3
MODEL 8120-100A/110A STRAIN GAGE CONDITIONER OPERATION MANUAL
3
stabilization of transducer characteristics.
(b) With the transducer unloaded, set the module
output to zero using the coarse (R
c
) and fine (R
f
)
BALANCE controls. In some instances, an integral
digital indicator will be used to display the
conditioner output such as our 8120-110A. When
only the conditioner is supplied (8120-100A), an
external indicator must be used to monitor the
conditioner output.
(c) Load the transducer to a convenient up-scale value,
which is greater than one half of full scale. Adjust
the coarse (c) and fine (f) SPAN controls until the
output signal causes a reading equal to the
dead
weight
value. Remove the
dead weight
, then press
the CAL button and note the indicator reading
obtained. This reading can now be used in future
calibrations since it is related to a value obtained
thru
dead weight
calibration. To calibrate the
instrument in the future, simply press the CAL
button and adjust the SPAN controls to obtain the
reading previously recorded after
dead weight
calibration.
(d) If
dead weight
calibration is not practical and the
transducer manufacturer has supplied a calibration
resistor
(or
resistor
value),
install
the
recommended calibration resistor. Now press the
CAL button and adjust the SPAN controls until the
module output is equal to the
Equivalent Input
value simulated by the installed calibration
resistor.
(e) If
dead weight
calibration is not practical and the
transducer-calibration data is unknown, the
Equivalent Input
value for the factory-installed
calibration resistor can be approximated as
follows, assuming that the mV/V sensitivity rating
of the transducer and the bridge resistance are
known.
X = 25000 R
b
K R
c
Where X = Equivalent Input, % of full scale
R
b
= bridge resistance, ohms
K = transducer sensitivity, mV/V full
scale
R
c
= calibration resistance, ohms (59 K
installed)
Sample Calculation: Assume that K = 3.000 mv/v for a
5000 pound load cell (full scale) with a bridge
resistance of 350 ohms.
X = 25000 x 350 = 49.44% of full scale = 2472 lbs
59000
x
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4.0. Verification of Normal Operation
It is the purpose of this section to aid the user in
determining, in the event of a malfunction, to which the
Model 8120 is suspected of contributing, whether the
module is functioning normally or whether it is a
source of the observed trouble. In the event the module
requires repair the user may contact the factory service
department or the local representative for assistance.
If the module is suspected of faulty operation, observe
the following steps:
(a) If the module is totally inoperable (front panel
power indicator does not light), check the primary
power fuse (F1) located on the conditioner printed-
circuit board. If the fuse is blown, replace it with a
0.50-ampere fuse. Before reapplying power,
visually inspect the power cord wiring and the
printed-circuit board for any discrepancy, which
could have caused the overload.
(b) If the transducer has some preloading, the
BALANCE controls may not allow successful
zeroing of the module output. This condition can
be remedied by connecting a resistor (50 K - 200
K range, metal film type) from the +
Signal
terminal of the transducer to the + or -
Excitation
terminal. The
Excitation
terminal to which the
connection is made is determined by the direction
of the loading or off-zero reading.
(c) The inability to balance correctly where the
module output reads totally off scale and the
BALANCE controls have no authority can very
likely be the result of a damaged or defective
transducer or interconnect cable. This possibility
can be confirmed (or eliminated) by substituting a
transducer and cable known to be in good
condition or by simulating a balanced transducer,
using either a commercially available transducer
simulator or the simple
Star Bridge
arrangement
supplied with the 8120. The
Star Bridge
simulates
a conventional 4-arm bridge in an exact condition
of balance. Plug the
Star Bridge
into the
connector, on the rear of the instrument, in place of
the interconnect cable. Attempt to balance the
substitute bridge on the instrument. If conditions
now appear to be normal, the transducer or cable is
at fault. If the previous difficulties persist, the
module is faulty.
