Abstract: Apparatus for measuring thickness, two light measuring heads (2, 3) being mounted by air bearings in housings on either side of the web (1). The measuring heads are provided with specially implemented nozzles blowing against the surface and maintaining a constant distance of 60-100 m to the respective paper surfaces. The thickness is measured by the distance between the nozzles being measured via an electrical measuring probe (13) in one, and a means (13') sensed by the measuring probe. By the special implementation of the nozzles (12, 12') and by their being pneumatically pressing (via 11) against the surfaces, great accuracy (better than 0.5 m) is achieved even if the position of the paper is altered, rapidity also being considerable. The time to indication for a thickness alternation of about 100.mu. is about 0.5 msec.

Abstract: A self-stabilizing position sensor for pneumatic measurement. A nozzle, mounted on one end of a tube, blows compressed air directly against a surface to be sensed. The tube has on its other end an outlet member with restricting openings beyond which is a throttle plate, and a member for controlling a known, essentially frictionless position sensor. The midsection of the tube extends through a compressed-air chamber and through the chamber end walls where the tube is born on air cushions. The portion of the tube inside said chamber has inlet holes for compressed air flowing towards and through the nozzle or reversely towards and through said openings. By virtue of a gap, the throttle plate constitutes a non-sealing cover for a second chamber where the openings open.