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Drawing number: 21089
Revision: NR
OPERATION MANUAL FOR
MODELS 122A, 122A21, 122A22, and 122A24
HELIUM BLEED PRESSURE SENSOR
2
3.0 INSTALLATION
Prepare mounting port in accordance with
instructions on installation drawing 122-1010-90.
Inspect the mounting port for burrs and tool marks at
the conical seal surface.
NOTE: A 75 end-cutting reamer, Model 040A02, is
available from PCB. Contact the factory for price
and delivery information.
Use recommended mounting torque when installing
sensors. Do not overtorque as damage to seal surface
could result.
NOTE: When attaching helium supply hose, do not
torque hose fitting through helium tube on sensor.
Back up fitting with wrench on 3/8 hex on helium
hose fitting. (Refer to note 3 on installation drawing.)
In the presence of high vibration, support helium
supply tube by clamping to rigid surfaces with
adequate strain relief loop to avoid stressing helium
tube on sensor.
For charge output Model 122A, connect 4 ft. cable to
charge amplifier and switch to "long" time constant
position and check for drift.
Clean offending cable connections with
Trichloroethylene
or equivalent solvent to restore
insulation resistance.
It is good practice to depress ground button (or place
switch in GND" position) on the charge amplifier
while attaching input cables to the amplifier. This
will prevent spurious charges which may accumulate
on sensors and cables from blowing out the input
circuitry of the charge amplifier.
Since the charge output sensors are susceptible to
decreased insulation resistance, use care to keep cable
connections clean by use of shrink tubing wherever
possible.
4.0 OPERATION
4.1 CHARGE OUTPUT MODEL
122A
For the 122A Charge Output Model, it is only
necessary to select the proper range on the charge
amplifier and proceed with the measurement.
The probe sensitivity is approximately 1.0 pC/psi.
(The exact sensitivity is given on the calibration
certificate supplied with each instrument.)
EXAMPLE: The dynamic pressure expected is
500 psi and the desired output voltage for this
pressure is 1 volt.
Simply set the range switch of the charge amplifier to
500 units per volt and dial the standardization pot to
the exact sensor sensitivity at 500 psi (from
calibration graph). The system will then yield exactly
1 volt for 500 psi of input pressure.
4.2 TIME CONSTANT
During in-place calibration, static methods (dead
weight tester) may be used in a drift-free system by
placing the charge amplifier in "LONG TC" position.
However, during actual use, the TC mode switch
should be in "SHORT" position for two reasons.
Often times, the dynamic pressure being measured
exists superimposed upon a higher static level. The
short TC position will allow the effect of this static
level to zero out quickly without effect on the
dynamic level indicated.
Also, the influx of large amounts of thermal energy,
as in rocket combustion measurements, can cause the
sensor to generate spurious transient outputs of
considerable magnitude. Using the charge amplifier
on SHORT TC alleviates this problem by shunting
these longer term charge outputs to ground while
measuring the faster dynamic pressure pulses.