Campbell Scientific RTDAQ Software - Manual (Page 255)

Section 9. Calibration and Zeroing
9.1 Calibration Essentials
9.1.1 Definition of Calibration
Calibration, in general, refers to actions taken on a measurement system to
increase its accuracy. This is usually done by matching the system’s outputs to
known, “control” values in order to increase confidence in the measurement of
future unknowns.
Campbell Scientific’s approach to calibration uses a datalogger’s measurement
and computational capability to calculate the multipliers and/or offsets to be
used by a measurement instruction to provide more accurate readings. The
process of calibration uses the datalogger to assist the operator in intelligently
and automatically setting the multiplier and offset to be used in a measurement
instruction in order to obtain more accurate output data.
Calibration is periodically necessary when there has been sensor drift or other
variation in sensor outputs. When a calibration scenario is properly in place
within the datalogger program, it is quick and easy to use a software Wizard to
change the measurement configuration at run-time. This saves time over
previously used methods, such as re-writing the CRBasic program or
interfering with measurements to obtain calibration constants manually. With
this method, changes to multipliers and offsets can be made quickly and
automatically without rewriting datalogger programs or interfering with sensor
measurements.
9.1.2 Basic Calibration Process
When calibrating with a Campbell Scientific datalogger, known and measured
values are given as inputs. The outputs of the calibration then become the new
values for the multiplier and offset variables in the CRBasic program. If
needed, these calculated multipliers and offsets can be permanently stored and
automatically reloaded upon program restart (such as when a power-cycle
occurs on the datalogger). The datalogger makes use of a calibration file
(*.cal) to store these calibration values, and load them as desired. This can be
done at datalogger startup, or at other times designated within the datalogger
program.
To evaluate calibration histories, a final storage output table can be configured
to store the results of calibrations that have been performed, and the date and
time at which those calibrations were performed. This data is separate from the
calibration file and forms a permanent history of calibration constants used
within the program.
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