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Ultrasonic Wall thickness Method

The ultrasonic measuring method is based on the pulse echo principles with the calculation of ultrasonic transition time through the body to be measured. Similar to the depth sounder, a short ultrasonic pulse is generated in the transducer head by a piezo crystal and transferred into the body to me measured. As transmission of ultrasonic energy by air is at this frequency range not possible, a coupling medium is required. A suitable medium for ultrasonic transmission between the transducer and the measured body is water. When an ultrasonic signal travels through areas of different density (plastic material-water -air), there are always reflections in the areas where different materials meet

wall thickness equation :These reflected pulses return to the transducer where they are recorded. The transmission time of the pulse, counted from where it enters the measured body to where it returns through the same spot equals two times the wall thickness of the measured body. Multiplication of this transit time by the material sound speed properties, gives the value of the wall thickness.

The mechanic coupling fixtures, which direct the transducer on the test specimen, are different. According to the application environment 3 different methods can be applied.


Direct-Coupling is applied in large pipe dimension. Here a good sound throughput is achieved. This method is therefor applied when other coupling methods do not succeed.

Waterpath-Coupling is the most widley applied method. It is applied in a wide dimension range.

Immersion-Coupling is general recommended with small pipe dimensions where here are non contact and non force measurement is necessary.

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Rolf Diederichs 18.Dec.1995, info@ndt.net

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