2. "Simulated (Shunt)" Calibration: SHP and
SHN
This is an easier though generally less accurate technique than
"TWO-POINT (DEADWEIGHT)" CALIBRATION. It is useful,
however, when overall "deadweighting" is impossible or incon-
venient, and is good for an accuracy of about 0.2% (depending, of
course, on the accuracy of the specified EQUIVALENT INPUT, and
on the resistor/bridge tolerance and temperature). The known
calibration input is not produced by loading the transducer, as in
the "deadweight" method, but by shunting a resistor of known
magnitude across one arm of the strain-gage bridge, thereby
"simulating" a particular up-scale value of mechanical input. This
known EQUIVALENT INPUT then serves to determine the
SCALING FACTOR for the channel.
The 4077 is equipped with a 100-kΩ, 0.1%
calibration resistor
which you may, if you wish, replace with a resistor of some other
value (strain-gage transducer manufacturers often supply such
resistors with their instruments, along with the exact values of
EQUIVALENT INPUT thereby produced—contact the factory for
installation instructions).
EQUIVALENT INPUT can be approximated from a knowledge of
the
Shunt Calibration Resistance (R
); the transducer's
Bridge
Resistance (B)
; and the transducer's
Full-Scale Sensitivity (K, in
mV/V, full scale).
To determine the EQUIVALENT INPUT (X) as an approximate
percentage of full-scale output, you may use the following
equation:
a. Apply an
accurately known
value of input loading to the source
transducer—a value (positive or negative) less than 50% of
the nominal full-scale rating. If it is possible to remove all load
from the transducer, thus establishing a true
zero input
for the
4077's Channel 1, you should do so.
X
=
%
25000(mV/V)B
KR
"Computed" and "Simulated" Calibration App. E
E.2
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