Let just say you are bored with sampling water to measure turbidity, a gross term indicating any small and suspended particles in water. The turbidity reduces light intensity with either an increasing light depth or a higher particle concentration or both. So, how to do that?
To measure turbidity or biomass density, I need to measure the light intensity in the detector side which is converted to a voltage readout. One obstacle I had in the beginning to find a suitable sensor that works with the microalgae species I worked with Synechocystis sp. PCC 6803. This is micro-algae that are in sub-micron size, well-suspended in water, and rather have an extreme lifestyle, converting light energy and reduce carbon dioxide into newly synthesized biomass. That is enough about the microalgae for now.
Turbidity can be measured by a spectrophotometer. The laboratory spectrophotometer is expensive and mostly measures one sample at a time. It has a precise tuning motor with a prism that splits to mono wavelength from the primary source generated from a hallogen bulb. Available approaches are splitted into two ways: measure photosynthetic biomass by using wavelength to target the pigment of that microalage species. For Chlorophyll, a popular wavelength is 680nm, but since there are 6 types of chlorophylls, other wavelengthes could be used as well. The other approach is to treat biomass as suspended particles, and use the wavelengths in the region of 730-750 nm or even 850 nm. The apparatus is similar in both case: an light source to emit a single wavelength and a detector to detect the reduction of light intensity travelled a fix distance (1cm, 5mm).
Below are several steps I took to build an automatic monitoring system for biomass density of microalgae. Somehow, I were in luck that I bought a cheap turbidity sensor found on ebay.com or amazon.com, and to my surprise, the sensor works well with the microalgae species I experimented. The wavelength is not specific other than infrared region.
and the cover photo for the system (not a good one, too crowded)