Campbell Scientific CR3000 CR3000 Micrologger - Page 295
Self-Calibration
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Section 8. Operation 8.1.2.9 Self-Calibration Read More! Related topics can be found in Offset Voltage Compensation (p. 287). The CR3000 self-calibrates to compensate for changes induced by fluctuating operating temperatures and aging. Without self-calibration, measurement accuracy over the operational temperature range is worse by about a factor of 10. That is, over the extended temperature range of -40°C to 85°C, the accuracy specification of ±0.12% of reading can degrade to ±1% of reading with selfcalibration disabled. If the temperature of the CR3000 remains the same, there is little calibration drift with self-calibration disabled. Note Self-calibration requires the CR3000 to have an internal voltage standard. The internal voltage standard should periodically be calibrated by Campbell Scientific. When high-accuracy voltage measurements are required, a two-year calibration cycle is recommended. Unless a Calibrate() instruction is present in the running CRBasic program, the CR3000 automatically performs self-calibration during spare time in the background as an automatic slow sequence (p. 143), 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 CR3000 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 CR3000 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 CR3000 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 295