Question: Modified TDS sensor as Conductivity Sensor?

MadTinker asked on March 10, 2018 21:36
111 | 3 answers | shortlink

(Cheap) conductivity sensors are difficult to find and there's considerable number of references in Public Lab and, for example, Ed Mallon in his Cave Pearl Project Blog (excellent!). I'm still struggling with finding a good, solid basic circuit and have some Coqui-ish thing that I'm working on now. However, I was wondering if there is any reason why a TDS monitor ( e.g. the one by DFRobot) couldn't be modified to collect EC data? Obviously the DFRobot TDS monitor could be temperature compensated, the input scaled appropriately, and the probe would still have the over-voltage/hydrogen problem, but it seems that at $13 it's not a bad option.

What am I missing?

water-quality-sensor conductivity tds



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3 Answers

I agree that the DFRobot TDS sensor looks like a useful device at a good price. My lame understanding is that this and other sensors which purport to report Total Dissolved Solids are just conductivity sensors. This one has two metal probes and is obviously measuring the current passing through water. When most of the solids dissolved in water are salts, electrical conductivity is well correlated with the concentration of dissolved solids. When a lot of the solids dissolved in water are organic compounds or other stuff, the correlation is weaker. It might be a stretch to call this a TDS sensor, but in some situations TDS is so similar to EC that converting EC to TDS provides adequate results.

I don't know how effective it is to use their trick of alternating the current to avoid the problem of hydrolyzing water near the probes (I guess that's the goal of this). I don't know what voltage they pass between the probes, but maybe it is very low. The sensor includes a well-populated PCB, so maybe they keep the current to the probes at a low voltage. They would probably tell us if we asked.


Ag8n 14 days ago

There are several other previous questions that discuss this. But crazy is is correct, all tds meters are conductivity meters. One side note- there are two different tds scales. So while the conductivity should read similarly, the tds may not.

The circuit in the old conductivity meter was basically an ac generator (running at 10 kh z if memory is correct) feeding a Wheatstone bridge. The two solution electrodes were one leg of the bridge. The resistances of the legs were adjusted until an a.c. Voltmeter read zero.
What the circuits are now, I don't know.

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@Ag8n - yes, I saw the other discussion, but it had seemed to go on a tangent. ‘crazy’ @cfastie I think hits it right on the head (gotta love that autocorrect). Ive ordered one, will follow-up with them on special operating characteristics, and follow-up here with my results. Probably a couple of weeks given my travel, etc.

Ag8n 14 days ago

You hit the nail on the head with auto correct. Good luck with your project.

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An quick update: The TDS sensor from DFRobots performs as describes. Very easy to connect to the MiniPearl logger. In general, it collects a voltage drop and converts that to a TDS value in PPM. As noted by DFRobots, conversion to conductivity is a simple scaling (i.e. multiply by 2). I got the TDS sensor working on the Mini Pearl Logger (with a hint from @cfastie) and the TDS values match reasonably to the values obtained via a TDS pen I had laying around but never used. Interestingly the TDS pen also converts to a Conductivity value by simply multiplying by 2. Instructions and code for calibrating the TDS sensor are provided by DFRobots. I briefly got the DS18B20 temperature probe and the TDS sensor sorta working together on the Mini-Pearl Logger (some significant power saving timing issues still needed to be worked out). Then I fried something and now cannot access the microUSB. I'm going to work understanding the TDS sampling code before I jump back into the hardware.

warren 3 days ago

@stevie - maybe this could at some point be a good kit for students to use? Pretty cool, @MadTinker !!

Interestingly the TDS pen also converts to a Conductivity value by simply multiplying by 2

Does this confirm @cfastie's thought that this and other sensors which purport to report Total Dissolved Solids are just conductivity sensors -- or just that TDS and conductivity are directly related?

MadTinker 3 days ago

Confirm? Hmmm... well, I have two data points, both of which coincidently use the same multiplier on EC to get TDS. From discussions with folks who are a bit more knowledgeable than me, the value of the multiplier is a major point of contention. It really depends on what the 'solids' are (eg. organics versus salts) and what you mean by 'dissolved'.

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