Campbell Scientific CR6 CR6 Measurement and Control System - Page 60
Current Measurements - Overview, 1.2.3 Resistance Measurements - Overview
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Section 5. System Overview at the point where the CR6 is grounded, a measurement error will result. For example, if the difference in grounds is 1 mV, with a copper-constantan thermocouple, the error will be approximately 25 °C. • If signal conditioning circuitry, such as might be found in a gas analyzer, and the CR6 use a common power supply, differences in current drain and lead resistance often result in different ground potentials at the two instruments despite the use of a common ground. A differential measurement should be made on the analog output from the external signal conditioner to avoid error. 5.1.2.1.2 Differential Measurements - Overview Reading List: • Differential Measurements - Overview (p. 60) • Differential Measurements - Details (p. 310) Summary Use a differential configuration when making voltage measurements, unless constrained to do otherwise. A differential measurement measures the difference in voltage between two input terminals. Its sequence is illustrated in the figure Simplified Differential-Voltage Measurement Sequence (p. 60), and is characterized by multiple automatic measurements, the results of which are averaged automatically before the final value is reported. For example, the sequence on a differential measurement using the VoltDiff() instruction involves two measurements - first with the high input referenced to the low, then with the inputs reversed. Reversing the inputs before the second measurement cancels noise common to both leads as well as small errors caused by junctions of different metals that are throughout the measurement electronics. Figure 20. Simplified Differential-Voltage Measurement Sequence Image not available. 5.1.2.2 Current Measurements - Overview Reading List: • Current Measurements - Overview (p. 60) • Current Measurements - Details A measurement of current is accomplished through the use of external resistors to convert current to voltage, then measure the voltage as explained in the section Differential Measurements - Overview (p. 60). The voltage is measured with the CR6 voltage measurement circuitry. U terminals can be configured to supply precise current excitation for use with resistive bridges. 5.1.2.3 Resistance Measurements - Overview See Reading List: • Resistance Measurements - Specifications (p. 93) • Resistance Measurements - Overview (p. 60) 60
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