Campbell Scientific CR10 CR10 Measurement and Control - Page 187

SECTION 13. CRlO MEASUREMENTS 13.6 RESISTANCE MEASUREMENTS REQUIRING AC EXCITATION Some resistive sensors require AC excitation. These include the 207 Relative Humidity Probe, soil moisture blocks, water conductivity sensors, and wetness sensing grids. The use of DC excitation with these sensors can result in polarization, which will cause an erroneous measurement, and may shift the calibration of the sensor and/or lead to its rapid decay. The AC half bridge Instruction 5 (incorporated into the 207 relative humidity measurement Instruction 12) reverses excitation polarity lo provide ion depolarization and, in order to minimize the time excitation is on, grounds the Excitotion Voltooe *vx OV -vx excitation as soon as the signal is integrated (Figure 13.6-1). The slow integration time should never be used with a sensor requiring AC excitation because it results in the excitation lasting about 1.5 times as long, allowing polarization to affect the measurement. INFLUENCE OF GROUND LOOP ON MEASUREMENTS When measuring soil moisture blocks or water conductivity, the potential exists for a ground loop which can adversely affect the measurement. This ground loop arises because the soil and water provide an alternate path for the excitation to return to CR10 ground, and can be represented by the model diagrammed in Figure 1g.A-2. Meosurement Seouence In tegrotion Time (milliseconds) FIGURE 13.6-1. AC Excitation and Measurement Sequence for AC Half Bridge X FIGURE 13.6-2. Modelof Resistive Sensor with Ground Loop 13-21