Detecting a certain matt density of freefloating macrophytes

Right now I’m working on a university project, which utilizes azolla or duckweed biomass production. For the automation process, we need some data to determinate the optimal harvesting moment. The Plants have different doubling times, for example one week, but after reaching a certain matt density they self inhibit their growth. For the Azolla plant this is reached after 2 kg/(m^2). So at this point the harvesting should be initiated.

My idea to determine this matt density was to measure the light reaching the bottom of the pond. For this i would install some LDRs inside a clear tube and collect the resistance over time. When the correct matt density is reached (measured by weighing the “pond”) I want to look at the data and hope, that there is a visible correlation.
In the attached PDF I have drawn the prototype.

Do you think a normal LDR (Gl5516)is viable and the 10 bit is enough?
Maybe you have a better idea for measuring a plant mat on a water surface.
Detecting the water level would also be quite helpful.

Thank you for your time!

Notiz 20.12.2018.pdf (1.72 MB)

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OPs pics;

Setting up a UNO and some LDRs would be possible, getting the tubes waterproof will be important.
The measured level will probably be an experimental process.
Not knowing your light source, which will have to be the same each time you take your readings, the sensitivity of the system cannot be predicted.

Tom.. :slight_smile:

LDRs are very nonlinear, and for best response, you need to choose a voltage divider resistor that roughly matches the LDR resistance at the point where you want to harvest.

For low light levels a photodiode with a transimpedance amplifier is a better option, as the response is linear over many orders of magnitude in light intensity.

Ok thanks for your input I need to consider! :smiley:

My light source will be a grow lamp, so the different mesurements should be comparable.

I will design a new setup with a photodiode. Sounds like a higher sensitivity, if i can figure out which spectrum is needed.

Rather than make you own photodiode amplifier, consider using an ambient light sensor, like this single-chip luxmeter from TI or similar. They use a very simple I2C interface and would be easy to turn into a waterproof, submersible probe.

consider using an ambient light sensor

At first, I thought that would not be a good idea. I thought many such sensors would be needed to measure an accurate average light reading, because the plants grow randomly, and a single sensor might be luckily/unluckily underneath a particularly dense or sparse patch of growth. But I suspect this is only because of the lab environment, where the light comes from a constant direction. In a production version, the light source would be the sun, I assume, so the light direction would vary through the day, and the exact positioning of the sensor would be less critical. In other words, you might need an array of sensors in the lab where only one sensor would be needed in production, because the sun moves.

The same considerations apply to the much less sensitive, highly nonlinear LDR, and the OP has clearly anticipated the need for multiple sensors.