Conductivity measurements with Bipolar Pulse

[Markos]

Hi,

I'm trying to implement a conductivity meter with the Arduino board. My
intention is to keep the electronic circuit as simple as possible.

I'm only using a voltage divider as shown:
http://www.c2o.pro.br/hackaguas/figuras/condutivimetro_00_bb.png

The alternating pulses are generated by switching the signals from pins
7 and 8 of Arduino:
http://www.c2o.pro.br/hackaguas/figuras/condutividade_efeito_frequencia_01.png

For now I'm using two steel wires as electrodes:
http://www.c2o.pro.br/hackaguas/figuras/eletrodos_00.png

My intention is to be able to control via software some measuring
parameters, such as electrode polarization time. And adjust these
parameters on the fly depending on the sample concentration range.

Recently I started to read some papers about bipolar pulse technique:
http://pubs.acs.org/doi/abs/10.1021/ac60285a015

The bipolar pulse technique for measuring solution resistance minimizes
the effects of both the series and parallel cell capacitances. The
technique consists of applying two consecutive voltage pulses of equal
magnitude and pulse width but of oposite polarity to a cell and
measuring the cell current precisely at the end of the second pulse.
(Source: Peter Kissinger, William R. Heineman, Laboratory techniques in
electroanalytical chemistry)

Does anyone has any experience with this conductivity measuring technique?

Thanks for any tip,
Markos

[Mathew Lippincott]

Markos,
I don't have experience with the technique but I do know that the regular digital pins of the arduino might not be up to the timing  accuracy you seek.  do you have an oscilloscope to check them with?

alternatively, the Open Water group has been using a 555 to generate their pulses, and with a digital potentiometer or two you could do pulse control you want.
https://publiclab.org/tag/coqui

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[Markos]

Hi Mathew,

I don't have a professional oscilloscope but used this one:
http://www.all-sun.com/en/d.aspx?pht=1184

And I got the following readings:
http://www.c2o.pro.br/hackaguas/figuras/condutividade_efeito_frequencia_01.png

http://www.c2o.pro.br/hackaguas/figuras/condutividade_efeito_frequencia_00.png

I started to mount an oscillator circuit for a conductivimeter
http://www.c2o.pro.br/automacao/figuras/Circ_Cond_Claudimir.jpeg

Following this paper:
http://pubs.acs.org/doi/pdf/10.1021/ed074p572

But right now I'm trying to keep the simplest circuit as possible and try to do more via software with Arduino.

With an electronic circuit oscillator the frequency is constant. But if I could generate pulses with the Arduino I would have the freedom to modify the frequency applied during a reading.

My intention is to evaluate whether it would help to increase the range of linear reading for samples with different conductivities without changing the electrode.

Very interesting the project Coqui and production of sound signals.

(https://publiclab.org/notes/donblair/09-30-2014/coqui-bbv1-0)

Will take a look more closely on this project.

Thanks for the tip,
Markos

[Don Blair]

Hi Markos!

Wow, you've made fantastic progress.  I haven't looked deeply into the paper you mentioned, yet (http://pubs.acs.org/doi/abs/10.1021/ed074p572?journalCode=jceda8)  -- but the circuit seems very simple indeed, and also very low-cost. If you would be interested to do so, it would be great to see pictures of your setup as a 'research note' on Public Lab (publiclab.org) -- there are a lot of people who would see it, and perhaps have comments and suggestions.  (Let me know if you need help with that process)

I look forward to learning more about your setup, as it seems very promising! 

As Mathew pointed out, we have been working on an approach using a '555 timer' (https://en.wikipedia.org/wiki/555_timer_IC).  The circuit diagram and build instructions for building the circuit on a bread board are here:

https://publiclab.org/notes/donblair/09-30-2014/coqui-bbv1-0

Basically, the 555 circuit is set up in such a way that the frequency of its output is related to the conductivity of the solution.  You can then measure the frequency of the output pulses using an Arduino 'interrupt' pin (D2 or D3).  Let me know if that approach sounds interesting to you -- we are testing it out ourselves, and it would be great to get your help :) In general, it would be very useful to see how these two approaches compare for different applications ... 

Thanks so much for your note ... I hope to look into your circuit (and read the paper) more soon ...

Cheers!

Don

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[Markos]

Hi Don,

I took a few steps in setting up a circuit to use the concept of bipolar pulse.

At the moment I am dedicating myself to study the possibilities of this technique:
http://www.c2o.pro.br/hackaguas/figuras/circuito_original_condutancia_pulso_bipolar.png

It is more difficult but I hope to have a circuit that allows me to make changes in reading conditions automatically.

It is a more difficult road, but I hope to have some compensation at the end. :^)

I also took some ideas from this paper:
http://www.c2o.pro.br/hackaguas/figuras/computer_controlled_bipolar_pulse.png

I mounted the following circuit with the Arduino and two opamp 741:
http://www.c2o.pro.br/hackaguas/figuras/Condutivimetro_Pulso_Bipolar_02.png

I made some adjustments in the resistances to get the expected signal:
http://www.c2o.pro.br/hackaguas/figuras/Condutivimetro_Pulso_Bipolar_04.png

Now I intend to prepare some solutions with NaCl and evaluate the linearity for different times of pulse polarization.

A difficulty when trying this method is the time taken by the Arduino to make an analog reading.

I'll see what I can do.

I intend to have something working to share a research note, because I'm still in the trial and error stage.

Any tip?

Thanks for your attention,
Markos

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