833 
PIEZOELECTRIC GAUGES 33 
gauges used on the same measurement, even when the 
sensitivity of each gauge appeared to be stabilized. 
These difficulties were largely overcome by adoption of 
an improved construction technique, the governing 
principle of which was to prevent disturbance of the 
electrodes during the later stages of gauge assembly and 
during subsequent use. 
An assembly drawing of a single-ended UERL gauge 
is shown in Fig. 1, and a picture of the different stages 
of construction is shown in Fig. 2. Properly lapped 
tourmaline disks receive a coat of electrode material, 
such as Du Pont liquid silver No. 4391, and are baked 
for about an hour at 550°C. Four disks are then as- 
sembled in a pile, two on each side of a steel tab as 
shown in Fig. 1, and the pile is sweated together into a 
single homogeneous unit by being placed in a spring 
clamp and again heated in an oven. Provision is made in 
this operation for electrode connection to the inter- 
mediate crystal faces. 
The peripheral edges of the pile are insulated by the 
application of a thin layer of dielectric material such as 
pure latex which can be painted around the edges and 
cured in situ by baking at ca. 120°C. With the periphery 
properly insulated, contact can now be made between 
the outermost crystal faces and the grounded central 
tab. Originally this was accomplished by direct soldering 
to the electrodes, and the technique was revised because 
of the deleterious effect on the intimacy of electrode- 
crystal contact. The present technique is to make con- 
tact by means of a conducting silver paint which can be 
baked on at temperatures low enough to be compatible 
with the insulating material used around the edges of 
the unit. 
In connecting a unit of this type to a length of 
electrical cable, it is only necessary to solder the central 
tab to the cable shield and spot solder the high electrode 
leads to the cable conductor. These connections can be 
made quickly, with a minimum of heating of the gauge 
element. In this manner the electrodes are afforded the 
greatest possible protection during mounting of the unit. 
(Elements of this type have also been mounted in 
threaded inserts which can be screwed into the wall of a 
container in which pressure variations are to be studied.) 
Gauges of this design have been used to measure pres- 
sure, impulse, and energy flux in waves ranging in 
amplitude from 0.5 to 100,000 lb./in.. 
It was originally feared that the introduction of the 
metal central tab might tend to introduce spurious me- 
chanical oscillations on the pressure-time records. How- 
ever, there has so far been no evidence of the presence of 
oscillations more serious than those ordinarily observed 
with gauges of earlier design which were not provided 
with central tabs. 
The central tab principle is also vitally important be- 
cause of the strength it gives the completed unit, 
particularly where measurements are to be made in 
gaseous media in which stresses caused by the ex- 
ceedingly high particle velocities are capable of causing 
severe damage to unreinforced gauges. 
The gauge design described is also well suited to the 
construction of push-pull units. If one pair of tourmaline 
disks is arranged with positive faces connected to one 
high potential lead while the pair on the opposite side of 
the central tab has negative faces connected to another 
high potential lead, the central tab and outer pair of 
faces can all be grounded to the shield of a two-con- 
ductor cable while the two high potential leads are con- 
nected to the two center conductors, respectively. This 
SSS) 
PLAN 
FULL SCALE 
i 
ELEVATION 
SSS) 
= 
SSSSSSSSSSSSSSSSSSSSY 
PISS 
NNNNNANAANANANNNNNNN 
mad 
iE 
EXPANDED 
TYPICAL 
CONNECTION 
EXPANDED SECTION (A-A) 
H (ELECTRODE FACES 
} SWEATED TOGETHER) 
KEY 
} = CURED LATEX INSULATION 
P-STEEL PLATE 
S-—8ILVER ELECTRODE 
%- SILVER SHIELD 
T — TOURMALINE 
W- WIRING POSITIVE 
Wr WIRING NEGATIVE 
Fic. 1. Assembly drawing of UERL-type tourmaline gauge. 
results in a perfectly symmetrical, completely shielded 
unit which develops signals balanced about ground. A 
less symmetrical, but more efficient push-pull gauge can 
be made by connecting a single-ended-type unit to the 
center conductors of a two-conductor cable without 
grounding any point within the gauge. 
In the case of gauges intended for measurements in 
gaseous media, the serious effects of perturbation of the 
flow around the gauge‘ can be minimized by surrounding 
the gauge element with a streamlined housing or baffle 
of suitable shape and proportions. Various aspects of 
this design problem are still under investigation in 
several laboratories. 
C. Coating of Gauges 
After the gauge element is assembled and mounted, it 
is necessary to apply a waterproof coating, since contact 
with even minute quantities of moisture results in low 
gauge resistance with consequent impairment of the low 
frequency response. c 
Numerous coating techniques have been tested by the 
UERL group, and only a few have proved satisfactory. 
The most useful method has been the application of a 
4J. K. L. MacDonald and S. A. Schaaf, “On the estimation of 
perturbations due to flow around blast gauges,” OSRD Report 
No. 5639. 
