Is contact strip thickness gauge better than laser?

Contact gauges do not mind an oily or dirty strip of any roughness, there is no problem changing the height of the strip during measurement in inches, changing the tilt or waving of the strip thanks to the floating head that copies the unevenness of the strip. Contact gauges measure materials of any composition very accurately, so they are also used for accurate on-line calibration of X-ray gauges. The materials used for contact sensors and their cooling allow measurements with a temperature dependence of less than 1μm/200°C. The shape of the measuring tips guarantees that even the sensitive surface of the strip will not be damaged, and the service life of the sintered diamond powder measuring contact is decades. With contact gauges, it is easy to check the accuracy continuously – with a value of 0.0 µm on the display after measurement. Thanks to simple calibration using commercially available standard gauges, direct metrological traceability is guaranteed. The advantage of contact gauges is their lower prices and low cost of spare parts. Their disadvantage is the limitation of the strip speed below 900 m/min and the maximum depth of measurement from the edge of the strip to 310 mm, which is usually sufficient for connection to the control loop even for wide strips.

𝐋𝐚𝐬𝐞𝐫 gauges have guaranteed measurement accuracy limited by many conditions, which need to be found in the parameters below the line, where at least some reputable manufacturers state them. For accurate measurement of laser gauges, a clean surface is required, no oil droplets, no emulsion, no fumes, a clean environment without dust, a strip tilt below 1°,a change in the strip height of only a few mm without vibrations. With triangulation measurement methods, the problem is to check the alignment of both sensors to one axis, changing colour, roughness and glossy surface of the measured strip. Therefore, the most modern laser gauges(e.g.LMT-750A)do not use triangulation laser sensors, but four-point laser sensors of a new generation. The electronics of these four-point laser sensors can automatically check the correct alignment of the sensors against each other in one axis, because misaligned measurement will worsen the accuracy of the measurement of the corrugated strips several times over, and the automatic calibration will only hide this measurement error. The advantage is the possibility of measuring strips with a temperature above 250°C, their depth of measurement, which can be scanned over the entire width of the measured strip, and the almost unlimited speed of the measured strip.