Campbell Scientific RTDAQ Software - Manual (Page 259)

Section 9. Calibration and Zeroing
scale (5 lbs), this is the first condition in which the scale should give a
“known” reading for its output. The calibration is triggered, the datalogger is
informed of the value of the known weight, and the measurement is read. The
datalogger then notifies the user that it is ready to measure the second point.
The second known weight is placed on the scale (15 lbs), and this is the second
condition in which the scale should give a known reading for its output. The
second point of calibration is triggered, the datalogger is informed of the value
of the second known weight, and the measurement is read. At this point the
new multiplier and offset are calculated by the datalogger and the variables are
adjusted accordingly to ensure that the scaling gives a properly matched
reading for those two conditions. For future measurements (unknowns), a
linear response will be used based on the line defined by those two points.
To perform a two-point multiplier and offset calibration, use an argument of 2
(the number two) for the calibration type in the FieldCal instruction of your
CRBasic program. The Calibration Wizard can be used to calculate and apply
the two different known conditions while the program is running in the
datalogger. The simplest way to set up a program that does two-point
multiplier and offset calibration is to generate it. See Generating Calibration
Programs later in this chapter.
9.3.4 Two-Point Multiplier Only Calibration
Two-point multiplier only calibration uses a linear fit technique against two
different known value conditions of the sensor’s measurement, but only the
slope value (multiplier) is calculated and changed. The offset is unaffected by
this calibration. The sensor is placed into the first condition, and the known
value for that condition is provided to the datalogger program. One or more
measurements in that first condition occur, and then the datalogger informs the
user that the second known condition should be applied to the sensor. When
that condition is applied, the second known value is provided to the datalogger,
and the datalogger measures the second condition. After completing the
measurement of the second point condition, a best fit of the two points is
calculated, resulting in a slope value (m value or multiplier) with the offset
assumed to be zero. Thus the simple form y=mx is a representation of the fit,
where m is the new multiplier value.
To perform a two-point multiplier only calibration, use an argument of 3 (the
number three) for the calibration type in the FieldCal instruction of your
CRBasic program. The Calibration Wizard can be used to calculate and apply
the two different known conditions while the program is running in the
datalogger.
9.3.5 Zero Basis Point Calibration
The purpose of this calibration type is to simply load the value of certain
variables into zero point variables.
This type of calibration would be used when the storing of values into variables
needs to be done as the result of a calibration event, when that value capture
needs to be triggered from the Calibration Wizard, and when the result needs to
be stored into the calibration file.
A common application for this type of calibration would be a measurement in
which a zeroing calibration is based not only on the zero offset for a single
sensor, but the zero state of multiple sensors. An expression using the loaded
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