SECTION VI ULTRASONIC LEAK TESTING

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 are 0.25 and 0.5 inch thick, scale divisions at 2.5 and 5.0 could be used, or for better resolution 5.0 and 10.0 could be used. In the first case, each major division on the scale equals 0.1 inch. In the second case, each major division equals 0.05 inch. In general, each major division equals 0.1 of the thickness for which the echo signal is placed at division 10. d. Obtain an echo signal from the thinner reference standard and adjust the instrument Delay control to position the signal at the chosen scale division. e. Obtain an echo signal from the thicker reference standard and adjust the instrument Range control to position the signal at the chosen respective scale division. f. Repeat paragraphs d and e until no further adjustment is necessary. Always place the same vertical position of the signals at the respective divisional markings; that is, place the point where the signal breaks the baseline, crosses the 10 percent vertical-scale line or any other convenient vertical-scale line. g. Place the search unit on the test part and obtain a signal. Read the thickness from the horizontal scale of the display, using the same vertical location on the signal that was used when placing the signals from the reference standards. On parts with irregular or pitted surfaces, take several readings and record the average. If the surface is curved, see paragraph 5.5.5.2 for additional instructions. As required, multiply the readings by a correction factor (see paragraph 5.5.3.3). h. The calibration should be rechecked each one-half hour during continuous use. If it is determined that the instrument is out of calibration, all measurements made since the last satisfactory calibration should be repeated after correcting the calibration. SECTION VI ULTRASONIC LEAK TESTING 5.6 ULTRASONIC LEAK TESTING. 5.6.1 Introduction. NOTE Std cm³/s is a unit of gas leakage rate. One std cm³/s means that one cubic centimeter of gas at atmospheric pressure (14.7 pounds per square inch) and standard temperature (20^oC) passes through the leak each second. A leakage rate may also be expressed in the derived SI units of pascal cubic meters per second Pa m³/s). Adding the unit of pressure, pascal provides a valid leakage rate without having to convert to atmospheric pressure conditions. 1 Pa m³/s = 10 std cm³/s. a. Gas leakage of f low greater than about 0.1 standard cubic centimeters per second (std cm³/s) produces ultrasonic energy with frequencies in the range of 30,000 to 50,000 Hz under most conditions. The gas leakage can be out of a pressure system or into a vacuum system. The ultrasonic leak detector utilizes a probe containing a microphone sensitive to this range of ultrasonic frequencies. The ultrasonic energy from leakage is detected. Background noise at other frequencies does not affect the detector. b. Ultrasonic energy in this relatively low-frequency range of 30,000 to 50,000 Hz travels easily through air. The leak detector can therefore detect leakage with the probe located away from the leak. The maximum detection distance depends on the leakage rate. A leakage rate of 0.1 std cm³/s can be 5-84 Change 2

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