Ultrasonic thickness gauges determine the thickness of a sample by very precisely measuring how long it takes for a sound pulse that has been generated by the transducer to travel through a test piece and reflect back from the inside surface or far wall. By comparing this measured interval of time with known data about the material type, it possible to calculate the thickness of the sample to an extremely high accuracy. Ultrasonic thickness gauges are quite similar in several respects to flaw detectors but, whereas, flaw detectors display A-scans of ultrasonic waveforms, thickness detectors generally provide a numerical format of exact measurements.
Ultrasonic thickness gauges have a number of applications in the field of non-destructive testing. Because sound waves reflect from boundaries between dissimilar materials they are excellent tools for measuring the thickness of paints and other coatings as part of a quality control program. Thickness gauges are also routinely used to scan storage tanks, pipelines, ship hulls, and many other things for any signs of damage or weakness that may have been caused by corrosion, environmental damage or just simply wear and tear.
Ultrasonic thickness gauges are able to work with a wide range of materials including metals, plastic, ceramics, composites, epoxies, and glass. The ability to work when able to access only one side of the test piece makes them easy to use across a range of application. A number of transducer types have also been developed to increase the versatility of the instrument.
Ultrasonic transducers usually operate at frequencies between 500 KHz and 100 MHZ. Typically, lower frequencies will be used to optimize penetration when measuring thick, highly attenuating, or highly scattering materials, while higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials. Transducers generally require an acoustic couplant, such as Propylene Glycol to ensure a complete acoustic pathway between the sensor and material being measured.
There are three types of transducers used with ultrasonic thickness gauges:
- Contact transducers are the general purpose transducer for ultrasonic thickness gauges. They, like all ultrasonic transducers, must be in direct contact with the material being tested and they emit sound waves perpendicular to the test material surface.
- Delay line transducers are used for testing very thin materials, are used as they incorporate a short plastic waveguide, or delay line, between the active element and the test piece which improves near surface resolution.
- Dual element transducers are optimal for improving resolution when test surfaces are rough or pitted from corrosion.
Calibration
Like most test and measurement equipment, ultrasonic thickness gauges need to be periodically calibrated to maintain accuracy. In the case of NDT equipment, calibrations are performed using precision test blocks that simulate conditions and serve as reference standards. Different types of test blocks simulate different conditions. Test blocks can include steps of different thicknesses for use with thickness gauges, holes and other “flaws” for use with flaw detectors, or predetermined hardness for hardness detectors.
Things to Consider When Selecting an Ultrasonic Thickness Gauge:
- What parameter (thickness, flaw detection, etc.) is being tested for?
- Does the equipment require specialized training?
- What accessories (transducers, couplant, calibration blocks) are required?
- Are there professional standards which must be adhered to?
If you have any questions regarding ultrasonic thickness gauges please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
