An optical alarm for vacuum traps

An optical alarm for large industrial vacuum systems has been developed to prevent overflow of vacuum filters and traps. The alarm prevents two problems: nuclear reactions when the dirt contains both radioactive materials and water; and overload of the vacuum system. The level of dirt in the trap is detected by the attenuation of light across the trap. Water in the trap is detected by light refraction at large angles of incidence to the water surface. To minimize sensitivity to dirty trap walls, a second detector measures light attenuation across the empty section of the trap and is used to normalize the first detector output. Logic circuits minimize sensitivity to large chunks of dirt moving in the trap. A logarithmic ratio technique is shown to work well at rejecting the light attenuation through the dirt film. The six-decade dynamic range of the detector circuitry together with a light source intensity more than five decades brighter than ambient light allows operation with more than a four-decade range of light transmission through the dirt film on the trap walls