INFICON Explorer Portable Gas Chromatograph - Manual (Page 118)

SiteChart Software
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Once you have stored multiple points you can select the best curve to fit the points.
If one concentration point is stored, SiteChart draws a line from this (0, 0) to the
single point and uses the line as the calibration curve. If the compound is stored at a
second, higher concentration, sample concentrations are calculated using the first
sensitivity when the concentration is below the lower concentration. For
concentrations above the lower value, SiteChart uses a sensitivity value calculated
from the slope of the line connecting the two concentration response points.
Figure 6 Linear Calibration Curve
If a third, higher concentration is stored, then the concentrations above the second
value are measured using a sensitivity value calculated from the slope of the line
connecting the second and third concentration response points. You can store up to
five concentration points.
A quadratic calibration curve fits a quadratic function to the points you have stored.
In order to use the quadratic calibration curve, you must store at least 2 or more
points. The curve will always start at (0, 0) and go through at least 2 points you have
stored and (0,0).
In order to obtain the most accurate results you should store a point that is at the top
end of the desired concentration range. If you are interested in the range 10 to 100
ppm, store a point at 100 ppm. Data past this point will be extrapolated based on a
tangent through the last stored point.