should in fact be even higher than 
1.5 x 10° cycles per second to pre- 
vent the pickup from overshooting 
the peak pressure. 
The pickup must be free 
of hysteresis and have good mechan- 
ical properties. It must not be 
brittle or have a low tensile 
strength or be soluble in fresh or 
sea water. It must operate without 
permanent deformations and it must 
573 
le 650: x io7® ——_ 
Figure 1 - Typical Pressure Record 
from an Underwater Explosion 
This record was obtained by a tourmaline gage 
supplied by the Underwater Explosives Research 
Laboratory at Woods Hole; the charge was 150 
respond linearly to the pressure. grams of tetryl at 3 feet from the gage. 
Finally, some way must 
be provided for transforming the mechanical variations of the pickup dimen- 
sions into electrical variations of voltage so that a record can be obtained 
with a cathode-ray oscillograph. 
Both high=- and low-impedance pickups were developed, to provide a 
check on the validity of the pressure measurements. The two types supplement 
each other, as will be discussed later in greater detail. 
EARLIER PIEZOELECTRIC GAGES 
Before the TMB gages and the manner in which they fulfill these re- 
quirements are described, the background of development should be filled in 
somewhat more completely than was done in the Introduction. As mentioned 
there, Sir J.J. Thomson was probably the first to suggest the use of piezo- 
electricity for the measurement of explosion pressures. An early form of 
piezoelectric manometer was devised by Keys, who used a dozen tourmaline 
crystals mounted in mosaic fashion inside a brass pressure vessel 6 inches in 
diameter. So large a voltage was developed by these crystals that the output 
could be impressed directly on the deflection plates of a cathode-ray oscil- 
lograph; no prior amplification was necessary. 
A more recent and much smaller tourmaline crystal pressure gage was 
devised by Professor E.B. Wilson, Jr., and developed in collaboration with 
Dr. R.H. Cole of the Underwater Explosives Research Laboratory at the Woods 
Hole Oceanographic Institution, Woods Hole, Massachusetts (5). It consists 
of a single tourmaline crystal in the form of a thin plate, with two copper- 
plated faces or electrodes to which the cable leads are soldered. The pickup 
is insulated from the water by a thin protective coating, usually of Bostik 
cement. Typical overall dimensions of the crystal are about 0.6 inch by 0.3 
inch by 0.08 inch. A shielded, rubber-insulated Belden cable connects the 
pickup to the amplifier. 
