376 
Brass Cover 
Brass Pan 
Solder Joint 
Quartz Crystal 
Electrode 
Mico 
Brass Tube 
Bokelite Bushing 
pies 355 
Insulated Wire 
from Electrode 
Copper Cable 
Filled with Wax 
Figure 3 - Components of the TMB Quartz Piezoelectric Gage 
The brass housing consists of three parts: a "pan" which contains the crystal; a cover; and a brass 
tube which is silver-soldered to the pan. A cylindrical, X-cut quartz crystal is cemented on one side 
to an electrode consisting of a thin copper disk, and on the other to the cover of the brass housing, 
through which it is grounded. The use of a copper disk for the electrode obviates the trouble of 
electroplating. Laterally the crystal is surrounded by an air gap which is required because quartz 
is insensitive to isotropic pressure. It is the need for this air gap which makes a housing necessary. 
aE = KA= 5 x HOG pounds Sea aR inch 
where AQ is the charge produced, in coulombs, 
AP is the pressure applied, in pounds per square inch, 
Kis the piezoelectric constant for quartz, about 
1.03 x 10-1! coulombs per pound, and 
Ais the area of one side of the crystal, in square inches. 
If a capacitance of 5000 micromicrofarads is used with a crystal 
0.05 square inch in area, the voltage sensitivity is 
KA. AV Gi sen0t!® volt = 1074 volt 
(6; AP~ 5x 10° pounds per square inch pounds per square inch 
For a pressure of 4000 pounds per square inch the signal AV is 0.4 volt. The 
voltage variation across the capacitance is led through an amplifier into the 
input of a cathode-ray oscillograph. Then the record produced on the fluo- 
rescent screen can be photographed. 
In any quartz gage intended for work under water, it is essential 
to have a watertight mount which will provide an air gap surrounding the lat- 
eral surface of the crystal. Rapid leakage of charge between the two elec- 
trodes must also be prevented. The leakage resistance of the TMB piezoelec- 
tric quartz gages is usually several thousand megohms. 
