Measuring Wall Thickness with Ultrasonic Gauges


By Rob Weber, Fischer Technology, Inc.

Unseen corrosion of steel structures poses a great risk to people and property. Water and chemical tanks have collapsed, bridges weakened and hand rails have failed, all from corrosion that cannot be seen or noticed from the outside. Therefore, a reliable, non-destructive test method is required in order to evaluate the integrity of these structures.

Handheld ultrasonic gauges have been available for over 50 years and are used in a wide variety of industries, including power generation, oil and gas, refineries and petro-chemical manufacturing plants. Ultrasonic gauges are proven and effective instruments for measuring the wall thickness of steel and other engineered materials. Similar to a boater’s depth finder, the ultrasonic transducer sends out an ultrasonic pulse and listens for the return echo. The time from the initial pulse, or “main bang,” to the return echo is used to calculate distance.

Measuring wall thickness with an ultrasonic gauge

Operational Challenges

There are challenges one faces when attempting to measure the thickness of structural steel. Steel is subject to corrosion and normally coated with paint. Therein lies the challenge in measuring the wall thickness of a coated steel surface: How do we differentiate the thickness of the paint coating from the thickness of the steel?

Conventional ultrasonic gauges don’t separate the paint from the steel and will yield errors because they show a total thickness (paint and steel). When using a conventional ultrasonic gauge, the inspector must first remove the paint coating from the test area, inspect the location and then repaint the surface — adding unnecessary steps, which raises costs.

Modern ultrasonic gauges offer solutions to this measurement challenge. Gauges analyze the acoustic signature of the material and separate the paint layer from the steel using the “echo to echo” evaluation technique. The inspector can measure the thickness of the steel (or other materials) through a paint coating, thereby saving the time and expense of paint removal and touch-up.

Fischer's line of UMP Ultrasonic gauges

Modern Developments

Several other features available in modern gauges provide invaluable tools to the inspection professional. A-scans show the actual waveform of the ultrasonic signal. A-scans are very useful in evaluating the quality of a measurement, ideal for evaluating thin materials, and may expose disbonded surfaces and material flaws. B-scans offer a graphical, side (cross-sectional) view of the measurement area and allow an inspector to “see” thin areas as compared to other measurement locations.

Current generation gauges may offer other features designed to improve their usability and functionality. Some gauges include vibrating, tactile alarms that provide the inspector instant feedback when working in loud environments. Visual, color-coded displays offer a user visual indication when a measurement is outside specified inspection limits. Large displays and easy-to-navigate menus improve the usability of the gauge, letting users easily tap into the full functionality of an instrument.

Fischer Technology offers several models of modern ultrasonic gauges to the corrosion inspector, from the base model UMP20 to the upgraded UMP40 and top level UMP100. Housed in a compact, IP54 rated case, Fischer’s ultrasonic gauges offer a wide selection of transducers to meet the most challenging measurement needs.

Usability Features

With an instinctive keypad and intuitive menu options, the UMP line provides a quick-to-master, modern user interface. Additionally, Fischer’s UMP line of ultrasonic gauges allows for field upgrades to the unit's firmware. This easy upgrade path unlocks new features, capabilities and functions and can be done over the phone.

As for data reporting, current trends are pushing the inspection industry towards paperless inspection reports. Inspectors and facility owners should certainly look to manufacturers offering software packages that allow for easy offloading of data to a PC. Software, such as Fischer’s PC UltraDatex, allows users to easily transfer data into an attractive and usable electronic report.

Inspectors and facility owners all rely on accurate thickness measurements to evaluate the integrity and safety of critical structures. It is vitally important to use accurate gauges and understand the critical components when measuring wall thickness in the corrosion industry.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

Rob Weber, Fischer Technology, Inc.

Rob Weber is Fischer Technology’s Senior Product and Technical Support Engineer. He has been working with Fischer Technology for over 10 years and
has extensive experience with coating thickness instrumentation and measurements. Rob frequently attends and presents at industry shows such as NACE, SSPC, and PCI.




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