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DAMPING FLUID
Fig. 5. Drawing of quartz diaphragm and damping assembly.
damaged. The diaphragm assembly shown in the extreme left is easily detachable, and diaphragms varying in range from 3 psi to 50,000 psi can be quickly substituted. This makes a single instrument that can record the pressure wave from a hand clap as well as the internal pressures of our largest rifles.
Diaphragm Construction
For pressures in the range of 3 to 100 psi it is possible and desirable to use a quartz diaphragm instead of the steel diaphragm shown in Fig. 1. Figure 5 is a drawing of this diaphragm assembly.
Fused quartz is an almost ideal material for a pressure diaphragm. Its ratio of Young's modulus to density is high, thus allowing a high natural frequency for a given pressure range. Fused quartz also has one of the smallest temperature coefficients known, and consequently its calibration is almost independent of temperature. The most interesting feature of the quartz diaphragm, however, is its ability to be optically contacted with another piece of fused quartz. This property allows us to build a diaphragm and a backing plate optically contacted together to form a single integral unit. This system is so stable that it requires only one careful static calibration for the life of the instrument.
To sum up the features of this assembly, we have the following characteristics :
1. High frequency response for a given pressure range.
2. A negligibly small temperature coefficient.
3. A stability that permits a single calibration for the life of the gauge.
4. No detectable hysteresis.
Damping
This quartz diaphragm, with almost perfect elastic properties, will vibrate at its natural frequency for a long time when subjected to a shock wave if not properly damped. One of the most difficult problems in the design of this instrument was to find the proper damping method for the quartz diaphragm. All sorts of schemes were tried, but all systems that gave adequate damping loaded a diaphragm so much that they reduced its natural frequency two or three times. This, of course, was highly undesirable, as one of the main features of the gauge is its high frequency response. Finally, almost by accident, it was found that if the direction of motion of the damping fluid was at right angles to that of the diaphragm, the mass of the damper did not add to the mass of the diaphragm and hence the frequency response was not destroyed. To accomplish this damping it was only necessary to bring a rigid metal support close to the front surface of the diaphragm in such a way that a drop of the proper viscosity fluid could
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November 1952 Journal of the SMPTE Vol. 59