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National Advisory Committee for Aeronautics, Technical Notes - A Miniature Electrical Pressure Gage Utilizing a Stretched Flat Diaphragm

A variable—air—gap inductance type of electrical pressure gage is
described that is basically 7/16 inch in diameter and 1/1» inch in thick—
ness. The gage was designed to measure pressures fluctuating at high
frequencies. It is also capable of measuring steady—state pressures
with errors of less than 1 percent of full scale and has proved to be of
value as a general-purpose electrical gage for aeronautical work where
small size and minimum reaponse to acceleration forces are important
factors.

Design equations and curves are presented which can be used to
predict the deflections and fundamental natural frequencies of stretched
flat diaphragms.

In recent aeronautical research where the trend is to higher air—
speeds, the measurement of fluctuating pressures has become more essen—
tial and at the same time more difficult. The measurements are more
essential because air—flow disturbances such as buffeting become more
pronounced with increase in Mach number. They are difficult because,
in general, pressures fluctuate at high frequencies, temperature changes
are wide, and accelerations are violent. In addition, high—speed air—
foils are thin, and models installed in high—speed tunnels are often
quite small.

For some time these trends have indicated an urgent need for elec-
trical pressure gages which will satisfy the following basic requirements:

(1) Very small size: It is often necessary to mount the gage near
the pressure orifice in small models to minimize the effect of connecting
tubing on the gage response.

(2) Good frequency response to several thousand cycles per second:
Very high mechanical resonant frequencies and either a flush diaphragm or
very high acoustical resonant frequencies are thus required.
2 NACA TN 2659

(3) Minimum response to accelerating forces: Vibratory acceler-
ations of 100g or more and centrifugal accelerations of 5000g or more
(on rotating devices such as propellers) may be expected.

(h) Minimum temperature effects: Temperatures from —500 F to 200° F
may be expected.

(5) Sensitivity at low pressures: Lowest full—scale range should be
fl pound per square inch or less; other ranges, up to $100 pounds per
square inch.

(6) Low internal impedance: Mounting space and conditions often
prohibit the use of a preamplifier or other impedance-changing device.

(7) Linear variation of pressure with output voltage: This charac—
teristic simplifies data reduction, especially where the pressures are
varying in a complex manner.

(8) Convenient output: The output should be such that amplifying
and recording equipment of standard design can be used.

(9) Simple and rugged construction: The construction should be
such that the cost of construction and upkeep is minimized.