Abstract: PCT No. PCT/CH78/00023 Sec. 371 Date May 23, 1979 Sec. 102(e) Date May 23, 1979 PCT Filed Oct. 2, 1978 PCT Pub. No. WO79/00180 PCT Pub. Date Apr. 19, 1979In the measuring and control technics, indicators for a limit value and measurement value working for example on an optoelectronic basis are used to determine magnitudes of physical and chemical nature. For solving such problems, particularly in fields where there are risks of explosion, a reference light beam (20 to 23) is used in parallel to the measuring light beam (13,15 to 17) representing the magnitude measured. A difference signal (36) is obtained resulting from both light beams and representing the magnitude of the information. With this signal, indicators of universal use for limit value and measurement value can be built, working in a precise and reliable way, even in severe operating conditions, by using preferably light guides (15,17,21 to 23).

Abstract: An optical-electrical liquid probe for sensing the presence or absence of a liquid comprises a light emitter and a light receiver housed within a translucent hollow body. When there is no liquid adjacent a sensing part of the body, the light rays from the emitter pass to the receiver by refraction through the walls of the body. The presence of liquid adjacent the sensing part of the body causes the light rays to be refracted in such a manner that they no longer reach the receiver with the result that the state of the receiver changes.