Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 301

Section 8. Operation Table 63. StrainCalc() Instruction Equations StrainCalc() BrConfig Code Configuration Full-bridge strain gage. Half the bridge has two gages parallel to + and - , and the other half to - and + : 6 where: • : Poisson's Ratio (0 if not applicable) • GF: Gage Factor • Vr: 0.001 (Source-Zero) if BRConfig code is positive (+) • Vr: -0.001 (Source-Zero) if BRConfig code is negative (-) and where: • "source": the result of the full-Wheatstone-bridge measurement (X = 1000 * V1 / Vx) 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 referencejunction temperature provides the determination of a reference-junction compensation voltage, corresponding to the temperature difference between the 301