Build a Turbidity Sensor to Monitor and Control Microalgae Biomass Density

Sensor Performance

To measure turbidity, there are many options offered by brand-name vendors such as HANNA, YSI, MT. Those sensors also come with many wavelength to choose, a detailed documentation, and even an internal memory and dual beams. One trade-off is high-cost, from a few hundreds to ten thousand dollars. For developers, investing a few hundred dollars without knowing how it will performs is an luxyry option. The lower cost sensor means a lack of support and documentation and possibly the accuracy.

The turbidity (TSD-10) used in this system is a commerical and for general use, mostly with washing machine or diswasher. The microalage culture can change density quickly, and this generic sensor seems to a better option, trading off a high precision in a narrow range for a wide-range of measurement. Here is the datasheet from Amphenol.

Turbidity Internal Sketch.
Internal structure of the sensor. The emitting side is an infrared LED and the detector side is a light-dependence resistor with a voltage divider.
Turbidity and Optical density.
The conversion of readout voltage to the optical density at 730nm, and NTU unit to the optical density at 750nm.

Here is what it really means for a turbidostat: The reading of the culture density (in green) is hugging the set value (in red). The Pump Status shows when the pump is on.

Finally, $15 sensor works fine to measure to biomass density of microalage.