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The techniques for performing leak testing with

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The techniques for performing leak testing with acoustic emission test equipment are similar to the technique used with the ultrasonic contact leak test probe. The ultrasonic contact leak detector differs in three ways from acoustic emission test equipment. 1. A contact piezoelectric sensor (transducer) is ultrasonically coupled to the device suspected of having a leak with a waveguide in the ultrasound detection system. In acoustic emission test systems, the sensor needs to be in direct contact with the test item. 2. In acoustic emission systems, the oscillator frequency is adjustable, enabling the user to tune in the frequency generated by the leak. Not all ultrasonic contact sensors have frequency tuning capability. 3. In acoustic emission systems, the bandpass is adjustable to allow selection of the frequencies unique to the leakage frequencies but to discriminate against artifact noise sources. Ultrasonic contact sensors do not usually have this feature. The flexibility of adjusting acoustic emission test and monitoring systems for operation in various environments makes these systems very attractive for isolating the particular sound sources of interest to an inspector. With both systems, the detected sound may be further analyzed with a spectrum analyzer to distinguish the normal machinery noises, such as sound emanating from sleeves and rotating bearings, from the noise of leaks or hydraulic flow. Instruments can be tuned to reduce background noise. Specialized Techniques Liquid Leak Amplification At leakage from 10 –6 to 10 –7 std Pa·m 3 ·s –1 (10 –5 to 10 –6 std cm 3 ·s –1 ), minimal turbulence is produced. For such low leakage levels, one manufacturer recommends using a liquid leak amplifier as an ultrasonic bubble test. The liquid with a low surface tension is faster and more reliable than classic bubble tests. The bubbles do not have to be visible for leak testing. As bubbles form and collapse they produce strong ultrasonic signals, which are easily detected by the ultrasound detection device. Bubbles form and collapse almost instantly so waiting time for bubbles in low level leaks is markedly reduced. Ultrasound Detection of Leakage in Immersion Bubble Tests One manufacturer has inaugurated a semiautomatic ultrasonic signal system for a bubble testing immersion system using a detergent in the water bath. An ultrasonic transducer is suspended above the water bath and the electronic circuitry serves as an acoustic signal amplifier to detect bubble emission by ultrasonic noises. In addition to the rapidity of the test procedure, ultrasound detection of bubble emission offers several advantages. The automatic alarm feature eliminates human judgment and the tedium of attempting to observe bubble formation in water or soap solutions. The system provides a sensitivity to leakage of less than 2 × 10 –4 Pa·m 3 ·s –1 (2 × 10 –3 std 3 ·s –1 ).
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  • Fall '19
  • Acoustic Emission

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