A typical application of photodetectors in analytical measurement is as a turbidity analyzer. The cloudiness of a liquid, called turbidity, is caused by the presence of finely divided suspended material. Turbidimetric methods involve measuring the light transmitted through a medium.
Turbidity can be caused by a single substance or by a combination of several chemical components. For example, the amount of silica in liquid may be determined in approximate concentrations of 0.1 to 150 ppm (parts per million) of SiO2. Sometimes composite material turbidities are expressed as being equivalent to silica.
In the typical application, a turbidity value is developed from a test sample under controlled conditions.
In this application, a laser beam is split and passed through two mediums to matched photodetectors. One medium is a carefully selected standard sample of fixed turbidity. The other medium is an in-line process liquid. If the in-line process liquid attenuates the laser beam more than the standard or reference sample, the electronic circuit triggers an alarm or takes some appropriate control action to reduce turbidity.