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Outer and Inner Diameter Measurement

Principles of measurement

This method has been used for many years in the testing of tubes for nuclear reactors. The accuracy of this method is about 1/100 and is independent on process condition and material properties.

Just as the wall thickness can be determined by measuring the transit time between two successive echo pulses, the distance between the transducer and the surface of the pipe can also be measured. The acoustic transit time from the sensor to the surface echo of the pipe determines the distance. If this is done with transducers which are installed across from each other, and if the distance between the transducers is calibrated just once, the outside diameter can be determined. When a number of transducers or reversible moving are used, correspondingly many test axes are obtained, and thus average value, minimum, maximum and ovalness can be determined.

The inside diameter is determined from the wall thickness measured using the same transducers. The wall thickness and diameter are measured practically simultaneously by means of rapid electronic multiplex process in the region around approx. 3 kHz.

As is well known, the water acoustic transit time is a function of the water temperature. This problem can also be solved using ultrasonic technology. An additional ultrasonic transducer is integrated in the test chamber, and measures the sound speed of the water across a reference path. A deviation of the sound speed due to variations in temperature is automatically compensated cyclically when distance is measured.

With the combination of wallthickness it is a economically complete solution achieved. Thus a separate diameter measurement unit can be saved. Further advantages is the central geometry data record, simple operation with no complicate interface solution. The measurement principles is simple, of course needs a sophisticated ultrasonic electronics.

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Rolf Diederichs 21.Dec 1995, info@ndt.net

/DB:Article /AU:Diederichs_R /IN:NDTnet /CN:DE /CT:UT /CT:thickness /CT:process /CT:plastic /ED:1996-01