In 1999 Indianapolis-based Shelby Materials set up a new ready-mixed concrete plant on the southeast side of the metropolitan area. The company included a wet concrete recycling system that collects returned concrete and allows the plant operator to add gray water to tightly controlled mix designs. The production team decided to focus on establishing a control system based on maintaining desired gray water density.

The system is simple to install and operate. Central to the system is a self-contained, totally enclosed density sensor that is immersed in the gray water pool, open basin, or even a pipe with a diameter greater than 3 inches.

The sensor cable runs through the pipe to a display box. The installer uses a special adapter that positions the sensor so that it faces the flow.

Once in its proper position, the sensor uses microwave-like technology to measure the amount of solids suspended in its surrounding water. Unlike a radar device that assumes that the surrounding volume is clear and looks for exceptions, the sensor assumes that the surrounding area is completely solid and measures the open space by relating the percentage of unreturned signal concentration to water free of suspended solids.

By detecting the initial signal concentration and the measured responses as well as the unit weight of the typical suspended solids, the measuring device can report the gray water density from 1.00 to 1.50 grams per cubic centimeter. The system's overall accuracy reportedly is better than 0.01 gram per cubic centimeter.

Another important feature on the sensor is the fact that it also measures water temperature at the same time.