Response elliptical contours

However, it is not possible to add °C and min In a normalized factor space the factors are unitless and there is no difficulty with calculating distances. Coded rotatable designs do produce contours of constant response in the uncoded factor space, but in the uncoded factor space the contours are usually elliptical, not circular. 

The response surfaces in Figs. 2 and 3b are a part of distorted parabolic cylinder, which show a minimum ridge in the experimental domain. The response surface in Figure 3a is an inverted paraboloid (dome), and the corresponding contour plot is elliptical. The stationary point, which is the point at which the slope of the response surface is zero when taken in all directions, on this surface is within the design domain however, it is a minimum point. The response surfaces in Figs. 4a and 4b have a saddle behavior or minimax nature. The stationary point is not a maximum or a minimum point, but a saddle point. 

The fast solid contact electrode discussed in Section 10.1.3.3. was used to record ammonium activity maps over a disk of urease held in a gel (about 25 pm diameter) in a 0.1 M urea solution buffered at pH 7.2. Figure 10.16 compares two images obtained (a) with a conventional NH4+ micropipette and (b) with a solid contact NH4+ micropipette. In both cases, the tips were 3 pm in diameter, the rastering speed was lOpm/s, and the tip-substrate distance was 70 pm. The equipotential contours obtained with the solid contact electrode are circular as expected for a disk-shaped ammonium ion source, while those recorded with the conventional micropipette are elliptical due to the longer response time of the probe. 

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