Abstract: The invention describes an apparatus for preventing gripping of objects having wrong dimensions or orientation. The apparatus comprises a part handler, e.g., a holder for parts to be treated in a chemical reactor, where the parts has to be transferred from a working position to a temporary storage. The holder may have different shapes, e.g., rectangular, elliptical, or circular, and is provided with positioning openings or recesses for engagement with pins or semispherical elements on the engaging surface of the part handler. The apparatus is provided with at least two through beam optical sensor units with adjustable divergence of the light beams emitted from the light emitting to the light-receiving element. The sensor units are located near the edge area of the holder.