The rotary grating method has been applied for rapid measuring of ultrasonic velocities in liquids. In Debye-Sears ultrasonic diffraction system, when an optical grating whose constant is equal to the wavelength is inserted in the light path so as to overlap the ultrasonic grating, the diffraction image is just the same as that when either one of the gratings singly exists. But when the constant of the optical grating is changed, each line of the diffraction image developes its satellite lines and the whole pattern becomes complicated. It is known that when an optical grating of the proper constant
d0 is rotated by the angle θ around the optical path, the effective constant
d of the grating is given by the law
d=
d0/sin θ. Hence, by rotating the grating and observing the position at which the satellite lines coincide with the main lines, the angle θ that corresponds to the wavelength can be determined and thus the velocity is calculated if the frequency is known.
The range of velocity measurement of the apparatus is 900 m/sec to 2000 m/sec. The accuracy of the measurement depends upon the accuracy of detecting the coincidence of the satellite lines with the main lines. In the present apparatus, the accuracy of coincidence detection is approximately ±0.2°. This means that the error of the velocity measurement is about 0.7% for medium velocity substances (for instance, water) and 0.4% for low velocity substances (for instance, chloroform).
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