🎈 Public Lab: Planning for a new Riffle conductivity circuit

Planning for a new Riffle conductivity circuit

by pdhixenbaugh | August 20, 2016 21:32 20 Aug 21:32

What I want to do

I want to build a conductivity sensor that can attach to the riffle inside of a water bottle, for use measuring conductivity changes from road salts during the winter season. The coqui is a good starting point, however there are two questions I have: * Can we use a much smaller voltage to measure the resistance across the water, since we don't want to risk hydrolyzing the water and introducing error? * How can you tell the riffle to output a voltage to the conductivity circuit

My plan for action

If I remember correctly, one of my physical chemistry experiments called for creating a circuit to pass a small current across a "cell" of arbitrary design (two electrodes), and then using an opamp to amplify the signal before it was read by our MATLAB software. I will see if I can find this lab procedure in my notes.
I will gather other research notes and emails detailing the methods others have used
I will read more about the arduino and the riffle to find out how to interface it with such a sensor.

Questions and next steps

If anyone has procedures for creating sensors to use with the riffle, or conductivity sensor designs, I would be interested in reading those.
My goal is to have an attempt by Friday 8/26

Why I'm interested

Stream salinity is a growing environmental impact. If salinity spikes in small streams from de-icing salts, it can be lethal to stream life. Also, stream salinity slowly increases year after year, and does not quickly fall back down when salt use stops. Stream salinity can be readily addressed by implementing BMPs to prevent de-icing salt overuse. The local Maryland Department of the Environment is developing research and possibly a Total Maximum Daily Load (TMDL) allowance for salt. I think developing a conductivity reader that is inexpensive and works well could help cut down on stream salinity.

5 Comments

Exciting! thanks for posting about your upcoming process. sounds like you've got it planned out.

I know you're very active in the discussion @donblair started previously about open conductivity questions, but I wanted to drop the link in for others to follow the research trail: https://publiclab.org/notes/donblair/01-07-2016/conductivity-sensing-open-questions

@pdhixenbaugh have you had a chance to make much progess on this? Here is a link to a circuit design from Analog Devices that claims to have pretty good performance. http://www.analog.com/en/analog-dialogue/articles/fully-automatic-self-calibrated-conductivity-measurement-system.html

This uses an Analog Devices processor, but I suspect this could be moved to an Arduino if desired. I'd be happy to help with that if you'd like.

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Hello @rfdave ! I would be interested in getting this rolling! Thank you so much for providing that link to the design! I think there are many folks on public lab who have this expertise in hardware making and science, and I hope we can develop this together. One thing that happened after I posted this note is I became aware of @bhickman (Ben Hickman)'s design here: https://publiclab.org/notes/bhickman/05-09-2016/conductivity-and-temperature-meter

This seems like it is already one solution to my questions. I wonder if it could be simplified in conductivity cell design so that it was easier to construct, and can me optimized for stream conductivity measurements. (Maybe it doesn't care about low conductivity measurements, but accurately measures a spike in conductivity, or maybe you make two sensors, one that measures typical conductivity well, and the other one measures the spike in conductivity well. Or maybe, with the range in conductivities we're looking at, we only need one cell constant to do both.

I see that your background is in electrical engineering -- I'm wondering how the Analog Devices' circuit compares to Ben Hickman's design in terms of complexity?

I think there may be two endgoals for designs here:

A conductivity probe that is able to be assembled by an end-user with limited experience and access to hardware (DIY solution)
An inexpensive, good, troublefree design that is able to be assembled on a circuit board in quantity and distributed to users.

The impatient part of me wants to focus on an easy, DIY solution that people can start building on their own, and start measuring conductivity in streams ASAP. But the better long term solution might be to work on a design that's an open hardware circuit board for measuring conductivity board that could be assembled by a factory.

What are your thoughts so far?

Hi @pdhixenbaugh I finally got some time to look at both schematics. The Analog Devices one is ~ 3x as many parts adn @bhickman design, although both of these schematics are missing the power supply sections, and the AD one is more complicated, it requires more voltage rails, so the complexity is going to go up more. From an accuracy point of view, AD goes through an analysis of the measurement accuracy, so from an expense point of view, the design based on the AD design will be more expensive, but possibly more accurate.

From a DIY point of view, starting with a fabricated board, there are more components, so more work and more money to build with the AD design.

Thanks @rfdave! This is a great start! I am going to inquire about what kind of accuracy @bhickman got with his as the next step.

@liz roping you into this here discussion. Are you aware of what type of work the mayfly developers are doing on conductivity sensors, or who to ask about that?

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