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Drawing number: 21089
Revision: NR
OPERATION MANUAL FOR
MODELS 122A, 122A21, 122A22, and 122A24
HELIUM BLEED PRESSURE SENSOR
3
4.3 OPERATION - ICP
MODELS
122A21, A22, and A24
Consult Guide G-0001B for a complete description of
the low-impedance concept in instrumentation.
All PCB power units designed for ICP
use contain
built-in fault monitor meters to aid in locating circuit
faults (shorts or opens) and to identify normal
operation of the miniature impedance converting
amplifier built into the sensor.
Do not attempt to apply voltage directly to the input
pin of the sensor from any other power source. The
input current must be limited to 20 mA maximum and
the input voltage must not be higher than 35 VDC.
Consult the factory if it is desired to build a power
unit rather than to use one of those available from
PCB.
The Models 122A21, 122A22 and 122A24 are
identical except for sensitivity.
The Model 122A21 has a sensitivity of 20mV/psi
which gives a full-scale range (for +5 V out) of 250
psi.
The Model 122A22 has a sensitivity of 1.0 mV/psi
and a full-scale range of 5000 psi.
The Model 122A24 has a sensitivity of 5.0 mV/psi
and a full-scale range of 1000 psi.
NOTE: Full-scale range is limited by the helium
source pressure. The helium source pressure must be
at least twice the peak combustion pressure (static
plus dynamic) to be measured, but not more than
about ten times the lowest pressure.
This is true for all models, charge or ICP
.
5.0 CALIBRATION
5.1 CHARGE MODEL 122A
To calibrate this model, install in pressure port of
dead weight tester and apply various static pressures
recording corresponding outputs at each pressure of
interest.
Discharge the charge amplifier before each reading
and return pressure to zero before each new pressure
input.
Use the charge amplifier on "LONG" time constant
and keep cables and connections clean to avoid drift.
A D.C. digital voltmeter is ideal for readout during
calibration.
NOTE: The helium tube must be plugged with a
pressure-tight seal before attempting to calibrate to
avoid losing pressure through the helium tube.
If a hydraulic calibration system is used, be sure to
remove all traces of oil from the diaphragm passage.
5.2 ICP
MODELS 122A21, 122A22
and 122A24
Because of the shorter time constants built into these
models, static calibration methods are much more
difficult to use.
Pressures must be applied rapidly and readout must
be made with fast recording devices such as light
storage oscilloscopes or similar types of data
acquisition hardware.
A pneumatic (compressed air or hydrogen) system
with quick opening valve and reference gage is an
excellent method of calibrating these sensors.
To take full advantage of the time constant built into
the sensor, use a DC-coupled power unit such as the
PCB Model 484B06.