Campbell Scientific CR6 CR6 Measurement and Control System - Page 327
Read More, Calibrate, Warning when Fast Scan x is running background, calibration is disabled
![]() |
View all Campbell Scientific CR6 manuals
Add to My Manuals
Save this manual to your list of manuals |
Page 327 highlights
Section 8. Operation Self-Calibration Read More Related topics can be found in Offset Voltage Compensation (p. 323). The CR6 self-calibrates to compensate for changes caused by changing operating temperatures and aging. Without self-calibration, measurement accuracy over the operational temperature range is worse by about a factor of 10. If the temperature of the CR6 remains the same, there is little calibration drift with self-calibration disabled. Note The CR6 is equipped with an internal voltage reference used for calibration. The voltage reference should be periodically checked and re-calibrated by Campbell Scientific for applications with critical analog voltage measurement requirements. A minimum two-year recalibration cycle is recommended. Unless a Calibrate() instruction is present in the running CRBasic program, the CR6 automatically performs self-calibration during spare time in the background as an automatic slow sequence (p. 158), with a segment of the calibration occurring every four seconds. If there is insufficient time to do the background calibration because of a scan-consuming user program, the CR6 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 PGIA (p. 308) and A-to-D (p. 489) converter of the CR6 is described by the following equation: COUNTS = G • Vin + B where COUNTS is the result from an A-to-D conversion, G is the voltage gain for a given input range, Vin is the input voltage connected to V+ and V-, and B is the internally measured offset voltage. Automatic self-calibration calibrates only 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 one segment for the wiring-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 CR6 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 value) + (4/5) • (old value) This results in the following settling percentages: • 20% for 1 new value, • 49% for 3 new values • 67% for 5 new values • 89% for 10 new values • 96% for 14 new values If this rate of update 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. 327
![](/manual_guide/products/campbell-scientific-cr6-cr6-measurement-control-a476969/327.png)