606 
36 
amplifier and a moving-coil galvanometer. These latter two methods have the 
disadvantage of requiring exceedingly high gage impedance, at least 30,000 
megohms, for successful operation. The oscillographic method, on the other 
hand, requires an impedance of only about 10 megohms. The various calibra- 
tion techniques will now be described. 
Oscillographic Recording 
In this method of calibration the pressure is released by a burst- 
ing diaphragm, and the output is recorded on the screen of a cathode-ray tube. 
The gage is shunted by a mica 
capacitor and connected into the 
amplifier of a Du Mont Type 208 
cathode-ray oscillograph. A hori- 
zontal time sweep is provided by 
1000 Ibsin@ 
a TMB sweep generator and synchro- 
[+ ol 1 mitsisecona nized with the pressure release by 
- a simple trigger circuit. The out- 
Figure 21 - Calibration Record Obtained 
with the Direct Oscillographic Method Put of the crystal as it is sub- 
The pressure inside a calibration chamber is released jected to a pressure change appears 
by puncturing a thin brass diaphragm. The time as a voltage-time curve on the 
ee te a screen of the cathode-ray tube; 
see Figure 21. It is photographed 
with an Ektra camera, and the film records are measured with a micrometer 
microscope. The time required to release the pressure by puncturing a dia- 
phragm, about 0.2 to 0.3 millisecond, makes the process especially amenable 
to oscillographic recording. The event is fast enough so that the required 
RC is not dnconveniently large; on the other hand, it is slow enough so that 
the demands of flat frequency response on the amplifier and adequate beam in- 
tensity on the cathode-ray tube are not excessive. The slow descent of the 
portion of the curve to the right of the peak gives an indication of the 
large RC, 40 milliseconds, which was used. This time constant is so large 
compared to the time required for the release of the pressure that the leak- 
age error in the measurement of peak pressure, as in Figure 15 on page 25, is 
less than 1 per cent. For time calibration a sine wave of known frequency is 
fed into the amplifier from a Hewlett-Packard oscillator. The amplitude is 
calibrated by putting a step voltage from a potentiometer into the amplifier 
of the Du Mont oscillograph. 
Table 4 gives the results of a typical set of calibrations on five 
tourmaline crystals. Each KAvalue cited in Column 3 is the result of the 
number of determinations or "trials" indicated in Column 2. Thus the KA value 
