As opposed to measurements on flat surfaces, convex surfaces produce larger measurement values and concave surfaces produce smaller measurement values.

Instruments do not need any additional calibration when measurements are made on surfaces whose curvature radii are larger than the reference values of the smallest convex curvature radii (r_min) listed in the table.

	rmin for steel/iron	rmin for non-ferrous metals
Surfix® Pro S with 1.5mm probe Surfix® S with 1.5mm probe Surfix® Pocket Surfix®	20 mm / 0.8 "	50 mm / 2 "
Surfix® Pro S with 3.5mm probe Surfix® S with 3.5mm probe Surfix® easy	30 mm / 1.2 "	75 mm / 3 "
Surfix® Pro S with FN1.5/90° probe Surfix® S with FN1.5/90° probe	25 mm / 1 "	65 mm / 2.6 "
Surfix® Pro S with F10 probe Surfix® S with F10 probe	50 mm / 2 "	-

With smaller curvature radii, an uncoated object should be tested having the same curvature to see whether the zero value or the measurement value can be accurately measured sufficiently with a calibration foil or whether a calibration must be carried out on the curved surface due to the excessive deviation.