Device for sensing and controlling optical density

Claim 1

In a device for sensing and controlling the optical density of a fluid, a pair of tube portions which have transparent tube walls of equal optical characteristics, said tube portions being arranged in series one after the other and one of said tube portions having a cross-sectional area greater than the other, means coMmunicating with said tube portions for directing therethrough a fluid the optical density of which is to be measured, so that the fluid flows first through one of the tube portions and then through the other of the tube portions, light source means situated adjacent said tube portions for directing light transversely therethrough, so that the light which travels through the tube portions of larger cross-sectional area has a longer path of travel through the fluid than the light which travels through the tube portion which has the smaller crosssectional area, a pair of photosensitive means respectively situated adjacent said tube portions at a side thereof opposite from said light source means for respectively receiving light travelling through said tube portions, said pair of photosensitive means respectively responding to light received thereby for converting the light into electrical signals the magnitude of which corresponds to the optical density of the fluid, and means electrically connected with a said pair of photosensitive means for detecting the differential between said signals, whereby as the optical density of the fluid increases said differential will also increase, and a regulating means operatively connected with said light source means for regulating the latter as required to maintain a constant intensity in the light which is received by that one of the photosensitive means which receives light which has travelled through the tube portion of smaller cross-sectional area.