Question: How much are Coqui measurements affected by hot/cold water?

warren is asking a question about coqui
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by warren | with kgradow1 September 13, 2014 20:40 | #11140

What I want to do

Some people don't drink water from the hot tap, because its sat in pipes and has more heavy metals. But could we use the 555-based <$5 Coqui sensor?


My attempt and results

First we had to let them equalize in temperature because that'll affect conductivity. Then we made a more durable version of the Coqui soldered on a perforated circuit board instead of a breadboard.

Questions and next steps

How much is temperature a factor? Can the Coqui compensate for temperature?

Why I'm interested

This'd be a great first thing people could do with such a sensor... A Hello World, and one which would have real environmental health repercussions for anyone drinking their tap water.


Hey @warren @donblair and Katie - Nice work - this is simple and exciting.
we did a home brewed test of Coquí here last weekend with hot/cold water, 2 kids and a Yoda Angry Bird:

Our judgement was that the sounds were "the same but different" and that both hot and cold "vibrated a lot".

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Ah, this is all so great!

This brings up a bunch of interesting issue for me that I'd love to get feedback on.

Naming conventions. Maybe it'd be good to start to figure out a useful naming convention for these various projects; I'm starting to get confused :) For me, I see the Riffle, which has now been used as a basic datalogger board for several projects that don't involve water quality, as a sort of general data logger. I love Mat's suggeston of the 'Sniffle' as the air quality version; and Craig recently made a 'Muffle' version that is intended as a mushroom grow room monitoring device. Maybe we should come up with a name for the basic datalogger that is separate from any of these names? At the risk of generating two many entities to keep track of.

So, Riffle Coqui starts to get even more confusing for me :) But that's par for the course when all of this fantastic development happens so quickly with a bunch of fun and enthusiastic folks!

Just thinking out loud, here: my notion of the Coqui:

had started to move in the direction of a device that "translates a resistance value into an audio-range frequency" -- where the resistance could be a photocell, a pair of electrodes in water, a thermistor, or many other things. I'd come to think of it as separate from the Riffle, which is expressly intended as a device that measures certain standard water quality parameters.

I'm also thinking of the 'Coqui' as more general in application, at this point, because I'm not yet comfortable with what exactly we're measuring when set it up with water as one of the resistors. The 555 sends an oscillating current through the water between the electrodes, and if the oscillation is too slow, the water will become polarized by the measurement, changing the apparent conductivity of the water; avoiding polarization is also a reason to try to use the minimal voltage necessary for the measurement (my understanding is that 3V is on the 'high' side, but we need to look into that). All that said, if all we're making is a comparative measurement (is the conductivity higher or lower in sample A than sample B), then it seems we could get away with not worrying too much about polarization from our instrument. But because we know that the temperature of the sample will affect its conductivity, as you say (high temp --> higher conductivity), we're going to have to be careful about making sure that any differences in 555 frequency aren't simply due to temperture ...

Tap water experiments. Okay, so here are some associated proposed to-dos / questions on my end, to clarify my own thinking, here:

  • If the conductivity of the water is observed to be different for tap water samples that were obtained through 'hot water pour' and 'cold water pour', and then maintained at 25 C, what might this be due to? Is lead in the pipes the only / most likely explanation?

  • If lead from old pipes is a concern, what concentrations are typically present in hot tap water due to lead piping, and how much would this affect the conductivity?

  • What is the affect of temperature on conductivity? I found a nice chart, here, that shows the effect in a graph:


These show the change in measured conductivity of solutions for different solutions of known conductivity, for a range of solutions that are on the high end of potable water in conductivity. Not sure it's reasonable to use this as a rough guide, but from the chart it looks to me like conductivity of these solutions changes by about 30 uS/cm per degree C. Since this temperature effect seems to be larger for higher conductivity solutions, and this is on the high end of potable water conductivity, let's use this as a conservative estimate.

Stumbling onwards: some typical examples for conductivity are in this chart (from the same reference, above):


My conclusion is: a difference of a few degrees Celsius (meaning a change of 100 uS/cm or so) isn't going make tap water (50 uS/cm to 800 uS/cm) appear to be in the conductivity range of industrial waste (10,000 uS/cm), or sea water (55,000 uS/cm); but it seems like it can push us around the typical range of tap water by as much as 15% of the expected range.

For assessing incursion of seawater into a freshwater water source, it looks a difference of a few degrees in the samples isn't going to matter much when making comparisons.

For the tap water - lead question, my next step here would be to find out how much we'd expect the conductivity to change in tap water, in terms of uS/cm, due to the presence of lead from pipes. It might be easy to find that information. This difference will then set our threshold around how close we need our sample temperatures to be. (There are probably a bunch of other confounding factors here that I'm not thinking of .... ?)

Places I'm reading next:

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Great, thanks Don. I can change the title/body to reflect whatever name you think is more appropriate -- "Coqui-based water conductivity sensor"?

I forgor to mentinon that this test made it really clear to me that you have to dip the probes all the way in to get a consistent tone -- making me want to put a 'water line' mark on the probe stick. Probes partially submerged or even submerged in bubbly water didn't generate a clear tone.

We didn't have a thermometer on hand, but left the water at room temperature for over 30 minutes, and finger-dipped to check the temperature difference. Finger-dipping, while a time-honored accessible method from the caveman days, should be tested itself -- if we want to make a simple kit, what's an acceptable temperature difference between two glasses, and does a finger-dip get you in that range?

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Hi Warren, I am new at this, so maybe I missing the ball, but isn't one of the reason why not to drink from the hot tap the possible presence of Legionella bacteria?

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Hi, Don - sorry, you never wrote back so I hadn't changed the title until now - it now says "Coqui" only - thanks.

The conclusion you draw:

it seems like it can push us around the typical range of tap water by as much as 15% of the expected range.

is great, even if you have since drawn a more precise conclusion. And the graphics are super helpful! Could you add them to the Coqui page, which is a bit sparse on these kinds of limitations at the moment? It mentions "conductivity" but not whether it's absolute or comparative, and not how it's affected by temperature etc. I added a Status and Limitations section to the Oil Testing Kit as we talked about on Wednesday; maybe these could be part of such a section on the Coqui page, too.

Hi, @GreenChemist - I don't know - do you have more information about that? If so, I don't think this would test for that, just the metals.

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Hi Warren, Indeed the test would not find the bacteria, you need a polymerase chain reaction (a PCR test), or grow a culture. I am not sure if this can be done in the scope of public lab. I just wanted to mention it here to point out that the conductivity might give some people a false assurance about the safety.

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It's a good point -- esp. always to be sure that the scope of the test is clear. The idea here was more exploratory, but in general this kind of conductivity test is not a test of drinkability or safety -- just an indicator of comparative change.

I've seen some DIY PCR work -- have you done any of that kind of thing? What's your interest in tapwater research?


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Hi Warren, Thanks. I am a chemical engineer who always worked in Medical and Safety industries. Currently I am looking for a new position anywhere on earth as long as it is in a green industry or education. Anyway since a few years we are living in France, and I regularly get annoyed by the completely biased french press, especially with regards to nuclear industry and diesel cars. So I started to browse around at open source websites for monitoring information. Public lab is doing great work. I made my own spectrometer, a bit different, when the tests are finished I will publish. A PCR test is probably the next item I will tackle. Best, Boris

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I renamed this and converted it to a "Question" in the new Q&A system

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So both questions and research notes can be marked as an activity? And at some point, folks replicating the activity can post their answers? I like this idea for this and bigger questions!

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Also: I'm not sure if you're testing (1) Hot Tap Water vs Cold Tap Water, or (2) Hot Tap Water vs. Hot Tap Water which has cooled down. Put another way, from the title it looks like you're testing the temperature dependence of conductivity (2), but from your description, it looks like you're interested in whether hot water has more conductive stuff in it (1). Cudos for pointing out that factor, and highlighting what the coqui is measuring through this experiment

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Actually i haven't tried marking a question as an activity; i guess it works? Maybe not a good practice? I dunno -- new territory for us! As to your other question, I think you're right and it's ambiguous. Changing the title now, but shall I post another question about the other?

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I'm actually having trouble editing this page -- must be a bug due to it being "converted" to a question instead of being one from the start.

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I've submitted a bug report with steps to fix: -- sorry!

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