Device on film processing machines

摘要

The invention relates to a device on film processing machines for generating control signals when film strips or film plates are present or not present in a feed slot. Control signals of this type are important because in modern processing procedures the individual steps are subject to an exact control with a precise temperature tracking and, in addition, the preparation and regeneration of baths are also to be executed automatically. The start and the end of a film and the film width are to be detectable. Previous systems which had mechanical sensors or reflected-light barriers had the inherent risk of damaging the film mechanically or the risk, at least in the case of infrared-sensitive films, of strips forming due to the light of the light emitting diodes. Mechanical sensors or switches can furthermore be corroded by the processing chemicals. These disadvantages are removed according to the invention in that compressed air is blown transversely through the feed slot onto NTC or PTC conductors which are cooled when no film is present. An inserted film interrupts the cooling. The rise in temperature causes a change in current which can be easily measured and is used to generate a control signal.