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Needless to say, an external calibration source can also be used for this technique of
calibration. Suitable devices (e.g. Model 4405, Model 4422) are listed in Section 4.
6.7 Calibration shunt
A calibration shunt can be connected directly between terminals 15 and 16. Such a shunt is
generally used to tune a strain-gauge bridge according to requirements. The strain-gauge
sensor must be connected for this purpose. If the value of the calibration shunt and the tuning
step generated by it are known, then this technique can be used to calibrate the measuring
chain. The specification sheets of most strain-gauge sensors state the value of the calibration
resistance and the related tuning step as a percentage of the rated characteristic value or
directly in mV/V.
Example:
Given: Strain-gauge sensor 350 ohms
Rated characteristic value 1.5 mV/V
Tuning step 1.2 mV/V
Calibration shunt 100 kohms
Sensor excitation 5 V
Required output voltage U
I
from the amplifier 10 V
Wanted: The amplifier output voltage U
OCAL
to be set if a 100-kohm calibration
shunt has been connected.
1
st
step: Calculate and set the gain v for 1.5 mV/V as described in the example in
Chapter 6
2
nd
step: Calculate the step in the input signal caused by the shunt
U
ICAL
= Tuning step x excitation voltage
In this example: 6.000 mV
3
rd
step: Calculate the amplifier output voltage U
OCAL
(to be fine-adjusted) when
U
ICAL
(= 6.000 mV in this example) is applied instead of the input voltage
U
I
supplied by the sensor (7.5 mV in this example):
=
=
10 0.006
0.0075
= 8.000
U
I
= Output of the sensor x excitation voltage
U
O
= Output of the amplifier with U
I
In this example , 8.000 V are to be set at the amplifier output.
The switch settings are described in Chapter 4.1.3 titled Meaning of the DIP switches Page 13.
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