must guarantee a long service life in the most demanding environments, therefore it must offer a compact and resistant cabinet.
Multilingual interface: This is an important detail that enables the use of the instrument by various users and in both measurement systems, both metric and imperial.
PC connectivity: the inclusion of a computer connection interface and appropriate software should satisfy the demands of professional data management. Today almost all ultrasound thickness gauges have a USB or RS232 interface and proprietary software included in the kit.
Incorporation of special technologies (for example, ThruCoat ™ and ThruPaint ™, depending on the manufacturer): Although they also use dual element transducers, these features are more advanced than the echo-echo measurement on paint, which measures the time interval between two successive bottom echoes. Rather, they use a single background echo to measure the actual thickness of the metal. Thus, the meters that incorporate them can simultaneously indicate the thickness of the metal and that of its coating (the eco-echo mode ignores this thickness), each one adjusted to its corresponding speed of sound propagation in the material. The gauge can be set to indicate only the actual thickness of the metal, so there is no need to remove paint or coating from the surface.
To these parameters we must add others that, although they are also important for the meter, pertain particularly to the selection of the appropriate transducer. To this end, when we think of a transducer we must take into account the following:
Accuracy: Many factors affect the accuracy of measurement in a given application, including proper instrument calibration, material sound velocity uniformity, sound attenuation and dispersion, surface roughness, curvature, poor coupling of the sound and lack of parallelism of the rear wall. All of these factors must be considered when selecting a meter and its transducer. With proper calibration, measurements can generally be made with an accuracy of +/- 0.01mm, and in some cases can be as close as 0.001mm. Accuracy in a given application can best be determined by using reference standards of known thickness. In general, meters that use dip or delay line transducers for Mode 3 measurements are able to determine the thickness of a part with much greater accuracy, so this detail will be critical if accurate measurements are what we are looking for.
Type of material to be measured: Common engineering materials, including most metals, ceramics, and glass, transmit ultrasound very efficiently and can be easily measured over a wide measurement range. Most plastics absorb ultrasonic energy faster and therefore have a more limited maximum measurement range, but can be easily measured in most situations. Rubber, fiberglass and many composites can be much more attenuating and often require high penetration meters with a push button / receiver optimized for low frequency operation. On the contrary, in precision measurements and / or of ultra-thin thickness it is advisable to choose transducers that operate at high frequency, in order to increase the absorption and dispersion effects of the energy of the ultrasonic pulse.
Measuring range: commercially available gauges have a measurement range of approximately 0.080mm to 635.00mm, depending on material, transducer, surface condition, and selected configuration. As stated above, thin materials are generally measured with high-frequency transducers and thick or attenuating materials are measured with low-frequency transducers. For very thin materials, such as sheet metal and sheet metal, delay line transducers are frequently used, although, like immersion transducers, they have a more restricted maximum measurable thickness due to possible interference from a multiple of the interface echo. . In some cases of wide thickness ranges and / or multiple materials, more than one type of transducer may be required.
Geometry: As we see in the figure below, as the curvature of the surface of a part increases, the efficiency of the coupling between the transducer and the part decreases, so the smaller the radius of curvature, the size of the transducer also should decrease. Measurement at very sharp radii, particularly concave curves, may require delay line transducers specially adapted to that concave shape, or dip transducers to achieve proper coupling. Delay and dip line transducers can also be used for measurement in grooves, cavities, and similar areas with restricted access. Even in some cases of measurements on a cylindrical surface, such as the cylinder wall of an engine, the use of a transducer with a rounded contact surface may be required.
Temperature: Common contact transducers can generally be used on surfaces up to about 50 ° C. The use of most contact transducers on higher temperature materials, eg freshly rolled steel, steam pipes, etc., can cause irreparable damage due to the effects of thermal expansion. In these cases, delay line transducers with heat resistant delay lines, immersion transducers or dual element transducers suitable for measuring materials up to 500ºC should always be used. Regarding the thickness gauge itself, some manufacturers offer devices with a temperature compensation function, which allows you to enter the temperature of the calibration block and manually or automatically enter the (high) temperature of the points where the measurements are taken . Thus, the device indicates the corrected thickness, which can be saved in the memory of the device.
Transducer connection type: transducers wear out with use, so it is not only advisable to have a range of frequencies and sizes to meet specific needs, but also to choose a type of connection according to the application, in order to obtain the maximum performance. In this regard, the most common connection types are (see figure above):
- Encapsulated: Permanently attached to the cable, they have a large projection area and are suitable for flat surfaces.
- Microdot type connector: removable from the cable, so that the cable can be changed if necessary, they have a smaller projection area and are indicated for surfaces with greater curvature.
- LEMO 00 type connector: offer a more wear resistant connection.