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

Question

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 https://github.com/analogdevicesinc/EVAL-ADICUP360/tree/master/projects/ADuCM360_demo_cn0409 , 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: 
 
 Concentrations:

MarkThoren · Accepted Answer

"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! 
 -Mark)

MarkThoren · Answer

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. 
 -Mark

Cement Slurry Concentration	LED1		LED2
Cement Slurry Concentration	90˚	180˚	90˚	180˚
0.40%	7353.98	6171.08	7330.48	6518.05
0.60%	7192.37	6363.16	7184.97	6690.13
0.80%	7299.1	6221.45	7271.09	6569.18
1%	4709.17	5306.67	4782.54	5479.3

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