Low-Cost Submersible Turbidity Sensors Using Low-Frequency Source Light Modulation

Submersible turbidimeter and fluorometer design generally involves tradeoffs between ambient light rejection, sensitivity, and cost. We introduce an optical backscatter transducer in which the light source is modulated with a low-frequency (tens of Hz) square wave, with a detection scheme that mitigates the effects of external interference from ambient light and from 50- and 60-Hz sources. With this design, high-pass filtering and subsequent amplification enable sensitive measurements using inexpensive circuitry. Additional accuracy-enhancing features include: a rail-splitting virtual ground, which eliminates the potential for clipping at low signal levels; a reference output, which provides correction for the intensity of the internal light source; and error-tracking provisions, which flag the output if circuit voltages reach saturation limits during measurement. We have designed, constructed, and evaluated submersible turbidimeters which incorporate this transducer. These sensors were inexpensive to fabricate (U.S. $70 of materials and 4 h of labor) and were proof tested to 7 atm of water pressure. They responded linearly and reproducibly to formazin turbidity standard, with negligible inherent offset. A calibrated sensor exhibited bias ≤0.11 NTU in suspensions ranging from 0 to 8 NTU. A similar sensor with reduced circuit gain exhibited a one-to-one response to increasing turbidity in suspensions ranging from 0 to 815 NTU.