Fig. 14. Normalized sensor reading N versus distance D between the access tube and the metal sheet: (a) measured at 10 distances up to D = 96.3 mm for = (_a = 1, 16.4, 20.3, _w = 78.38); and (b) N(D) calculated from the electromagnetic model. The distance D between the access tube and the metallic cylinder is varied between 0 and 162 mm, and the permittivity was between the air and the water value. The solid lines are the best fits with the exponential model (Eq. [23]).