Campbell Scientific RTDAQ Software - Manual (Page 257)

Section 9. Calibration and Zeroing
*.cal file), use the SampleFieldCal table output instruction with the
NewFieldCal system variable as the trigger.
For more information about how to use these instructions, refer to the FieldCal
instruction topic of your datalogger manual, or use the online help topic for
FieldCal within the CRBasic Editor.
9.2.2 Calibration File Details
It is important to understand the purpose and function of the calibration file
created by a CRBasic program when using the FieldCal instruction. The
calibration file has the same name as the program that creates and uses it,
except that it ends with a .cal extension. For example, myProg.CR1 would
generate a calibration file called myProg.CAL. The calibration file is located
on the same datalogger storage device as the program that creates it (e.g., CPU,
CRD, USR). The calibration file is created when the program runs and doesn’t
find an existing calibration file that it can use, and it is updated upon each
successful calibration. The calibration file contains information about the
latest calibrations performed during program execution and information that
the RTDAQ Calibration Wizard needs to step users through the calibration
process.
9.3 Five Kinds of Calibration
The FieldCal instruction family can perform five basic kinds of calibrations:
Zeroing Calibration, Offset Calibration, Two-point Multiplier/Offset
Calibration (Linear Fit), Two-point Multiplier Only Calibration, and Zero
Basis Point Calibration. These calibration types are described below.
9.3.1 Zeroing
Zeroing is the act of placing a sensor into a state where the output condition is
known to be zero and changing the measurement’s offset variable so that the
sensor output reads as zero. By measuring the output of the sensor in this
specialized condition (the zero condition), the offset variable will be changed
to ensure that the known zero condition results in a measurement value of zero.
Note that this process only changes the offset variable that is shared between
the measurement instruction and the FieldCal instruction. The multiplier is
unaffected.
A simple example of zeroing would be taking off all items from a scale
designed to measure the mass of objects. With nothing on the scale, this is the
condition in which the scale should give a “zero” reading for its output. The
calibration is triggered and the offset is adjusted to ensure the scale gives a zero
reading for that condition.
To perform a zeroing calibration, use an argument of 0 (the number zero) for
the calibration type in the FieldCal instruction of your CRBasic program. The
Calibration Wizard can be used to calculate and apply the proper offset while
the program is running in the datalogger, or code can be configured within the
CRBasic program to trigger the zeroing event based on flags or other user-
defined conditions that occur while the program runs.
The simplest way to set up a program that performs zeroing calibration is to
generate it. See Generating Calibration Programs later in this chapter.
9-3