The N-Point algorithm is based on an approximation of the derivative by the slope of a regression straight line generated through the point of interest and 1, 2, or more contiguous points on each side. You must specify the total number of points to use in the derivation, which can be 3, 5, 7, or more. The N-Point algorithm is best suited when X values are equally spaced in the coordinate system of interest, as often occurs when reading mechanical gauges.
The window algorithm approximates the derivative by the weighted average of the slopes of two straight linesdrawn from the point of interestto the extreme right and left points inside a window of constant width. To use the window algorithm specify the window size, which is defined as a fraction of the full range of X values. The window algorithm is used by the system by default; the window size (derivative control) value that was used previously is shown in the dialog box. It is better to use the window algorithm when X values are not equally spaced in the coordinate system of interest, as with electronic gauges. A problem with the window algorithm is that it may yield erroneous results within the last window if the last pressure point is erroneous. That problem exists to a lesser extent if the N-Point algorithm is used.
