Campbell Scientific CR9000X Measurement and Control System - Manual (Page 357)

Section 9. Program Control Instructions
9.2 Datalogger Status/Control
BiasComp
Measures bias current and adjusts the bias current DACS accordingly. This
instruction is done automatically at user program compile time. The bias
current is the amount of current that is required to flow into the input channel
in order to make the measurement. This is reduced to a minimum (<3
nanoamps) when the bias current compensation is adjusted correctly. If the
bias current compensation is not adjusted correctly, the current could rise as
high as 100 nanoamps. The major factor affecting the bias current is
temperature. When there is adequate time for all measurements, BiasComp
and Calibrate are typically run in a scan in the SlowSequence section of the
program to provide continuous adjusting of the bias current compensation and
the calibration as temperature changes. If executed in the SlowSequence, an
RC filter is applied with the previous bias compensation weighted .95 and the
new weighted .05. BiasComp uses 120 measurement slots in the task
sequencer.
The DAC values that are the results of the bias compensation appear in the
Status Table.
This instruction must not be placed inside a conditional
statement. NOTE
Calibrate
The Calibrate instruction is used to force calibration of the analog channels
under program control. Calibration is typically performed to compensate for
errors in voltage measurements due to temperature.
During calibration, the datalogger measures offset and gain on voltage ranges
and calculates calibration coefficients. Calibration occurs when a datalogger
program is compiled (typically, when the datalogger is powered up or when a
watchdog error occurs).
The major factor affecting the calibration of the analog
measurements is temperature. If calibration is not done as part
of the program, a typical shift in the calibration is 0.01 % per
degree C change from the temperature at which the program
compile calibration occurred resulting in measurement errors. NOTE
When there is adequate time for all measurements, BiasComp and Calibrate
are typically run in a Scan in the SlowSequence section of the program to
provide continuous adjusting of the calibration as temperature changes. If
executed in the SlowSequence, an RC filter is applied with the previous
calibration weighted .95 and the new weighted .05.
Calibrate uses 54 measurement slots in the Task Sequencer.
This instruction must not be placed inside a conditional
statement. NOTE
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