WPI Bridge-8, Instruction Manual

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4 World Precision Instruments
Transducer Calibration
Bridge-8 provides fixed and accurate decade amplification steps. Of most interest to
the user, however, may be the conversion factor from the physical variable—grams,
mm of Hg, mm of displacement—to output voltage. Since the transducer’s conversion
factor is often not exactly known and varies with the voltage applied to the sensor
(10 volts from Bridge-8), it is therefore necessary for the user to calibrate the system
using known forces, pressures or displacements to scale the output of Bridge-8
quantitatively with the original physical parameter being measured.
Example
A typical pressure transducer is nominally rated at 5 microvolts out per volt of
excitation per mm of Hg. To compute the approximate output from Bridge-8 for this
transducer:
5 µV/V • 10 V • Amplification = 0utput µV per mm Hg
Thus, if we had selected x100 Amplification, the output per mm Hg would be
approximately 5 mV per mm Hg. Since the nominal rating of 5 µV per volt is usually
not exact, the user must, in the end, calibrate the transducer with a known hydrostatic
pressure to obtain an accurate conversion factor. (WPl’s model PM015 Pressure
Monitor/Calibrator is a suitable hydrostatic pressure calibration device.)
General Amplification
For transducer types other than resistive bridges, such as active transistor circuits,
magnetic, photocell or piezoelectric devices, the instrument’s differential amplifier may
still be used for signal amplification in differential and single-ended modes. Note that
pin 7, instrument ground, must be connected to the circuit ground of the input source
to achieve effective amplification. Careful shielding of signal generating devices will
result in lower noise “pickup” by the signal input wiring.
SHIELD
-
+
3
2
7
signal gnd
+ 5
-5
ext.
sig.
source
Specifications
Voltage Amplification .....................................x1, x10, x50, x100, x500, x1000 & adjustable
Input Impedance .................................................................................................................> 10
12
Ω
Input Leakage Current ...............................................................................< 0.1pA at 25°C, typ.
Voltage Offset Adjustment ............................................................. ± 50mV (low offset range),