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ETL 1110-8-11(FR)
15 Jul 91
f. Most water shock gages used in the past, and
those presently in use by the US Navy, employ tour-
maline gages connected by coaxial cable to the asso-
ciated signal conditioning, amplifying, and recording
systems. Tourmaline crystal water shock gages
employed in this manner are high-impedance devices.
As such, they are not compatible with normal
low-impedance instrumentation amplifier circuits. To
achieve compatibility, the high-impedance gage is
coupled to the low-impedance amplifier through a
charge amplifier or impedance converter. These are
available from several commercial sources. The
charge amplifier or impedance converter is placed as
near as possible to the gage to reduce the length of
the connecting cable. However, the connecting cable
can be several hundred feet long for the impedance
converter and possibly 1,000 to 2,000 ft long in the
case of the charge amplifier.
Figure 7. Idealization of circuit components which
g. Water shock measurements of this type are
determine RC-time constant of tourmaline gage
particularly susceptible to two noise sources: (1) that
induced by low-circuit resistance to ground (gage and
parallel with the input impedance of the charge
cable in contact with the water), and (2) water shock-
amplifier, the combined capacitance of the crystal and
induced cable noise.
the coaxial cable in parallel, and cable compensation
networks. The RC-time constant is the time it takes
h. Care must be taken to ensure that the electri-
for the voltage from the gage due to a suddenly
cal resistance between the water shock transducer
applied hydrostatic pressure to fall to 1/e = 0.368 of
circuit (gage and connecting cable) is as high as
its maximum value. Thus, the larger the RC-time
possible to limit system background noise. Electrical
constant the better the low-frequency response of the
resistance to ground and across the tourmaline crystal
gage. RC-time constants of several seconds are pos-
should be 10,000 M ohms or greater. Circuits sub-
sible with proper precautions (Cole 1948).
mersed in water with low resistance to ground pick
up stray electrical noise from the surroundings and
k. To determine the calibration factor, KA, in
are susceptible to noise from electrical energy given
picocoulombs per pounds per square inch, for the
off by the detonation of the explosive charge. Leak-
tourmaline water shock gage, a dynamic pressure
age across the crystal produces drift in charge ampli-
environment is desirable but usually not practical.
fiers and degraded low-frequency response in imped-
The calibration method normally adopted, providing
ance converters.
the RC-time constant of the gage and calibration
circuit is long enough, is to statically pressurize the
i. Coaxial cables used to connect the tourmaline
gage to some desired pressure, and then quickly
water shock gages to the charge amplifier should be
release the pressure (Cole 1948).
antimicrophonic or "low noise" cable. Standard
coaxial cable generates electrical noise when sub-
l. An electrical calibration, which takes into
jected to the high-shock environment which exists in
account amplifier gain settings and other attributes of
water near a detonation. Antimicrophonic coaxial
the electrical circuit that is connected to the gage,
cable has graphite between the outer shield and the
must also be performed. The standard technique is
inner insulator. This greatly reduces the amount of
the "Q-step" calibration circuit shown in Figure 8
charge generated on the cable due to water shock.
(from Cole 1948). If a voltage, Vs, is applied to the
capacitor, Cs, then the resulting voltage, Vc, at the
j. Figure 7 is an idealization of a tourmaline
open end of the circuit is given by:
gage connected to a charge amplifier via a coaxial
cable. The RC-time constant for the circuit is deter-
mined by the electrical resistance across the crystal in
7
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