Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 276
Voltage Range
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Section 8. Operation 8.1.2.5 Voltage Range In general, a voltage measurement should use the smallest fixed-input range that will accommodate the full-scale output of the sensor being measured. This results in the best measurement accuracy and resolution. The CR800 has fixed input ranges for voltage measurements and an auto range to automatically determine the appropriate input voltage range for a given measurement. The table Analog Voltage Input Ranges with CMN / OID (p. 276) lists input voltage ranges and range codes. 8.1.2.5.1 AutoRange For signals that do not fluctuate too rapidly, range argument AutoRange allows the CR800 to automatically choose the voltage range to use. AutoRange makes two measurements. The first measurement determines the range to use, and is made with the 250-μs integration on the ±5000 mV range. The second measurement is made using the appropriate range with the integration specified in the instruction. Both measurements use the settling time programmed in the instruction. Autoranging optimizes resolution but takes longer than a measurement on a fixed range, because of the two measurements required. An auto-ranged measurement will return NAN (Not-A-Number) if the voltage exceeds the range picked by the first measurement. To avoid problems with a signal on the edge of a range, AutoRange selects the next larger range when the signal exceeds 90% of a range. Auto-ranging is recommended for a signal that occasionally exceeds a particular range, for example, a Type J thermocouple measuring a temperature usually less than 476°C (±25 mV range) but occasionally as high as 500°C (±250 mV range). AutoRange should not be used for rapidly fluctuating signals, particularly signals traversing several voltage ranges rapidly. The possibility exists that the signal can change ranges between the range check and the actual measurement. Table 52. Analog Voltage Input Ranges with Options for Common Mode Null (CMN) and Open Input Detect (OID) Range Code Description mV5000 mV25001 measures voltages between ±5000 mV measures voltages between ±2500 mV mV2502 measures voltages between ±250 mV mV252 measures voltages between ±25 mV mV7_52 measures voltages between ±7.5 mV mV2_52 measures voltages between ±2.5 mV AutoRange3 datalogger determines the most suitable range 1 Append with C to enable CMN/OID and set excitation to full-scale (~2700 mV) 2 Append with C to enable CMN/OID 3 Append with C to enable CMN/OID on ranges ≤ ±250 mV, CMN on ranges > ±250 mV 276