Build a Conductivity Probe

A typical component of most RET programs is that the participating teacher create an activity or lesson derived in part from the work they have participated in.  To that end, I have created a lab activity where students create a simple DC conductivity probe that they can use to detect if a solution is conductive (electrolytic) or not (non-electrolytic).  The probe consists of a pen barrel, copper and nichrome wire, and electrical tape.

DC Conductivity Probe

DC Conductivity Probe

The activity then has students construct two different circuits, using common electrical components and a breadboard. The first circuit is a replication of commercially available “all or nothing” conductivity testers.  The functionality essentially works like this: if the solution conducts, the red LED will light up, if not, the LED remains unlit.

LED based conductivity circuit

LED based conductivity circuit

LED circuit with conductive solution

LED circuit with conductive solution

The circuit is straightforward to construct, with electrical components costing less than a dollar overall (assuming you have the breadboard and 9V battery).  The circuit schematic looks like this:

Circuit schematic for the LED circuit (N.B. the battery should be 9V, not 3V as labeled in the image)

Circuit schematic for the LED circuit (N.B. the battery should be 9V, not 3V as labeled in the image)

After constructing the circuit and connecting the probe, students test the conductivity of several common solutions: salt water, sugar water, distilled water and tap water.

The complexity increases as students move into the second circuit.  This circuit utilizes an Arduino UNO and LCD in order to display a calculated “relative conductivity” value, which is based upon the voltage drop across the conductivity probe.  The setup looks like this:

Setup for using an Arduino UNO and LCD to connect to the conductivity probe and display relative conductivity values.

Setup for using an Arduino UNO and LCD to connect to the conductivity probe and display relative conductivity values.

Note that the black and red wire pointing upwards in the upper right corner of the image would be connected to the conductivity probe.  This is a more intense wiring project, but is fulfilling when the student is able to get their apparatus working.  After troubleshooting any issues, students re-test the same solutions that they had previously when using the LED circuit.

If you’d like to try out this activity, feel free to download the pdf of the student handout.  I’d be happy to send along supporting materials with setup costs, implementation and assessment criteria as well; just send me an email or contact me via twitter.

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