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ETL 1110-8-11(FR)
15 Jul 91
is used between the gage and power supply. Rise-
t. A variety of recording systems exist for stor-
times of 2 to 3 sec have been achieved with cable
ing the measured electrical records from water shock
lengths of l,000 ft between gage and amplifier.
gages. FM magnetic tape machines offer acceptable
frequency response (80 Khz) and provide extended
r. Amplifier and recording systems selected to
record times (minutes) so that the sequencing of tape
record water shock induced pressures, for both the
machine turn-on and explosive charge detonation
basic tourmaline crystal gage and the tourmaline
presents little problem.
should have sufficient sensitivity to provide a low
u. Digital transient recorders operating at a mini-
signal-to-noise ratio for the recorded pressure-time
mum of 500,000 samples a second, with 12-bit accu-
history.
racy, offer better resolution than tape recorders as
well as ease of data handling with computers. These
systems, however, record for much less time
frequency response of the individual components of
(generally from 16,000 to 256,000 data points, or
the system. Piezo-electric gages and properly
32 to 512 msec) than tape machines, and the sequenc-
terminated coaxial cables, in general, pass higher
ing between charge detonation and data capture is
frequencies than amplifiers and recording systems.
more critical.
Therefore, care must be taken to ensure that the
amplifiers and recording system have sufficient fre-
quency response to reliably record the phenomena.
resolution (8-bit) but have higher frequency response
The smaller the explosive charge or the closer the
(several megasamples per second) than the transient
free surface of the water to the measurement, the
recorders.
shorter the duration of the water shock pulse. For
example, water shock pressure measurements from a
w. Analog storage oscilloscopes, or analog oscil-
10-lb TNT equivalent explosive charge in 4 ft of
loscopes with photographic capture, provide fre-
water at a 32.5-ft range show pressure durations of
quency response into the 100's of megahertz, but
the initial pulse in the order of 20 sec (Hendry and
have much worse signal amplitude resolution than the
Welch, to be published 1991). From a practical stand
digital recording systems. Similar to the digital
point, instrumentation amplifiers with 5-percent down
recording systems, the sequencing of data capture is
points of 50 to 100 khz are readily available.
critical with the oscilloscopes.
FOR THE DIRECTOR:
JOHN A. McPHERSON
Acting Chief, Engineering Division
Directorate of Civil Works
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