0009im.jpg)
ETL 1110-8-11(FR)
15 Jul 91
the cable, then the "surge" resistance, R0, is defined
CsVs
(6)
Vc
to be:
C
Co
Cs
(8)
R0
LC
by an applied pressure P to the gage is then given by:
n. In Figure 9, R1 and R2 are made to be
V (KA)
approximately equal to the "surge" resistance R0 of
P c
(7)
the cable, and the capacitors C1 and C2 are equal to
Vp
CV
s s
the distributed capacitance, C, of the cable.
o. Some commercial water shock gages (e.g.
PCB Piezotronics, Inc.) have in-line impedance con-
verters built into the gage housing. This eliminates
the need for low-noise cable, reduces the require-
ments for high impedance between the cable and the
water, and eliminates the need for charge amplifiers.
These types of instruments are more convenient than
the older circuits. They also usually do not have as
good a low-frequency response as the tourmaline
crystals connected via properly handled coaxial cir-
Figure 8. "Q-step" calibration circuit (from Cole
cuits to a charge amplifier. There appears, however,
1948)
to be no intrinsic reason why the commercial gages
could not be configured to have longer RC-time
m. A potential problem which exists for very
constants.
high-frequency, short duration water shock measure-
ments is "ringing" or oscillation of the electrical
p. Although high cable insulation
signal in the coaxial cable. Circuit ringing manifests
(>100 M ohms) is not required for the commercial
itself as oscillations in the signal output. Proper
gages, care must be taken not to allow moisture con-
termination of both ends of the coaxial cable with
tamination of the cable and gage. If this occurs, then
capacitor/resistor networks, as shown in Figure 9,
the recorded pressure histories will fall off more
rapidly than the actual phenomena. Another symp-
tom is the recording, during part of the pressure his-
tory, of apparent negative pressures below the combi-
nation of atmospheric and hydrostatic pressures of the
overlying water.
q. The "Q-step" method is not used to provide an
electrical calibration of the transducer circuit when
using tourmaline crystal gages with the in-line
impedance converters. Instead, the manufacturer
provides a calibration constant, in terms of pounds
per square inch per volt of gage output, which takes
Figure 9. Schematic showing cable compensation
into account the characteristics of the tourmaline
network R1, R2, C1, and C2 which minimizes cir-
crystal and the charge amplifier (assuming proper
cuit ringing (from Cole 1948)
power is provided to the in-line amplifier). For this
situation, a known voltage step is normally applied to
prevents this oscillation while still providing accept-
the input to the recording system and used as a cali-
able frequency response (from Cole 1948). If L is
bration step or scale. A signal generator, available
the total distributed inductance of the cable (i.e., the
from the manufacturer, is used to simulate the water
inductance per unit length times the total cable
shock gage and validate the signal amplifying system.
length), and C is the total distributed capacitance of
To provide proper circuit dampening, a series resistor
8
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