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Arduino Conductivity Probe


This sketch shows the wiring diagram for the conductivity probe.  The electrodes are made from 2 no. steel washers.

Conductivity is a measure of the ability of water to conduct electricity which is a measure of the amount of charged particles it contains

In this project we are going to make device that can measure the conductivity of water.  What for you may say!  Well often if water in our rivers and stream has a high conductivity it may be indicate that the water is polluted.  Simplistically the more stuff (ions) in the water the higher the conductivity.  This stuff can arise naturally,i.e. from the soil, but is all too often due to discharges from drains or fertiliser from fields.  In a later post I will say more about this and use the probe to investigate the water in a stream running near to my house.  Conductivity meters are also used to measure the conductivity of water in aquarium and hydroponic systems and thus to judge the state of their health.

So what is water conductivity?

Put simply conductivity is the ability of the water to conduct electricity.  This is influenced by the amount of ions in the water such as calcium, nitrate, ammonium etc. but also by the temperature.  The SI unit of conductivity (specific conductance) is the Siemen per metre or S/m.

Probe design

This project owes much to the conductivity probe example given in the great book Environmental Monitoring with Arduino by Emily Gertz  and Patrick Di Justo.  I’ve adapted the design to make the unit a bit more robust.  This should ensure that the results are more reproducible.  The probe is good enough to give you an indication of whether different waters are more or less conductive than others.

Conductivity probes work by measuring the resistance in a circuit.  In this instance the circuit includes a gap filled by our water between two electrodes.  This is essentially like adding a resistor in series.

The more conductive the water (i.e. the more charged particles it contains) the lower the resistance, but equally the smaller the gap and the larger the electrode the lower the resistance.  In science we often standardise methods of measurement so they are consistent between instruments.  If you do it one way and me another we couldn’t compare our results.  So we take this into account iby determining the specific conductance that should be consistent between different probe designs.

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