Hach DR/2010 Instrument Manual page 60

Figure 18
Calibration Curve
If only points A and F are entered, the instrument bases its calculation on
a straight line (A-F), and the result is grossly in error. By adding the data
point D, the curve is greatly improved with the two straight lines A-D and
D-F, but in some regions of the curve, measurements still may not be
accurate enough. Further improvement is gained when data points B, C
and E are entered, giving straight lines A-B, B-C, C-D, D-E and E-F.
Table 7 briefly describes the Create Program menu. In-depth instructions
of the menu selections follow the table.
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