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Conductivity sensors

L

linnix

Jan 1, 1970
0
mooseo said:
Hmm, no, I really don't want to hear that! Given the realities of my
environment, I'm guessing these things will survive about 2 weeks
before they get destroyed by waves, so cheap is essential.

I'll try some power filtering and see if that helps.

Otherwise, I guess it might be time to look for a new solution...

How many probes do you need in each unit and how many units are you
planning to build? If building enough units, a low impedence solid
ground plane circuit board would help.
 
D

Donald

Jan 1, 1970
0
mooseo said:
Otherwise, I guess it might be time to look for a new solution...

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
 
L

linnix

Jan 1, 1970
0
Donald said:
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.

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.
 
M

mooseo

Jan 1, 1970
0
How many probes do you need in each unit and how many units are you
planning to build? If building enough units, a low impedence solid
ground plane circuit board would help.

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
 
M

mooseo

Jan 1, 1970
0
Donald said:
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.

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.
 
M

mooseo

Jan 1, 1970
0
linnix said:
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.

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

linnix

Jan 1, 1970
0
mooseo said:
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.
 
L

linnix

Jan 1, 1970
0
mooseo said:
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...

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.
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?

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

Christopher

Jan 1, 1970
0
I'm looking to measure when locations on the shore get splashed by
waves. I've tried lots of different things (direct voltage,
capacitance), but none have been ideal.

I've come across an interesting schematic, at:
http://www.emesystems.com/OL2mhos.htm

which uses a 555 timer to read the impedence across a probe. I've
breadboarded this up, and it works perfectly, measuring splashes of
water very quickly, but accurately detecting when the water has drained
away. I have connected the circuit ground to the black wire on the
schematic and am reading the voltage across the 1K resistor, which
gives a measure of the current consumed by the 555 (proportional to the
oscillation rate).

The problem is that I need to have several of these in close proximity
to each other. This means that when they are all splashed by a wave,
the probes will be shorted to each other. In practice, if I have 2 of
these probes running from the same power circuit, when they are both
immersed in the same solution, they act as if neither of them are
submerged.

I can't quite figure out what parts of the cirtuits are interacting
with each other. I have tried connecting the power to them through a
multiplexer, so they aren't on at exactly the same time, but this
doesn't seem to help.

Does anyone have any suggestions?

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
http://www.oldtemecula.com
 
J

James Waldby

Jan 1, 1970
0
mooseo wrote:
....
I'm looking to measure when locations on the shore get splashed by
waves. I've tried lots of different things (direct voltage,
capacitance), but none have been ideal. ....
The problem is that I need to have several of these in close proximity
to each other. This means that when they are all splashed by a wave,
the probes will be shorted to each other. In practice, if I have 2 of
these probes running from the same power circuit, when they are both
immersed in the same solution, they act as if neither of them are
submerged.

I can't quite figure out what parts of the cirtuits are interacting
with each other. I have tried connecting the power to them through a
multiplexer, so they aren't on at exactly the same time, but this
doesn't seem to help.
....

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 http://www.emesystems.com/OL2mhos.htm 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
 
M

mooseo

Jan 1, 1970
0
James said:
mooseo wrote:
...
...

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 http://www.emesystems.com/OL2mhos.htm 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
 
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