Campbell Scientific CR6 CR6 Measurement and Control System - Page 335
Resistance Measurements - Accuracy
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Section 8. Operation will cause erroneous measurement, shift calibration, or lead to rapid sensor decay. Other sensors, for example, LVDTs (linear variable differential transformers), require ac excitation because they require inductive coupling to provide a signal. Dc excitation will result in no measurement. CRBasic bridge-measurement instructions have the option to reverse excitation polarity to provide ac excitation and avoid ion polarization by setting the RevEx parameter to True. Note Take precautions against ground loops when measuring sensors that require ac excitation. See Ground Looping in Ionic Measurements (p. 114). 8.1.2.4.2 Resistance Measurements - Accuracy Read More Consult the following white papers at www.campbellsci.com for indepth treatments of several topics addressing voltage measurement quality: • Preventing and Attacking Measurement Noise Problems • Benefits of Input Reversal and Excitation Reversal for Voltage Measurements • Voltage Measurement Accuracy, Self- Calibration, and Ratiometric Measurements • Estimating Measurement Accuracy for Ratiometric Measurement Instructions. Note Error discussed in this section and error-related specifications of the CR6 do not include error introduced by the sensor or by the transmission of the sensor signal to the CR6. The accuracy specifications for ratiometric-resistance measurements are summarized in the tables Ratiometric-Resistance Measurement Accuracy. Table 74. Ratiometric-Resistance Measurement Accuracy 0 to 40 °C -40 to 70 °C -55 to 85 °C ±(0.015% of voltage measurement + offset)1 ±(0.025% of voltage measurement + offset)1 ±(0.03% of voltage measurement + offset)1 1Voltage measurement is variable V1 or V2 in the table Resistive-Bridge Circuits with Voltage Excitation (p. 330). Offset is the same as that for simple analog-voltage measurements. See the table Analog-Voltage Measurement Offsets (p. 315). Assumptions that support the ratiometric-accuracy specification include: • CR6 is within factory calibration specification. • Excitation voltages less than 1000 mV and excitation currents less than 1000 mV are reversed during the excitation phase of the measurement Effects due to the following are not included in the specification: o Bridge-resistor errors o Sensor noise o Measurement noise 335
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