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So I want to measure water level under a bridge, or inside the city drain.

Would it be possible to run two long (4 meter) metal probes, each one enclosed inside a PVC pipe, and placed one next to the other separated by a few cm, and use that as a capacitive sensor? I know it works on a small scale (two insulated PCB pieces inside a water bottle). But will that work on a bigger scale?

The idea is that the metal probes work as a capacitor, and as water level increases and occupies the space between the two PVC pipes, capacitance should change.

Would it be better to make the probes out of copper foil ? I would prefer 2" x 3/16 steel bars just to make it more rugged, but don't know if that would affect the electronics.

The sensor will be read with an Arduino + http://playground.arduino.cc//Main/CapacitiveSensor?from=Main.CapSense

I also have an ultrasonic sensor, but another application for this would be to put it inside a hole (drain) that sometimes gets flooded (there is a chance that the sensor gets wet).

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    \$\begingroup\$ You might be better off using an ultrasonic distance sensor, pointed down from the bridge perpendicular to the water surface. \$\endgroup\$ – Anindo Ghosh Apr 15 '13 at 19:15
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Nice project. Any conductor, even relatively weak ones, will do because the capacitance will be in series with either <1 ohm or 100 ohms+ and the resistance won't make a deal of difference. Water temperature, contaminant levels, salinity and eels may all conspire to give you a false reading though but good luck and you should get fairly decent reults. Do you plan to use the "capacitor" in an oscillator circuit?

One more thought - just in case the water physically touches the probes avoid any DC levels in case these give you "other" effects like killing an endangered species of blanket weed.

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  • \$\begingroup\$ 5v from the arduino only. Probes completely enclosed inside the PVC pipes. BTW will that current be enough? \$\endgroup\$ – marianov Apr 15 '13 at 19:28
  • \$\begingroup\$ @marianov what circuit will interface the probe to the arduino? You need to have some kind of oscillator so that "capacitance" or chages thereof will be measurable. \$\endgroup\$ – Andy aka Apr 15 '13 at 19:31
  • \$\begingroup\$ I thought of using the CapSens Arduino library (arduino.cc/en/Tutorial/CapacitanceMeter) but given the very low capacitance that apparently the steel bars will give, I'll use the solution proposed in this answer: electronics.stackexchange.com/a/45457/16720 \$\endgroup\$ – marianov Apr 18 '13 at 18:55
  • \$\begingroup\$ @marianov 2nd solution looks cool \$\endgroup\$ – Andy aka Apr 18 '13 at 19:02

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