Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 273

Section 8. Operation Sensors with a low signal-to-noise ratio, such as thermocouples, should normally be measured differentially. However, if the measurement to be made does not require high accuracy or precision, such as thermocouples measuring brush-fire temperatures, a single-ended measurement may be appropriate. If sensors require differential measurement, but adequate input channels are not available, an analog multiplexer should be acquired to expand differential input capacity. Refer to the appendix Analog Multiplexers (p. 538) for information concerning available multiplexers. Because a single-ended measurement is referenced to CR800 ground, any difference in ground potential between the sensor and the CR800 will result in an error in the measurement. For example, if the measuring junction of a copperconstantan thermocouple being used to measure soil temperature is not insulated, and the potential of earth ground is 1 mV greater at the sensor than at the point where the CR800 is grounded, the measured voltage will be 1 mV greater than the true thermocouple output, or report a temperature that is approximately 25°C too high. A common problem with ground-potential difference occurs in applications wherein external signal conditioning circuitry is powered by the same source as the CR800, such as an ac mains power receptacle. Despite being tied to the same ground, differences in current drain and lead resistance may result in a different ground potential between the two instruments. Hence, a differential measurement should be made on the analog output from an external signal conditioner. Differential measurements MUST be used when the low input is known to be different from ground. 8.1.2.3 Measurement Sequence The CR800 measures analog voltage by integrating the input signal for a fixed duration and then holding the integrated value during the successive approximation analog-to-digital (A/D) conversion. The CR800 can make and store measurements from up to 3 differential or 6 single-ended channels at the minimum scan interval of 10 ms (frequency of 100 Hz) using fast-measurementprogramming techniques as discussed in Fast Measurement Rates (p. 232). The maximum conversion rate is 2000 per second (2 kHz) for measurements made on a single channel. The timing of CR800 measurements is precisely controlled. The measurement schedule is determined at compile time and loaded into memory. This schedule sets interrupts that drive the measurement task. Using two different voltage measurement instructions with the same voltage range takes the same measurement time as using one instruction with two repetitions. Note This is not the case with legacy CR10(X), 21X, CR23X, and CR7(X) dataloggers. Using multiple measurement "reps" in these dataloggers reduced overall measurement time. Several parameters in CRBasic voltage measurement instructions VoltDiff() and VoltSE() vary the sequence and timing of measurements. Table CRBasic Parameters Varying Measurement Sequence and Timing (p. 274) lists these parameters. 273