Campbell Scientific CR6 CR6 Measurement and Control System - Page 114
Ground-Potential Differences, Ground Looping in Ionic Measurements
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Section 7. Installation • Connect excitation grounds to the closest ( ) terminal on the same terminal block. • Connect the low side of single-ended sensors to the nearest ( ) terminal on the same terminal block. • Connect shield wires to the nearest ( ) terminal. Note Always connect a device ground next to the active terminal associated with that ground. Several ground wires can be connected to the same ground terminal. If offset problems occur because of shield or ground leads with large current flow, tie the problem leads directly to the ground lug. 7.6.3 Ground-Potential Differences Because a single-ended measurement is referenced to CR6 ground, any difference in ground potential between the sensor and the CR6 will result in a measurement error. Differential measurements MUST be used when the input ground is known to be at a different ground potential from CR6 ground. Ground potential differences are a common problem when measuring full-bridge sensors (strain gages, pressure transducers, etc), and when measuring thermocouples in soil. 7.6.3.1 Soil Temperature Thermocouple If the measuring junction of a thermocouple is not insulated when in soil or water, and the potential of earth ground is, for example, 1 mV greater at the sensor than at the point where the CR6 is grounded, the measured voltage is 1 mV greater than the thermocouple output. With a copper-constantan thermocouple, 1 mV equates to approximately +25 °C measurement error. 7.6.3.2 External Signal Conditioner External instruments with integrated signal conditioners, such as an infrared gas analyzer (IRGA), are frequently used to make measurements and send analog information to the CR6. These instruments are often powered by the same Vacline source as the CR6. Despite being tied to the same ground, differences in current drain and lead resistance result in different ground potentials at the two instruments. For this reason, a differential measurement should be made on the analog output from the external signal conditioner. 7.6.4 Ground Looping in Ionic Measurements When measuring soil-moisture with a resistance block, or water conductivity with a resistance cell, the potential exists for a ground loop error. In the case of an ionic soil matric potential (soil moisture) sensor, a ground loop arises because soil and water provide an alternate path for the excitation to return to CR6 ground. This example is modeled in the diagram Model of a Ground Loop with a Resistive Sensor (p. 115). With Rg in the resistor network, the signal measured from the sensor will be: 114