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Arduino Conductivity Probe Stage 2

CLOSE UP OF WASHER

This is a close up of one of the washers with a wire soldered to it.

CLOSE-UP of ELECTRODE

Here is the completed electrode.  I am assuming that both sides of the washer will conduct electricity.

ELECTRODE

Another view of the electrode.

The washers are not fixed to the plastic only held there due to the wire

Conductivity is determined by measuring the resistance across two electrodes (metal washers) immersed in the water

Building the probe

We will use the Arduino to measure the resistance in the circuit.  We will then estimate the specific resistance (this takes into account the distance between the electrodes and their size) and then use this to calculate the conductivity.  Below are the equations we will use.
Where
l = distance between the electrodes
A = area of the electrodes  
R = resistance of the circuit

The specific conductance (the conductivity) ,

Therefore to estimate conductivity we need to measure the dimensions of the probe or find a fluid of known resistivity.  Whichever is easier is up to you!  Personally I’m going to try to measure the distance between the electrodes and their size.

Area of the washer, (π/4)[Doutside2 – Dinside2] =  (π/4)[0.0082 – 0.0032] = 0.000043

Area of the electrodes = 2 x front and back area + say 15% for the solder, wire and edges = 0.0002 m2

Distance between the washers = 0.01m or about 1 cm.

To add another complication the conductivity will increase as temperature increases.  The higher temperature makes the ions more mobile.  We are going to have to accept this limitation to the accuracy of our probe at the moment.  
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