Section 8. Operation
8.1.2.4 Auto Self-Calibration — Details
Related Topics
• Auto Self-Calibration — Overview (p. 89)
• Auto Self-Calibration — Details (p. 337)
• Auto Self-Calibration — Errors (p. 475)
• Offset Voltage Compensation (p. 323)
• Factory Calibration (p. 86)
• Factory Calibration or Repair Procedure (p. 461)
The CR800 auto self-calibrates to compensate for changes caused by changing
operating temperatures and aging. Disable auto self-calibration when it interferes
with execution of very fast programs and less accuracy can be tolerated.
With auto self-calibration disabled, measurement accuracy over the operational
temperature range is specified as less accurate by 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 auto self-calibration
disabled. If the temperature of the CR800 remains the same, there is little
calibration drift if auto-calibration is disabled. Auto self-calibration can become
disabled when the scan rate is too small. It can be disabled by the CRBasic
program when using the Calibrate() instruction.
Note The CR800 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, the CR800 auto self-calibrates during
spare time in the background as a slow sequence
(p. 156) with a segment of the
calibration occurring every four seconds. If there is insufficient time to do the auto
self-calibration because of a scan-consuming user program, the CR800 will
display the following warning at compile time: Warning: Background
calibration is disabled.
8.1.2.4.1 Auto Self-Calibration Process
The composite transfer function of the PGIA (p. 349) and A-to-D (p. 489) converter of
the CR800 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.
Auto self-calibration calibrates only the G and B values necessary to run a given
CRBasic program, resulting in a program dependent number of auto self-
calibration segments ranging from a minimum of six to a maximum of 91. A
typical number of segments required in auto self-calibration is 20 for analog
ranges and one segment for the wiring-panel temperature measurement, totaling
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