Both sensors are much more sensitive to changes in CO 2 and C 2 H 6 than to changes in N 2 . Note the three faces of each cube in Fig. when one mole fraction is set to zero are equivalent to Fig. . Fig. is a visualization to understand the mapping a : x → m, with x in a 3D gas composition space and m in a 2D sensor response space. The image of the sphere of radius = 0.04 in gas composition space (blue, left) is the solid ellipse-like shape (blue, right) in sensor response space; they are (approximately, for the case of the ellipse-like shape) centered, respectively, at the reference gas composition x op and its associated response m op . Unlike the M = 2, G = 3 case in the toy sensor array Fig. , the image of the sphere is not exactly a solid ellipse owing to the nonlinearity of a(x).