GE Turbidity Sensor

[David Bengtson]

I've pulled the Vout/NTU curve out of the GE turbidity sensor, and graphed it here

(Google Sheets)
https://docs.google.com/spreadsheets/d/1ycy1SN2QQEwb1S3M-fj8m3glTbXQE60-naHHi3COfPM/edit?usp=sharing

The Range is pretty good, measuring up to 8000 to 10000 NTU, but it's pretty compressed at the low end, which I suspect is the interesting range. I'll look at this more in detail, and figure out the performance from 0 to ~ 2000 NTU, as well as the take a stab at the necessary circuitry to read it accuratly.

Is there a target range of NTU?

Dave

[shi...@stroudcenter.org]

I actually experiment with several GE dishwasher turbidity sensors a few years ago.  I built some housings for the sensor so I could submerge them in our simulated stream channels where we can simulate different turbidity levels.  I find this to be much more accurate and closer to real world settings since we're using actual stream sediment.  I compared the dishwasher turbidity sensors to the research-grade Campbell Scientific OBS3+ sensors we usually use in our deployments.  I bought several different models, each with different ranges.  I think the one I used most was rated to 4000 NTU.  I used the graph in the datasheet to calculate a formula for voltage to NTU.  I just looked in my Arduino sketch, this is the formula I used to convert measured sensor voltage to NTU:  GEturbidity = (-197.03 * pow (GEvoltage, 3.0)) + (2112.5 * square (GEvoltage)) - (8062.4 * GEvoltage) + 10878;

What I found was that the dishwasher sensors were surprisingly accurate and gave very similar readings to the OBS3+, however the plastic of the dishwasher sensor is very good at attracting algae, slime, biofouling, etc, and even just little specs of clay (we have lots of clay in our sediment here).  Because the beam width of the dishwasher sensor is so narrow, it only took very small amounts of material on the sensor window to create enough blockage that the signal was seriously attenuated.  With the OBS3+, it sends out a wide beam of light and looks for reflections by particles in the water.  I observed the beam with a pair of night-vision goggles (it's IR so it shows up nice and bright with the goggles) and it is indeed a wide pattern like a flashlight.  So because of this wide pattern and sensing window, the OBS3+ can withstand significantly more fouling before we start to see problems with the signal.  For the dishwasher turbidity sensor, it either needs to be cleaned very frequently, or a wider beam pattern should be used.  In either case, the housing on the dishwasher sensor is a soft type of plastic that scratches easily, which will lead to other issues in the IR signal transmission.  So for our deployments where the sensors are located in streams with sometimes high sediment loads and high occurrences of biofouling, it would take a significant engineering effort to come up with modifications to the dishwasher sensors so we have decided to focus our efforts elsewhere for now.

We started enviroDIY.org awhile ago to share things like this, but unfortunately I've been so overworked with other tasks that I haven't been able to post much about my previous experiments and tests because I've been so busy working on new stuff.  While we are very interested in developing new low-cost methods for turbidity measurements, we also conduct a variety of research experiments that require us to use high quality research-grade instruments so many times that's where my attention is focused.

Steve

[David Bengtson]

Steve-

Thanks, that's pretty helpful information. I'm glad the results are good, but the long term reliability of the sensor sounds problematic. On the other hand, they're only $5.00, which is pretty cheap. If you can figure out a way to swap them out. Obviously not ideal for a long term deployment, but for short term sampling, it might be interesting.

I wonder how they deal with biofouling in dishwashers? There might be an interesting research avenue.

Dave

[George M. Gallant, Jr.]

Dishwasher vs stream/pond

1. Dishwasher environment has a rinse cycle with a relatively high rate of flow.
2. Dishwasher uses soaps and hot water.
3. Purpose is to shorten wash cycles for economy. Failure probably means longer wash cycle.

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[Jeffrey Warren]

Hi folks, sorry just a quick question- what's NTU?