Hello, everyone,
I have a learning kit with an Arduino uno r3 as of this week. At the moment I am still trying out various simple things and doing tutorials. But I already have my first own project in mind and I would like to hear your help and opinion before I jump in and order the first things.
It is primarily about measuring the nitrate content (later also others) of aquarium water. For this purpose a sample is taken and with a simple droplet test the nitrate content is determined by optical comparison with a scale. The solution takes on a pink to reddish colour. The more intensive the colouring, the higher the nitrate content. The picture shows four samples with different contents. In the aquarium I expect values between 0 and max. 40 mg/l.
Since the comparison with the colour scale is often very inaccurate, I would like to improve this procedure with a DIY photometer by measuring the decrease of light intensity through the sample. For this purpose I have tested with a green LED of the kit (exact wavelength unknown) in a very simple test setup (see picture, please don't judge) with a LDR, if this is possible at all. So I checked the four upper samples and the LDR gave me values from ~550 to 400 depending on the nitrate content, so it seems to work.
Now I would like to know if you can do something like this with an LDR or if there are better approaches. Does it make sense to use only one green LED or should one measure with another one? Or are there any completely different approaches that would give me better results?
Basically my next steps would be a decent housing and a rectangular glass cuvette so that the LED and the sensor are very close to the sample.
Any advice or help is very appreciated. Thank you very much!
Number one advice is to work through as many parts of the learning kit as possible. That makes You more fit to know how to proceed instead of the stakato, asking Forum and waiting for replay.
What is Your knowledge regarding light, wavelenghts, spectrum etc? Googling for various sencors is of cource good.
So I checked the four upper samples and the LDR gave me values from ~550 to 400 depending on the nitrate content, so it seems to work...
...Now I would like to know if you can do something like this with an LDR or if there are better approaches.
If it's working I'd say you're good-to-go!
The "main complaint" with LDRs is that they are slow but that's not an issue in your application. A photo-transistor is more linear* (but that's probably not important), it probably has a different bandwidth response and it might have a wider measurement range but I really don't know.
The LDR itself is fairly linear but the output of a voltage divider is not linear with linear resistance changes.
You can use a green LED as the photodetector as well. It will act as a green-sensitive photodiode and thereby eliminate some of the contamination from other sources of illumination and absorbing compounds in the sample.
Photodiodes and LEDs are much preferred over LDRs as detectors, as their response to changes in light intensity is linear (LDR reponse is logarithmic), and therefore linear in response to dye concentration.
If you can, take samples like those you show in the photo to a chemistry lab at a nearby university or commercial lab and ask a researcher to run absorbance spectra of a few different samples. You should find that many people are happy to help (once the pandemic slows).
Ideally, you will want to detect absorbance changes at the wavelength of the absorbance maximum.
Please read the post at the start of any forum , entitled "How to use this Forum".
OR http://forum.arduino.cc/index.php/topic,148850.0.html.
Then look down to item #7 about how to post your code.
It will be formatted in a scrolling window that makes it easier to read.
As my next step i'm going to print myself a proper housing with hopefully zero external light. Then I'm going to test this setup with a LDR and a photodiode to see and understand their differences a little bit better.I was thinking of an photodiode BPW34 since it covers the whole spectrum of visible light. I didn't find any sensors which are most sensitive to green wavelengths. Does anyone have a recommendation?
I also found this nice project which is very helpful:
I will probably just copy his setup just to see if it fits my needs. Maybe i can find out the absorbance maximum by myself with that. But the idea with a chemistry lab is very good and i'll try to find one.
