Campbell Scientific CR1000 Measurement and Control System - Manual (Page 301)

Section 8. Operation
301
Table 63. StrainCalc() Instruction Equations
StrainCalc()
BrConfig Code Configuration
6
Full-bridge strain gage. Half the bridge has two gages parallel to + and
-
, and the other half to - and + :
where:
• : Poisson's Ratio (0 if not applicable)
• GF: Gage Factor
• V
r
: 0.001 (Source-Zero) if BRConfig code is positive (+)
• V
r
: -0.001 (Source-Zero) if BRConfig code is negative (-)
and where:
• "source": the result of the full-Wheatstone-bridge measurement (X = 1000 *
V
1
/ V
x
) when multiplier = 1 and offset = 0.
• "zero": gage offset to establish an arbitrary zero (see FieldCalStrain() in
FieldCal() Demonstration Programs
(p. 153)
).
StrainCalc Example: See FieldCalStrain() Demonstration Program
(p. 162)
8.1.4 Thermocouple
Note Thermocouples are easy to use with the CR1000. They are also
inexpensive. However, they pose several challenges to the acquisition of accurate
temperature data, particularly when using external reference junctions. Campbell
Scientific strongly encourages any user of thermocouples to carefully evaluate
Error Analysis
(p. 302).
An introduction to thermocouple measurements is located in
Hands-on Exercise: Measuring a Thermocouple
(p. 42).
The micro-volt resolution and low-noise voltage measurement capability of the
CR1000 is well suited for measuring thermocouples. A thermocouple consists of
two wires, each of a different metal or alloy, joined at one end to form the
measurement junction. At the opposite end, each lead connects to terminals of a
voltage measurement device, such as the CR1000. These connections form the
reference junction. If the two junctions (measurement and reference) are at
different temperatures, a voltage proportional to the difference is induced in the
wires. This phenomenon is known as the Seebeck effect. Measurement of the
voltage between the positive and negative terminals of the voltage-measurement
device provides a direct measure of the temperature difference between the
measurement and reference junctions. A third metal (e.g., solder or CR1000
terminals) between the two dissimilar-metal wires form parasitic-thermocouple
junctions, the effects of which cancel if the two wires are at the same temperature.
Consequently, the two wires at the reference junction are placed in close
proximity so they remain at the same temperature. Knowledge of the reference-
junction temperature provides the determination of a reference-junction
compensation voltage, corresponding to the temperature difference between the