Public Lab Research note

Making a DIY conductivity probe from a water bottle and metal screws

by donblair | September 30, 2014 23:20 30 Sep 23:20 | #11217 | #11217

Creating a conductivity probe:














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@donblair is there text that goes along with this too?

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Looks great! maybe we could add a picture of how the cut ends of the alligator clips go into the terminal block?

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I found a power drill very useful to get the screws into the caps.

Here's what you need to finish the ends of the wires from the alligator clips:


First you cut the two headed alligator clip wire in half:



Then put about a quarter inch or half centimer of the end of the wire in one of the circle cut holes. Squeeze the wire strippers and pull them sideways to cut off the rubber casing -- without cutting the wire inside!:


You'll have the exposed wire ends:


Stick the ends into one of the holes in the terminal block:


And use a screwdriver to screw down the screw and smoosh the wire so that it's held tightly:


Do the same for the other side (strip the rubber off the end of the wire, then screw the wire into the second hole in the terminal block), and you're done!


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So, I'm making 15 of these conductivity probes to go along with the Digikey order that makes 15 Coquis. Last night cut open 15 plastic bottles with caps, washed the upper parts of the bottles and the caps, and left them to dry on paper towels overnight. Today i started testing whether water leaks through/around the screws in the caps, so i floated two of the bottletops and screw caps on two different cups of water. After 4 hours, the caps did not leak, which is great!

Although now i realize this is not best practice, I took the opportunity while all the caps are dry to put silicone caulk around the base of the screws on the rest of them. This is no longer advised!


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Hey Liz!

Wow -- this looks great! Since this research note was originally posted, there are a few insights that have emerged:

  • Silicone doesn't seem to work very well as a sealant with the typical water bottle plastic -- it tends to leak -- and there's also the worry that some silicone sealants might include materials that would tend to corrode the probe materials over time (provide a conductive path between them). To avoid this, we're currently focusing on trying to use metal screws / bolts with rubber washers on both sides of the plastic, and a nut to tighten the assembly down -- as seen below:


  • We've also realized that running the circuit at > 1.2 volts might induce electrolysis (to some unknown extent) in the liquid, affecting the measurement. So we might try to use a 555 timer with the lowest operating voltage we can find -- thus far, this one seems to run at 1V. We'll have to figure out how we might easily prototype this on a breadboard (might just required a voltage divider to drop whatever battery voltage we're using down to 1 V). Not sure how big an effect this is, or whether 1V is sufficiently low to avoid the problem. The circuit as-is does generally respond to changes in conductivity, so it's still useful as a demonstration of the concept.

Thanks for the great feedback!

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Ok great, thank you for the feedback! My next workshop is a demo only, so the voltage / electrolysis issue won't matter in this case thank goodness. Excited to see a future prototype from the Open Water team that addresses this issue!

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