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.
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.