
160 Industrial control systems
temperature of 76°F. From this result one can conclude that the minimum
MAG we can obtain is 20 at a temperature of 76°F and concentration of 28%.
Figure 6.5 shows a contour plot of temperature and concentration with
MAG values plotted within the experimental region. As can be seen, a mini-
mum MAG value of 5 can be achieved at a region of 80% concentration and
40°F temperature. This example, therefore, demonstrates how a one-variable-
at-a-time approach can fail to estimate the effects between two or more fac-
tors because the effect of temperature depends on the levels of concentration
in this example. This effe