Patent GB-981753-A (GB)
Title: Improvements in or relating to measurement of density
IPC: G01N21/43
Filed: 1961-02-13
Abstract: 981, 753. Photo-electric measurement of density. HONEYWELL Inc. Feb. 13, 1961 [Feb. 17, 1960], No. 5306/61. Heading G1A. [Also in Division G13] A probe for sensing the concentration or density of a fluid comprises a transparent rod insertable in the fluid and having a reflective coating one end, a source positioned to transmit radiant energy into the other end of the rod, a radiant energy detector shielded from the source and positioned to receive radiant energy emerging from said other end of the rod after it has travelled along the rod by internal reflection and been returned by the reflective coating. The arrangement operates to detect the concentration or density by reason of the variation of the amount of light reflected back to a photo-cell arising from the change in refractive index of the fluid and critical angle at the interface of the rod material and the fluid. The rod 34, which may be of sapphire, or of the material known under the Trade Mark "Pyrex", is mounted in a sleeve 14 of the tube 12 for the fluid by a threaded nut 24 compressing a sealing ring 32 engaged by the thread extension 20 of a sleeve portion 16 of the probe outer casing 18 which has screw-threaded engagement with the sleeve 14. The casing 18 is provided with a housing member 40 for the light sources and photocells and has adjacent radial holes in which two lamps 194, 196 are supported by metal rings 218, 220 with cut-away portions for the lamp conductors 226, 228. An L-shaped shield 76 secured to the member 40 covers the side and upper surfaces of the photo-cell 78 having its lower surface exposed to half the end surface of the rod 34. A reference photo-cell 108 is fastened to the upper surface of the member 40 with the interposition or a filter 106 providing a spectral distribution similar to that of the light passing to the cell 78 by way of the rod 34. The lower end of the rod 34 is provided with a reflective coating 162, for example, of aluminium protected by a cap 164 of "Pyrex" or synthetic sapphire secured by fusing at 166. In a modification, Fig. 3 (not shown), the two lamps 194,196 are replaced by a single lamp of higher power, and in a further modification, Fig. 6 (not shown), the inner circular surface of the cap 164 is also coated with silver paint, the painted areas being fired and the cap 164 subsequently secured by soldering. In a further arrangement, Fig. 7, 8 (not shown), the end protecting cap is formed by a disc secured to an arcuate protecting screen member extending over the length of the rod 34 and secured at its upper end to the nut 24. In a modified arrangement for securing the rod 34 of the tube 12, Figs. 9, 10 (not shown), the main housing 18 is threaded into a ring secured by a divided and releasable clamping ring to the flanged upper position of a sleeve extending from the tube 12. The signals from the detectors 78, 108 are passed to a comparator circuit which may be operative to control a valve to pass more or less water into the fluid passing through the tube 12 and to be maintained at a predetermined density, as indicated in Fig.11 (not shown).