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Strange readings using the 180 degree sensor on the EVAL-CN0409-ARDZ

Hi, during our master thesis we are testing different methods for preforming turbidity measurements on concrete slurry water.  When doing tests on the CN0409 we expect the 180 degree measurement to decreases when the slurry concentration increases, according to the Beer-Lambert Law. This is however not the case using our setup, between the concentration 0.4 and 0.6 the measured value are increasing. We are wondering if it's some error in the hardware, software or anything else?

(We have made 180 degree measurements using a cheaper sensor in combination with Arduino, and those results follows the Beer-Lambert Law.)

We are using EVAL-CN0409-ARDZ in combination with EVAL-ADICUP360.
The code is taken from, with the small modification that the sensor readings are sent using AppPrintf(). This is done in the function CN0409_CalculateRawData() in src/CN0409.c using au16DataSlotA[u8Count] and au16DataSlotB[u8Count].

A plot of the sensor readings as well as photos of the different concentrations are included below.

Sensor readings:


Image didn't show, made it smaller
[edited by: Karlsson at 12:30 PM (GMT -4) on 12 Apr 2021]
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  • I want to start by thanking for all the help, it's nice knowing the results can be replicated using a similar setup.

     I read the articles you linked and I'm not really convinced. The size and concentration changes the way that the light is scatters, but I think that any scattering should lower the light intensity that reaches the 180° sensor (compared to no scattering at all). 

    We also found another way to replicate the increased sensor reading. By taring a standard A4 printer paper into smaller pieces and place a single layer of that paper in front of the sensor (or in front of the LED) we get a higher reading than without the paper. If we place two layers of paper in front of the sensor (or LED) we get a lower reading than without the paper. The same is true if both sensors (or LED) is covered at the same time (higher for single layer / lower for double layers). This makes us skeptical to the clear cylinder theory.

    Our best theory so far is that the sensors have a bad alignment and that we accidentally measure forward scatter instead of transmitted light, since this should increase with concentration and then decrease. However it seams unlikely that both LEDs and sensors would be badly aligned and that it could be replicated on another device. I also don't think that this theory supports the decreases in sensor measurements for the concentrations 0-0.4.

    In short, we don't have any good theory of what casing this behavior but we are relay happy about the time you have put in to help us.


    (Image from

  • Hi Karlsson,

    I wouldn't be surprised at the alignment - note that we're just relying on the LED's built-in lens, nothing fancy. It's intended as a reference design for customers that would eventually be using their own optics. As you have demonstrated, different customers have widely different situations. The CN0409 was tested extensively with turbidity calibration solutions (referenced in the circuit note), but never with "real-world" corner cases.

    Take a look at the CN0503 for comparison. this circuit has a much more constrained optical path, a focused beam, and uses square cuvettes.

    The best answer depends on your price/performance/quantity requirements. If your end goal is simply to take the measurement and the DFRobot sensor you reference above does the job, the it's probably the right answer.

    However - if your goal is to design and build a better turbidity meter, the CN0409 and CN0503 are intended as starting points. The ADPD105 and ADPD4101 (and similar mutlimodal sensor front ends) greatly simplify the circuitry that would have been traditionally employed - choppers, lock-in amplifiers, discrete ADCs, etc.


  • Hi Mark, I just want to clarify that we don't think that the CN0409 is a bad product. It's really fast and has a good signal to noise ratio. As mentioned in the circuit note the CN0409 performance is well tested and has good results on liquids < 1000 NTU. We suspect that our measurement at 0.3% concentration is about 1000 NTU and under 0.4% the sensor performs as expected.

    Since we writing our master thesis on different ways to measure turbidity and not a specific sensor, it's more interesting to us why the measurement looks likes it look than if there is a better product. It's hard to write a theses report when the test results don't correlate to the physics explained in the theoretical part of the report.

    That the alignment of the sensors isn't exact could explain the increase in light intensity, but then the decrease in light intensity between 0-0.4% becomes a problem.

    So we aren't complaining about the performance of the sensor. We have just encountered a measurement that we can't explain. Since we ain't any experts on this area and don't know if there is some physics we don't know about or if this is a result of some filter in the sensor, we contacted the community.

  • "good" and "bad" are relative terms - good for one application might be terrible for another, and vice-versa :) But if you're writing a thesis, then indeed the CN0409 (And CN0503) are excellent reference points to compare against other techniques. Not sure how much you're focusing on the electronics vs. optics, but as I mentioned the photometric front-end ICs reduce the (external) circuitry to a trivial level. But a quick glance through the datasheets will give you some idea of how configurable these parts are - optimizing settings for different sensors, measurements, and interfering factors (sunlight, 50/60Hz light, ~10kHz-100kHz flicker light) could easily fill another thesis.

    Good luck!