Ultrasonic thickness gage features live, color A-scan. Source: Danatronics Corp.
Ultrasonic thickness gages have progressed since their early development in the 1960s. The first thickness gages were large and bulky. However, they used the same conventional longitudinal, or compressional wave, techniques still in use today. Thickness gages are used in a wide variety of industries including automotive, manufacturing, refineries, power plants, process control, transportation, and oil and gas. Ultrasound can be used on most engineering materials, including steel, aluminum, glass, plastics, composites and rubber. Ultrasound, by its very name, means high frequency sound. In the case of conventional ultrasonic thickness gages, the sound does not pass through air. As such, a fluid known as couplant must be applied to the test surface, much like the gel used in medical ultrasound to image babies. Also, because millions of cycles per second are used in the process, ultrasound is generally not applicable to wood, concrete or porous materials because of their air pockets. Ultrasonic thickness gages can be used for applications where access to only one side is possible. The formula used to calculate one-sided ultrasonic thickness measurements is: T=V*t/2 where (T) is thickness (V) is the acoustic sound velocity of the test material (t) is the transit time Divide by 2 to represent the round trip in the test material. Early gages only displayed the numeric value representing the thickness of the test part, for example, 0.250 inch. Later gages offered additional innovative features such as probe recognition and automatic gain control (AGC). Probe recognition automatically sets the gage to match the electronics to the new transducer. AGC is a common function used to automatically control the gain of the gage. For example, low-level signals can be automatically boosted for echo detection while saturated echoes can be decreased.
Color Coded B-Scan shows a cross sectional representation of a five-step test block with colors corresponding to alarm; red (to left) exceeds high alarm, yellow signals caution as readings are getting close to a high or low alarm and green represents a good reading. The cursor (white line on right of screen) can be used to scroll left and right to display thickness value. Source: Danatronics Corp.
Today’s gages are equipped with 50,000 to 100,000-reading data loggers, B-Scan or a cross-sectional representation of the test piece, echo-to-echo to ignore coatings and now, live waveforms, known as A-Scans.
Live A-Scans are required by many industries including shipping, refineries and nuclear plants to ensure safety. Most gages are set up to measure to the first returned echo. Without a live waveform, there is the chance a gage with only a digital readout and no waveform could be reading the second return echo-referred to as doubling. This is particularly dangerous when the operator is measuring thin parts and may not know the part is half the thickness of the gages’ reading and may be susceptible to rupture. If a digital thickness gage reads 0.040 inch, is the gage reading the correct echo at 0.040 inch, or is the gage reading the second echo and the test piece, which is only 0.020 inch?
Live waveforms also are critical in applications where a large delamination-a void inside the material-is possible.
Another potential use of gages with waveforms includes applications where disbands can occur. A disband occurs between the interface of two materials, such as rubber bonded to steel. In some instances, a gage with a live waveform can detect both delaminations and disbands.
Thickness gages with live waveforms can be used effectively in some applications; however, they should not be considered a replacement for ultrasonic flaw detectors, which have much higher pulser power, scanning speeds, bandwidth and gain control. Flaw detectors operate in a different way in terms of signal scan speed, vertical linearity and code compliance with organizations such as the American Welding Society or American Society of Mechanical Engineers.
Lobe skipping is a common error that can occur on a conventional digital thickness gage. The waveform alerts the operator when half cycles on the valid echo are not being detected, leading to erroneous thickness values.
Another problem that can occur with conventional thickness gaging is the ability to detect the presence of mode-converted echoes. A mode-converted echo is one that has changed its initial mode of compressional waves to shear waves, based mostly on geometry. Sometime between the first and second received longitudinal waves, a smaller shear wave, or mode converted echo, exists. By using the waveform, an operator can easily correct the echo by adjusting the gain and blanks.
RF waveform, displayed in green, represents a good bond. Source: Danatronics Corp.
The latest breakthrough in handheld ultrasonic thickness gages is a color display for the A-Scan. By employing the use of color in a handheld unit, the operator can quickly and easily tie the color of the waveform to problem areas. The operator completes some simple tests to determine whether a thickness is above or below a pre-determined value. If a reading goes below a certain thickness value, the color quickly changes from a green echo to a red echo, indicating an alarm. Another common test is to view the radio frequency waveform to detect a phase reversal change on bonded materials. The operator can determine whether a dis-bond condition exists because the color will change from green-a good bond-to red-a bad bond. Another technique enhanced through the use of color is to superimpose the echoes of old readings and compare them to the current waveform. For example, a waveform from a test conducted six months ago can be displayed with less contrast-in one color-while the live current waveform for the test, at the same location, is superimposed to compare signal degradation and potential wall thickness loss. Other advantages of using color include easy-to-read displays, multiple font choices, increased pixel resolution, backlight for indoor applications, direct sunlight readability, custom color palates, and large font sizes that allow one to view the thickness readings and waveform from several feet away.
Future gages will continue to employ the latest in display technology. In fact, most manufacturers will capitalize on the technology used in other industries such as that used for producing cell phones. Look for manufacturers to be innovative and offer competitive solutions with useful features. Color displays will drive this new demand for ultrasonic thickness gages that incorporate live A-Scans.
In addition, quality control professionals can look for smaller packages, faster microprocessors and the latest in measurement algorithms to ensure that each measurement, critical or routine, is backed by the latest in available technology, thus ensuring precise thickness measurements, increased production and, most importantly, improved operator safety. ndt
Dan Carnevale is president of Danatronics Corp. (Danvers, MA). For more information, call (978) 777-0081, e-mail email@example.com or visit www.danatronics.com.