Sensing pH in a way that makes sense (for the other 90%), by T.H. Culhane

For years now, many of us have been engaged in various discussions and experiments to determine the best way to continuously monitor the pH of our biogas systems.  Those of us doing this at home on limited to nonexistent budgets have dreamed of an inexpensive DIY microcontroller based system that we can stick in a port in our tanks and forget about (except for the readings on our smart phones which would alert us when things are going sour, of course!).

A discussion has been going on recently on our facebook group reviving this discussion with a lot of practical ideas emerging now that technologies are multplying along with expertise and prices are falling.  I post the thread below for archival purposes, but to summarize it, we are learning several key things:

1) Beyond finding ways to measure H+ ion concentration, which requires expensive glass bulb probes that are robust to sitting in solution for a long time or need to be constantly removed and cleaned (or require extraction of fluid for manual testing), there are optical measurements using colorimetry that may provide a good alternative (using LED based visible color sensors) and optical measurements using spectroscopy (employing red laser diodes).

2) There may be proxies to pH measurement since we are not concerned with small shifts in this logarithmic measure, but rather in gross changes (from the normal digestat neutral of 7) to a dangerously sour 5 or below.  Other state changes may alert us this shift is happening that are easier to measure (I can use my nose when I am standing next to a tank, so we need to figure out what that indicator is and how it can be sensed electronically.

Regardless, this is an exciting project that is X-prize worthy in our field since temperature and pH are the two main determinants of a healthy biodigester (and Nick has already readied our microcontroller based temperature probes that we were working on when he visited New York last year!).

Once we have a low cost easy to implement and effective pH meter deployed on all of our tanks around the world in this small biogas community, we can display the data on our geographical information systems maps and start solving problems with real time data. How fun that will be -- and how effective in combating the prime reasons for biodigester system failure that have kept aid organizations (like the SNV when I met with them in Rwanda) from supporting home kitchen-waste fueled biogas systems.

Let's make it so easy to keep our digesters alive and kicking that nobody can "throw the baby out with the bathwater".  In this fight to stop deforestation and indoor air pollution and energy and food insecurity and poverty we need everything and the kitchen sink. And a reliable cheap constant pH meter that doesn't require supervision will be a game changer!

If we build it... they will measure!


Here is part of the  recent discussion on facebook:

Nick Chase

21 hrs · Essen

When our digester go acidic we have some problems. Though we'd like to measure the pH. But measuring the pH isn't that simple. There are continuous probes that can stay in the solution for long but are very expensive. Other probes, the cheaper once, can't stay that long in solution. Another value is the EC - the electric conductivity. Where EC catches all Ions, the pH only measures H+. Measuring EC is simply and cheap. But EC doesn't simply correlate with pH as far as I know. Do you see a measurement that gives us a clue about the pH while measuring EC

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Dominic Kahumbu Wanjihia Can I ask why you need to continually monitor the ph? If your system is designed well and allows the continual digestive process, it should not go rancid.

11 hrs · Like

Jeffrey L. Frusha KISS principle? Take a sample and measure with something like the pH dip strips. on a schedule?

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Nick Chase hmm, and how do it get the values in a database?

10 hrs · Like

Ariel Reznik electronic aquarium ph meter not gonna do the trick? they're less than 20$ a piece?

10 hrs · Like

Nick Chase need to be calibrated from time to time. pretty difficult within a digester

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Ariel Reznik and you want it to log all the data automatically?

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Jeffrey L. Frusha Manual data entry isn't a lost art. Beside that, the chem strips don't need batteries, transmitters and receivers, or anything else. You can still take a reading when the power is out, too.

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Ariel Reznik also, laboratory electronic ph meter are pretty irritating to operate

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Jeffrey L. Frusha I don't see how, without extensive testing, you can determine the source of the ions, measured as pH, anyway. Some sort of spectroscopic analysis?

10 hrs · Like

Jeffrey L. Frusha Could you use something simpler like a fluorescence source, and a filtered light sensor?

10 hrs · Like · 1

Ariel Reznik if we're talking about ions, it means they're dissolved, and most likely don't absorb light. they might refract the light. but, if it's just H+ ions, then they probably act just like water.

9 hrs · Like

Ariel Reznik how many samples you wanna take every day?

9 hrs · Like

Nick Chase I would say 4 is enough

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Jeffrey L. Frusha Saturation spectroscopy?

9 hrs · Like

Jeffrey L. Frusha Hydrogen excitation is frequency specific.

9 hrs · Like

Ariel Reznik i say, just use a tap. 4 drops a day. ph paper

9 hrs · Like · 2

Jeffrey L. Frusha If I understood the principle of the article, then a clear tube with a peristaltic pump could cycle the fluid from a selected hollow probe placement. Red lasers are cheap. Stimulated Hydrogen is excited in a red band width. Maybe it could be tuned for better accuracy?

9 hrs · Like

Jeffrey L. Frusha Just got me thinking of ways to automate it, now, since y'all were talking probes and data-logging.

9 hrs · Like

Jeffrey L. Frusha Fiberoptic pH based on colorimetry...

8 hrs · Like

Thomas Henry Culhane I agree that we could use color as the sensor trigger for getting voltage differences and hence determine pH, but my reading of the paper on colorimetry is that they still had to take a fluid and subject it to a procedure that yielded solutions of different colors that corresponded to pH. They say early on in the paper that their method (using the fiberoptic colorimeter) allows for "in situ" measurement, but in the methodology they are using sample solutios. Am I missing something from my first read through? I'm not aware that biodigestate spontaneously changes color in any predictable way with varying pH (in my home tank in Germany the color to the eye was always the same, but it was the smell that let me know it was acidic, literally going "sour"). Perhaps rather than optics (which would require taking a sample of digestate and dripping a pH indicator into it and then using a colorimeter as an electronic eye -- doable but not "automatic") we will find an electronic 'nose' whose voltage will very with acidity?

8 hrs · Like

Thomas Henry Culhane

Electronic nose - Wikipedia, the free encyclopedia

Over the last decade, "electronic sensing" or...

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Nick Chase electric noses detect gases. gas sensors are cheap and easy to use (most of them) - but they can't detect pH ...

8 hrs · Like

Thomas Henry Culhane Something in the souring of a digester produces a gas or volatile that is really really easy to detect by nose. If we could find out what it is... the sensor could sit right above an effluent drip set at a slow rate. You can't mistake the smell at all. My understanding of all the sensors we've used with our Arduino boards is that ultimately whatever they pick up is translated into a voltage, to electroconductivity of some sort is involved, right? Either light or noise or chemistry or pressure, all create a variance in electrotension or in current of electrons to be read by our microcontrollers. A pH sensor's glass ball is selective to H plus ions making the electrons change. But maybe it isn't H plus ions we need to measure to make a meaningful change. What is it in "souring" that we can measure (it certainly makes my eyes water!). Something is "burning" the microbes when it goes acid, and usually we think "that is the protons". But protons react with things and cause changes... what changes can we measure without directly measuring the protons, and without having to add something to get an observable color change...? that's where my mind is going...

8 hrs · Like

Jeffrey L. Frusha I think, the spectrometer with the tubing and peristaltic pump is your solution, then. Red lasers are cheap, even disposable, and may be close enough to calibrate for the absorption and excitation needed.

8 hrs · Like

Nick Chase From my understanding a surplus in h+ kills the membranes proton transport. It simply blocks it. The methanogens can't work against that gradient and the methane and other products are kept inside. Hence the archea die.

8 hrs · Edited · Like

Nick Chase looks promissing

mySpectral Arduino Spectrophotometer: Spectruino

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Thomas Henry Culhane

pH Sensing Kits - Ocean Optics

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Thomas Henry Culhane Yes, let's explore the mySpectral. There is something there....

8 hrs · Like

Eimhin David Callanan If I don't get back here to tag Tiberius can you do it once you have approved him Nick? Tiberius will have friends and a healthy interested network who may be able to help with this problem, the scientific lab background will help I guess.

8 hrs · Like · 1

Thomas Henry Culhane Tiberius Brastaviceanu welcome to our group and our discussion!

8 hrs · Like

Eimhin David Callanan Tiberius, there is a community link between the methanogens and the emergent grower networks in that the effluent is a grade A , highly bioavailable organic fertilizer and also in the fact of the application to food waste recovery programs and so on, the community context here has a lot of potential and the networked context is firmly established, the processes are all Open, the structure is peer production, and the value system highly compatible with what you are doing.


8 hrs · Like

Nick Chase Eimhin David Callanan our processes are not yet open. Our workflows are nearly to 100% within facebook...

7 hrs · Like

Eimhin David Callanan Nick, Tibi can help you with that. Sensorica is one among the leading open peer workflow management systems out there.

7 hrs · Unlike · 1

Eimhin David Callanan aswell as being a p2p OS innovation lab of sorts.

7 hrs · Like

Thomas Henry Culhane Nick is right... we are trying to move our discussions ultimately to our website forum so it can be truly open... for a while I started copying and pasting the most active threads to our website but then we had the issue of whether people who post to this group would permit true openness of their posts, since they joined this "open group" knowing that it is really only open to other facebook users...

7 hrs · Like

Eimhin David Callanan There is a patch from groups on facebook through to outside systems, I know that Ishan from Metamaps did this with the Next Edge right ?

7 hrs · Edited · Like

Nick Chase Thomas Henry Culhane that's not the problem. The team doesn't want to use anything else than Facebook. Its not the "public" it's the team that doesn't like other tool...

7 hrs · Like

Eimhin David Callanan I added Ishan just now, maybe he could tell us more.

7 hrs · Like

Thomas Henry Culhane The team is simply used to getting facebook alerts on their phones and tablets and computers that are seductively easy to respond to, allow instant sharing and cross flow and peering in by others from other groups... no need to open any other apps or windows, hence the default to using it...

7 hrs · Like

Eimhin David Callanan But 100% of use value is captured by Facebook, sooner or later we have to rise to the challenge.

7 hrs · Unlike · 1

Eimhin David Callanan

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Closing keynote of the OuiShare Fest by Michel...

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Thomas Henry Culhane I'm reading Jaron Lanier's book right now!

Web resources related to the book Who Owns the Future by Jaron Lanier

"The most important book I read in 2013 was Jaron...

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Eimhin David Callanan I don't much like his stuff... maybe its just the tone.

7 hrs · Edited · Like

Nick Chase I wonder how the spectrometer thing works.. I mean I worked with spectrometers at the TU Berlin laboratory... I know the principles. I' have to get some hands on the myspectral...

7 hrs · Like

Nick Chase I have checked but couldn't find any price


Ocean Optics is more than spectroscopy. We are rooted in new optical measurement technologies, active in...

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Thomas Henry Culhane Couldn't find price either.

7 hrs · Like

Jeffrey L. Frusha Sounds like the 'sensor' reacts to the pH by changing color, then they probably use spectrum absorption to detect the color and take the reading.

7 hrs · Like

Nick Chase yes, that's it. the question is: how much does it cost and how easily can it be integrated in the system

6 hrs · Like

Eimhin David Callanan This is exactly the kind of thing Sensorica works on

6 hrs · Like · 1

Klaus Warzecha Continuous measurement of pH in suspensions using standard electrodes will certainly reduce their lifetime. The electrode might clog, bleed out, etc. Industrial type electrodes with a gel electrolyte are supposed to work in turbid or intransparent, muddy media, but they will certainly much more expensive. A possible model might be the Prominent DULCOTEST PHEX

Sensoren - ProMinent Gruppe - DULCOTEST® pH Elektroden

Mit unserer Produktlinie DULCOTEST® pH Elektroden...|By Dr. Winkler

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Klaus Warzecha Back in the days, played with earlier versions of Ocean Optics UV/VIS spectrometers. At that time, the signal-to-noise-ratio was worse than that of a standard spectrometer. On the other hand, the foorprint is really small and the modular concept is pretty cool. I'm sure they have improved much over the years and also have started to offer Linux-based development kits.

13 mins · Unlike · 1

Tiberius Brastaviceanu There are optical PH meters. Here are two papers from our database More

Optical PH meter

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Tiberius Brastaviceanu At some point I wanted to make a SENSORICA project to develop an open source version... but I don't know how much interest there is.

We are now working on an open source, low cost optical soil moisture sensor. If someone is interested, please join the project.

Optical soil moisture sensor - SENSORICA

SENSORICA: an open, decentralized and self-organizing value network, designing, producing, and distributing sensing and sensemaking solutions.

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