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Transient Pressure Recording with a HighSpeed Interferometer Camera
By WILLARD E. BUCK
This paper describes a transient-pressure recording camera with a full scale pressure range (by changing diaphragms) of 3 psi to 50,000 psi, and an accuracy of one-half per cent of full scale for any range. Its stability and hysteresis are such that a single static calibration suffices for years of dynamic measurements, and its frequency response varies from 10,000 cycle/sec to 100,000 cycle/sec, depending on the pressure range of the diaphragm used. The paper includes records of interesting applications.
G
IONVENTIONAL PRESSURE gauges which have high frequency responses are built as follows: A pressure-sensitive device with the required natural frequency (usually a diaphragm or a form of Bourdon tube which has a minute displacement or rotation proportional to pressure applied) is the heart of the instrument. This small rotation or displacement is converted into an electrical response by an electromechanical transducer of the designer's choice, usually either a variable condenser or variable reluctance device. The small electrical impulse thus obtained is amplified and recorded without losing the characteristics of the original signal.
For frequency responses above about 2000 cycle/sec, the most convenient presentation is on a cathode-ray screen. However, if these fleeting signals are to
Presented on October 10, 1952, at the Society's Convention at Washington, D. C., by Willard E. Buck, University of California Los Alamos Scientific Laboratory, P. O. Box 1663, Los Alamos, N.M.
be studied, they must be recorded on photographic film; and further, if the event lasts longer than a very small fraction of a second a continuous moving film camera is usually required.
It is obvious that recording the movement of a diaphragm directly on the photographic film is highly desirable if the system is sufficiently sensitive and has the required frequency response. Such a system exists in the familiar form of interference fringes which can be recorded directly with a moving film camera.
The unique properties which make this system an ideal amplifier are worth further discussion. The amplification factor, defined as the ratio of distance moved by the center of the diaphragm to the corresponding change in fringe diameter, is 14,620 for a fringe pattern using the 5461 A line of mercury and having a distance between light maxima of 2 mm. The frequency response of such an amplifier is approximately half the frequency of the light used. In this example it would be approximately
November 1952 Journal of the SMPTE Vol. 59
369