Magnitude-changes in the gas composition in the sensitive and intermediate directions in gas composition space-changes of v 1 and v 2 , green and yellow vectors in Fig. are mapped to the endpoints of the semimajorlike and semiminor-like axes of the ellipse-like shape-σ 1 u 1 and σ 2 u 2 , respectively. Among all magnitude-changes in the gas composition on the sphere (left), a change ∼ v 1 produces the largest-magnitude sensor array response.