Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 290
MeasOff, RevDiff
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Section 8. Operation 290 Unless a Calibrate() instruction is present in the running CRBasic program, the CR1000 automatically performs self-calibration during spare time in the background as an automatic slow sequence (p. 138), with a segment of the calibration occurring every 4 seconds. If there is insufficient time to do the background calibration because of a scan-consuming user program, the CR1000 will display the following warning at compile time: "Warning when Fast Scan x is running background calibration is disabled". The composite transfer function of the instrumentation amplifier, integrator, and analog-to-digital converter of the CR1000 is described by the following equation: COUNTS = G * Vin + B where COUNTS is the result from an analog-to-digital conversion, G is the voltage gain for a given input range, and B is the internally measured offset voltage. Automatic self-calibration only calibrates the G and B values necessary to run a given CRBasic program, resulting in a program dependent number of selfcalibration segments ranging from a minimum of 6 to a maximum of 91. A typical number of segments required in self-calibration is 20 for analog ranges and 1 segment for the panel temperature measurement, totaling 21 segments. So, (21 segments) * (4 s / segment) = 84 s per complete self-calibration. The worst-case is (91 segments) * (4 s / segment) = 364 s per complete self-calibration. During instrument power-up, the CR1000 computes calibration coefficients by averaging ten complete sets of self-calibration measurements. After power up, newly determined G and B values are low-pass filtered as follows. Next_Value = (1/5) * New + (4/5) * Old This results in • 20% settling for 1 new value, • 49% settling for 3 new values • 67% settling for 5 new values • 89% settling for 10 new values • 96% settling for 14 new values If this rate of update for measurement channels is too slow, the Calibrate() instruction can be used. The Calibrate() instruction computes the necessary G and B values every scan without any low-pass filtering. For a VoltSe() instruction, B is determined as part of self-calibration only if the parameter MeasOff = 0. An exception is B for VoltSe() on the ±2500 mV input range with 250 μs integration, which is always determined in self-calibration for use internally. For a VoltDiff() instruction, B is determined as part of selfcalibration only if the parameter RevDiff = 0. VoltSe() and VoltDiff() instructions, on a given input range with the same integration durations, utilize the same G values but different B values. The 6 input-voltage ranges (±5000 mV, ±2500 mV, ±250 mV, ±25 mV, ±7.5 mV, ±2.5 mV) along with the three different integration durations (250 μs, 50-Hz halfcycle, and 60-Hz half-cycle) result in a maximum of 18 different gains (G), and 18 offsets for VoltSe() measurements (B), and 18 offsets for VoltDiff()