PICARRO G2302 User Manual page 35

APPENDIX E – CALIBRATION
Calibrating the Picarro Analyzer
Since the Picarro Analyzer is extremely linear, it is only necessary to use
three calibration standards to calibrate each gas species (two points
define the calibration line and a third intermediate point is used for
verification). The exact value of each calibration standard is not of
particular importance as long as they span a representative range of
values over which the analyzer will typically be operated. It is reasonable
to use a concentration of zero for the low calibration value, for example.
Although it is not necessary to use more than three standards, additional
standards can be used to further constrain the linear calibration
coefficients.
To perform a calibration or verification of calibration, the user simply
introduces the first calibration standard into the analyzer for an interval
long enough for the analyzer to yield a stable measurement of that
sample. The stated concentration of the calibration sample (a calibrated
gas bottle, for example) and the value the analyzer reads for that sample
are recorded for each calibration standard used. These values can then be
plotted, as shown below, in a spreadsheet, for example, to determine the
linear relationship between the known calibration values and the
analyzer's reported values. A linear best-fit equation can be calculated
from the data. It is important to plot the analyzer's reported concentration
on the horizontal axis and the gas standards' stated concentrations on the
vertical axis. The slope and intercept of the best-fit line through these
points are the two values that are used to calibrate the analyzer. By
determining what the linear relationship is between the known calibration
values and the analyzer's reported concentration values in this way, a
calibration offset (slope and intercept) can be calculated so as to add a
correction term to the analyzer's factory or previous calibration.
Changing the analyzer's calibration is intended to be done infrequently.
Instead of recalibrating frequently to increase the accuracy of the data,
users often just verify the calibration by measuring three or more gas
standards and use the same regression procedure described here to
calculate an offset by which to correct their data offline. Using the equation
in the graph below, this would be accomplished point-by-point by
calculating the corrected data "y" by using the analyzer's data "x" so that:
Datacorrected = 0.9866•Dataraw+5.268.
Picarro G2302 Analyzer User's Guide
Rev. B 1/14/11 35