Figure 2-17.—Deriving LD aim values for a process-monitoring chart

5. Subtract the initial reference values from the averaged, processed control-strip density values. When a control parameter of high density minus low density is specified for the process, determine that also. The results of these subtractions are known as the aim values. The example in figure 2-17 summarizes steps 1 through 5 to show you how the aim- or center-line values are derived for an LD step. In the example below, LD red plots four lines below the center line, LD green plots one line above the center line, and LD blue plots one line below the center line. 6. Adjustment tolerances may be applied to the aim values, as appropriate, to bring the control-strip values closer to the aim values. These adjustment tolerances are specified in each process-monitoring manual. For example, the red low-density (LD) value for your processed control strip is 0.28. The adjusted reference value is 0.32. The process-monitoring manual indicates that the aim-value adjustment tolerance for LD is ±.04. You can adjust the red center-line value by subtracting 0.04 from the adjusted reference values (0.32 – 0.04 = 0.28). The adjusted aim value of 0.28 is your new center line or aim value on your control chart. NOTE: The adjustment tolerances change for each step and each type of process. You must consult the appropriate process-monitoring manual to obtain these tolerances. 7. Tolerance limits are defined and prescribed in each process-monitoring manual. When the average control values obtained in the preceding step fall within the tolerance limits, production work may be processed and the initial reference values may be used to determine future control values for that particular code of control strips. 8. When average control values fall outside tolerance limits, it is likely that there is a mistake in the following: reading the control strips, performing the calculations, setting up the densitometer, or starting up the process. If errors are not detected, there may be something wrong with the control strip. Improper storage or handling may be the problem. If possible, start over and use a new batch of control strips. Red Green Blue Initial reference strip readings of LD (averaged) 0.31 0.52 0.70 Correction factors of LD +0.01 -0.01 Adjusted reference values 0.32 0.51 LD readings of five processed control strips (averaged) 0.28 0.21 Adjusted reference values minus processed control-strip values +0.04 +0.01 0.00 0.70 0.69 -0.01 Figure 2-17.—Deriving LD aim values for a process-monitoring chart. 2-33