Xylem XL Series H-522 - Plus - Manual (Page 101)

XL™ Series
Analog Inputs / 5 Volt Excitation. 11-7
Slope = 1, Line = 45 degrees for the same scale for X and Y.
11.6 Two Point Calibration Basics
In general math terms a linear equation or straight line equation has the basic form as shown
below.
Y=MX+B. Where Y = the calculated results.
M = the slope of the equation.
X = the input variable.
B = the offset of the equation.
Sometimes it is easier to see the relationship of the input variable and the calculated output in
graphical form. When this equation is graphed it will always produce a straight line. The angle
or steepness of the line is determined by the slope value. On a graph that uses the same scale for
the X and Y axis and a slope of 1.00 the line will have a 45 degree angle.
The slope is calculated by dividing the rise by the run. The rise is the difference between two
points on the Y axis, Y2 - Y1. The run is the difference between two points on the X axis that
correlate to the same points on the Y axis, or X2 - X1.
Slope = M = (Y2 - Y1) / (X2 - X1)
When the values for the two points are manually obtained or manually measured, use two points
with as big a difference as possible. This will help limit errors based on errors in the
measurements. For example if a sensor has a 0.0 to 5.0 output and the values for the two points
was obtained at 0.0 volts and 1.0 volt, any error would be multiplied five times when the output
was at 5.0 volts.
Slope values less than 1.00 will be less than the 45 degree angle and slopes greater than 1.00 will
have an angle greater than 45 degrees when the X and Y scales are the same. Often the scales for
the X and Y axises are changed to produce a better looking graph.