We've just published our DIY underwater loggers: http://www.mdpi.com/1424-8220/18/2/530 and the #1 thing we would like to add to the platform is conductivity. The loggers themselves will easily run longer than a year, but unfortunately we can't find any other projects that get much more than a couple of weeks out of traditional contact probes [like the Atlas series] before they drift quite badly. Does anyone know of other projects building conductivity probes which hold calibration for months to a year?

  • $\begingroup$ I've built DIY conductivity probes for subglacial measurements. It is also aurduino based, it measure conductivity, turbidity, pressure, reflectivity and more, and transmit to a surface logger. For calibration stability I used graphite probes with high frequency alternating current. In DIY probes usually degradation comes from using direct current or easily corroded materials. However, my sensors remain below ~100m of ice, so I haven't been able to test changes in calibration. At least they have produced sensible data for 2 years now. If you want I can post an answer with details of my setup. $\endgroup$ – Camilo Rada Feb 13 '18 at 22:52
  • $\begingroup$ Sounds like you are in fresh water. I'm definitely interested in how you built your graphite probes (and determined the k constant?), but I forgot to mention I'm trying to cover the entire fresh - marine range. Our work is in coastal caves that regularly get flow reversals at high tide. Are you generating AC from the Arduino PWM, or using a separate oscillator circuit? The fellow at gasstationwithoutpumps.wordpress.com/2013/12/21/… has an interesting approach, but he gets drift - even with high frequency AC. $\endgroup$ – Ed Mallon Feb 14 '18 at 17:14
  • $\begingroup$ Ok, I'll add an answer later today. I do generate the AC from the arduino PWM, but with a false ground as you need the pulse to be symmetrical ground 0 V in order to minimize probe degradation. $\endgroup$ – Camilo Rada Feb 14 '18 at 17:18

I've built DIY conductivity probes for subglacial measurements. Unfortunately my sensors remain under ~100m of ice so I haven't be able to recover them to check the calibration, but for the same reason, I've taken multiple measurements to reduce calibration drift, as I'm unable to re-calibrate them. So far, they have produced sensible data for 2+ years, but I haven't check the calibration. However I can share the results of my tests and research in order to produce a stable sensor.

There are two main types of conductivity measurements: The first one is the potentiometric method. This is the most common one, and measures conductivity by creating an electric current between two or more electrodes embedded in the medium to measure. The second type is the inductive method, in which is a magnetic field what goes trough the medium, and you measure conductivity through variations in the induction effect that that magnetic field have in a coil. enter image description here

(picture taken from Knight)

Inductive probes are contactless, therefore, the most stable ones. However, they are more complex to build and don't work well with very low conductivity (< 100 uS/cm). Glacier water is often in that very low conductivity range, something that, in addition to the higher complexity made me choose the potentiometric method (you can see a bit of my attempts to put together an induction sensor in this forum).

For potentiometric probes, oxidation, deposition of salts or others chemicals can change the calibration parameters. This would be enhanced when direct current is used, as that will create acids in the water that will quickly corrode the probe. To reduce that problem, probes of inert materials are used, like platinum, gold or graphite (more common in DIY projects due to its lower cost). But to reduce deposition of chemical compounds in the probes you still need to avoid direct currents and use high-frequency alternating currents with zero mean, otherwise if there is a net current towards one electrode, you will accumulate acids or ions there that can degrade or coat the electrode and change its properties (i.e. change the calibration).

enter image description here

The picture above shows an example of an electric signal that can be generated using a false ground and the PWM output of an Arduino board. The setup could be as follows: enter image description here

Where R1 and R2 have in general to be equal to ensure a signal with zero mean, and the value have to be chosen depending of the range of conductivity values you want to measure and the resolution. If you want to cover a wide range (pure cave water to marine water), you will have very low resolution (un less you use a AD converter with more resolution than the 10-bits provided by Arduino boards). Alternatively you can have multiple sensors each one optimized for a different range.

For probes meant to be deployed outside of lab conditions, concentric electrodes are recommended, otherwise natural currents going trough the ground/water can affect the measurements. The setup I used is this one: enter image description here Where a central graphite electrode is surrounded by a ring of graphite electrodes. As you can't solder to graphite, I used conductive epoxy to glue it to a piece of plumbing copper tube modified with legs so it can be solder to a PCB. It looks like this: enter image description here

To avoid gluing, some people choose aluminium or stainless steel electrodes, that are affordable and do not suffer much of chemical corrosion.

Some of the data that was produced by this probes were used for this publication.

Many comercial setups use three or four electrodes. I don't understand all the advantages of those sensors. In some cases they are just multiple sensors each one calibrated for a specific range. I've also seen three concentric electrodes. I haven't do the math, but my suspicion is that they measure the current in two independent electrodes with different surface area. Then, you can model and calculate the resistivity of the layer of deposits/corrosion that covers the electrodes and compensate your reading accordingly, reducing the calibration drift issues.

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    $\begingroup$ Wow! that is one of the best probe builds I ever seen. I especially like the way you've integrated the potting with the overall build. Do you have any pubs yet? I'd love to put a reference to it on our blog. Does that RS breakout mean you are hard-wired to the surface? I originally thought you were transmitting wireless, but I was scratching my head about how you pushed the signal through that much ice. We tried "thermally conductive" epoxy on some temperature sensors a while back, but the saltwater ate through it in a couple of weeks. Which conductive epoxy have you had luck with? $\endgroup$ – Ed Mallon Feb 15 '18 at 17:14
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    $\begingroup$ @EdMallon We have a publication in review process, but it doesn't give much details of the probes: the-cryosphere-discuss.net/tc-2017-270 We are indeed hardwired to the surface. Using RS-485 allow cable lengths in excess of 1 km. However there exist wireless probes that manage 1km+ of ice thickness. The epoxy we use is the "MG Chemicals Epoxy - Clear Encapsulating & Potting Compound", we have recovered a few sensors after a few years and they seem unscratched. It is commonly used for encapsulating marine electronics, so I guess it handles sea water with no problem. $\endgroup$ – Camilo Rada Feb 15 '18 at 17:35
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    $\begingroup$ Let me know as soon as that paper gets through, and I will add a ref to it from thecavepearlproject.org/2017/08/12/… There would be many other researchers interested in the build details for that probe, as conductivity remains one of the most expensive sensors to buy. In fact that would be a good way to start up your own research blog, if you don't already have a decent place to host that info at your university. $\endgroup$ – Ed Mallon Feb 15 '18 at 17:43
  • $\begingroup$ @EdMallon Thanks Ed. I'll let you know. However, the paper is citable already, check the PDF here the-cryosphere-discuss.net/tc-2017-270/tc-2017-270.pdf If you try a similar setup and do further testing and validation please let me know. I have also done the math to optimize the measurements and the values of the resistors for a given conductivity range, let me know if you want more details. The idea of modelling the corrosion with a three electrode setup is something I would like to pursue for the next batch of sensors, that might come up once I finish my Ph.D. $\endgroup$ – Camilo Rada Feb 15 '18 at 17:49
  • $\begingroup$ Now that I'm thinking about it, how battery power supplies react to cold temperatures "in the real world" is another detail often left out of formal publications. It's almost impossible to get good metrics for battery behavior at low temps with the kind of "pulsed loads" you typically see in logging applications. $\endgroup$ – Ed Mallon Feb 15 '18 at 18:03

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