An input by-pass circuit for a current probe has first and second switches coupled between current probe inputs and current sensing circuit inputs. The hydrophobic andor dirt-repellent substance, which is a silane of the formula \u2014n\u2014m\u2014Si\u20143, wherein R is an optionally heteroatom-containing, C1\u2013C8-alkyl or aminoalkyl group and x and y each equal 0, 1, 2 or 3 and x+y=3, and a, a\u2032 and b, b\u2032 each equal 0. This AC signal is timed to determine its sustained rate. The inlet housing includes a contoured wall including an end portion positioned opposite the inlet opening, an aperture extending through the wall transverse to the first axis, divergent side wall portions on opposite sides of the inlet opening, and a necked portion having a reduced cross-section positioned downstream of the inlet opening and upstream of the aperture.