Conductivity sensors

It seems to me that an optical solution would be better.

A clear window with a IR LED and IR sensor against the back side of the window. When water is splashed on the window the IR would reflect onto the sensor.

Some experimenting would be necessary, but this would isolate each sensor.

Good luck.

donald

It's possible to do that. But IR would measure more than just water, like bird poops?

I was thinking about the same 555 with internal IR isolations.

Each unit has a single probe, and for this stage of the experiment, there will be 8 probes in close proximity, but they will be in separate waterproof housings.

This is just a pilot project, where I am cabling multiple units to a single DAQ... if it works, the next stage will be to build autonomous loggers that will be battery powered, so no physical connections between them. Which should eliminate all these problems?

Still, I can't justify doing that (designing and building the loggers) unless I've got some initial data to show the questions are interesting. This way seemed simple.

Thanks, mike

I've thought about this before, and it seems like an ideal solution, but I ultimately rejected it... the problem with an optical solution is biofouling... green algae will grow on a clean piece of plastic within a few days at some locations. That's an even bigger problem than bird poop because it doesn't wash away easily.

I sort of understand how IR isolators work for digital lines, but can they work with analog?

Yes, to a certain degree. Do you need analog values or just on and off (with or without water). For more accuracy, you can IR the A2D values with fibers.

That could be your problem. You are building an antenna, not sensors. I would put a $2 microcontroller in each probe and transmit the result via isolated media.

You already verified that this won't work. What else do you need?

Hello Mike,

I think I am a day late on this but interesting.

Your project is so close to the garage hacking I do I will throw in my thought. For the wave sensing transducer I would use a piezo element mounted against the wall inside of a sealed protective enclosure. Splash and wave movement would be easily detectable and distinguishable from one another as audio. This would give you multiple options to interface or convert the final signal. Using a bit of wire, those small inexpensive 462hz walkie talkies would allow you to plug into the mic jack and spread your sensors out over several miles of beach if needed.

Stop laughing,

Christopher

• * * Christopher

Temecula CA.USA

mooseo wrote: ...

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I read the other responses and don't see any convincing arguments re what the problem is exactly, which in part may be because your description of what you are multiplexing and how is rather vague. Re the schematic in

that you referenced, did you build one whole circuit per sensor and then attach each - (white) output to an A-to-D multiplexer input, with all = (black) lines in common to the other multiplexer input, or what?

Perhaps you could see if the problem still occurs with the following hookup: Build one 555 circuit, but without the 2.2 uF caps at the sensor lead connection points. Locally at each sensor, put a 2.2 uF cap in series with each lead and waterproof the connections. Use relays or manual switches to switch one pair of sensor wires at a time into the connection points.

-jiw

Hi James, Thanks for the suggestions. I'll try to provide some clarification. I've been doing some experimenting with your ideas to see how they work.

I've tried a number of different multiplexer scenarios, all based around a complete circuit per sensor. The simplest setup has been to connect all of the black wires together as a common ground with the power supply and the A/D board. The red wires were all connected to the positive of the power supply, and the white wires went through the multiplexer to a single channel on the A/D. This caused all of the afformentioned problems.

I've also tried switching the red and black wires through the multiplexer with the white wires going to different A/D channels... this still had the same problems. When I try this with a "monkey relay" (i.e., running them all through a breadboard and physically plugging and unplugging the red and black wires while the white stay connected to the multiplexer) everything works fine. This suggests that using mechanical relays to switch the power should work (I'm going to try this next week), but running mechanical relays at 10Hz will probably cause problems in the not-too-long term. Solid state relays are a possibility, but I don't have any around that I can breadboard up.

I've tried your suggestions of switching the probes into a single circuit, but with no luck. I tried it in a couple of configurations, both with the capacitors at the circuit end of the wires (which was the default) and out at the probes as you suggested. In the first case, the voltage signal drifted all over the place... in the second case (with the caps out at the probes) the voltage didn't respond to the probes being submerged... this was with the probes running through the multiplexer, so I'm guessing that the resistance and capacitance of the circuit become highly variable.

At this point, a relay solution might be the best bet, but I'm still waiting on an order that will have some inductors to use to try to isolate the AC noise off the ground. And I'm very much open for other things to try.

Thanks again for everyone's suggestions.

mike