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.
Obtain an echo signal from the thinner reference standard and adjust the instrument Delay control to
position the signal at the chosen scale division.
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.
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
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 188.8.131.52 for additional
instructions. As required, multiply the readings by a correction factor (see paragraph 184.108.40.206).
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.
ULTRASONIC LEAK TESTING
ULTRASONIC LEAK TESTING.
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 (20oC) 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.
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.
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