Drawing Number: 21101
Revision: NR
OPERATION MANUAL FOR
FREE FIELD BLAST PRESSURE PROBE
Model 137A
1
1.0
INTRODUCTION
The Model 137A Free Field Blast Pressure Probe,
charge mode, is designed for measuring field blast
and shock tunnel pressure time profiles.
This pressure probe incorporates the ST-2 element
that is virtually free from mechanical noise frequently
produced by shock, flying particles or sensor
resonance.
For applications requiring measurements from a wide
angle, a PCB-designed quartz-free field blast pressure
sensor (Series 113B50) in the shape of a pancake or
lollipop is recommended.
2.0
INSTALLATION
In field blast measurements, mount the Model 137A
incident pressure probe in an axial direction to the
blast source with the sensing surface of the probe in a
vertical plane.
In some cases, where flash temperatures such as those
generated by blasts and shock fronts are present, it
may be necessary to thermally insulate the diaphragm
to minimize signals generated by these effects.
Common black vinyl electrical tape has been found to
be an effective insulating material in many cases.
One or more layers may be used across the end of
diaphragm and adaptor.
A silicone rubber coating approximately .010
”
thick
has also been proven effective in many applications.
General Electric RTV type 106 is recommended.
Apply the rubber coating to the surface of the
diaphragm and allow it to cure in accordance with the
manufacturer
’
s instructions. (If you have ordered the
ablative coated models, further protection will not be
necessary.)
3.0
OPERATION
Using suitable cable (low-noise coaxial cable, PCB
Model 003A or equivalent), connect the sensor into
an electrostatic charge amplifier such as the Model
462A.
NOTE: Keep the input cable to the charge amplifier
as short as practicable because electrical noise at the
output of any charge amplifier is directly related to
the input cable length (capacitance).
Depress ground button of charge amplifier and adjust
electrical zero if necessary.
Range the
amplifier as desired to give the necessary
full-scale voltage.
For normal, drift-free operation, switch charge
amplifier time constant selector to "medium" or
"short" during use.
4.0
POLARITY
When subjected to increasing pressure, the Model
137A produces a positive-going output voltage.
Because most charge amplifiers are signal-inverting,
the resulting system is negative-going.
5.0
CALIBRATION
Do not try to calibrate the Series 137A statically. The
Series 137A is calibrated dynamically by PCB using
a hydraulic pulse technique.
Use charge amplifier to "medium" time constant and
allow sensor to thermally stabilize before attempting
to calibrate. Several charge amplifiers are designed
especially for use with ceramic pressure sensors for
higher frequency measurements.
For best accuracy, use the calibration certificate
supplied. Factory calibration is available for a
nominal charge.
6.0
MAINTENANCE
If the Model 137A Pressure Sensor is used in
extremely damp environments, it is advisable to
protect the cable connections with shrink tubing. If
the cable connections need cleaning, use Benzene on
a lint-free cloth.
Содержание 137A
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