Campbell Scientific LoggerNet Datalogger Support Software - Manual (Page 558)

Appendix F. Calibration and Zeroing
F.3.2 Offset Calibration
Offset Calibration is the act of placing a sensor into a state where the output
condition is known to be a certain value and then changing the measurement’s
offset variable so that the sensor output reads as exactly that value. It is similar
to a zeroing calibration, except that the known value is a non-zero value. By
measuring the output of the sensor in this specialized condition (the known
offset condition), the offset variable will be changed to ensure that this
condition results in a measurement value that matches it. Note that this process
only changes the offset variable that is shared between the measurement
instruction and the FieldCal instruction. The multiplier variable is unaffected.
A simple example of offset calibration would be placing an object of known
weight (such as 10 lbs.) on a scale designed to measure the mass of objects.
With a known weight on the scale, this is the condition in which the scale
should give a known reading for its output. First the calibration is triggered,
then the user informs the datalogger about the value of the known weight, and
finally the offset is adjusted to ensure that the scale gives a properly matched
reading for that condition.
To perform an offset calibration, use an argument of 1 (the number one) 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 offset event based on flags or other user-
defined conditions that occur while the program runs.
F.3.3 Two-Point Multiplier and Offset Calibration
Two-point multiplier and offset calibration uses a linear fit technique against
two different known value conditions of the sensor’s measurement. 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 of that first
condition are stored, and then the datalogger informs the user that the second
known condition should now be applied to the sensor. The second condition is
applied and its known value is then provided to the datalogger. The datalogger
then measures the second condition. When the measurement of the second
point condition is complete, a linear fit of the two points is calculated. The
results are a slope value (m value, or multiplier), and a y-intercept (b value or
offset). Thus the simple form y=mx+b is a representation of the linear fit,
where m is the new multiplier value used and b is the new offset used.
A simple example of a two-point multiplier and offset calibration would be
placing two objects of known weight (such as 5 lbs. and 15 lbs.) on a
conventional scale at two different times. With the first known weight on the
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
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