Campbell Scientific CR6 CR6 Measurement and Control System - Page 115
CR6 Configuration - Details
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Section 7. Installation , where Vx is the excitation voltage Rf is a fixed resistor Rs is the sensor resistance Rg is the resistance between the excited electrode and CR6 earth ground. RsRf/Rg is the source of error due to the ground loop. When Rg is large, the error is negligible. Note that the geometry of the electrodes has a great effect on the magnitude of this error. The Delmhorst gypsum block used in the Campbell Scientific 227 probe has two concentric cylindrical electrodes. The center electrode is used for excitation; because it is encircled by the ground electrode, the path for a ground loop through the soil is greatly reduced. Moisture blocks which consist of two parallel plate electrodes are particularly susceptible to ground loop problems. Similar considerations apply to the geometry of the electrodes in water conductivity sensors. The ground electrode of the conductivity or soil moisture probe and the CR6 earth ground form a galvanic cell, with the water/soil solution acting as the electrolyte. If current is allowed to flow, the resulting oxidation or reduction will soon damage the electrode, just as if dc excitation was used to make the measurement. Campbell Scientific resistive soil probes and conductivity probes are built with series capacitors to block this dc current. In addition to preventing sensor deterioration, the capacitors block any dc component from affecting the measurement. Figure 36. Model of a Ground Loop with a Resistive Sensor 7.7 CR6 Configuration - Details Reading List: • CR6 Configuration - Overview (p. 82) • CR6 Configuration - Details (p. 115) The CR6 ships from Campbell Scientific ready to communicate with Campbell Scientific datalogger support software (p. 89) on the USB port. Some applications, 115
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