Water Water Everywhere or It Makes No Sense.

[itsmoked]

I need to sense water level in a small tank with tap water.

I set out to detect it via conduction. I capacitively excite a probe with 1kHz 24V pulses. I look at the same probe by charging a capacitor via a simple diode rectifier. If the probe touches the water in the grounded metal tank the DC pulse train is supposed to diminish dramatically. The loss should appear on the sensing cap and the comparitor should signal the loss.

It's not working.. I have reduced the caps from 2.2uF clear down to 1000pf. At 1000pf the comparator trips every pulse for about 1/3 the period. It still doesn't definitively trip.

By "caps" I'm referring to both C25 and C27 at the same time.
The comparator's trip point is 4V.
The comparator is running the LED you see.
TP28 Has the 1kHz pulse train on it. This is cycling the drive comparator at the top.

http://www.box.net/shared/static/bopujnd0xu.png

Suggestions?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zappedagain]

I'm just curious because I've wanted to do the same test - what resistance did you measure for your tap, filtered, and RO water?

John D

 

[btrueblood]

"The combination of low current and AC avoids electrolysis"

I really think that is crapola. From a chemistry standpoint, once the metal ions leave the solid cathode and become aqueous ions, even if for a few milliseconds, they aren't gonna go back to the cathode in the same order as they left, and other ionic species are just as likely (possibly more so) to plate before the metal ions you wanted. Otherwise, why do battery plates in wet-cell batteries deteriorate with increasing charge cycles? Smoked's method is really the best, only sample the probes for a brief moment, when necessary, and leave the current off in the between times.

 

[waross]

I may have misspoken. I meant to say that issues based on electrolysis are largely avoided.
See

http://www.gemssensors.com/content.aspx?id=2258

I have used these and similar units from other manufactureres for water level control.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

zapped; I got:
~700k with tap
~860k with filtered
1.4M with RO.


btrueblood; I couldn't agree more.
Once while staying in a cabin at Yosemite's Ahwahnee Hotel I needed to heat some chicken soup. No hot plate.

All I had was a pot and... well nothing. I used a coat hanger I cut in half. I scraped the shellac off the ends and poked them in an outlet bent appropriately to sit in the pot without touching bottom. Then I added the soup.

There was some satisfying sizzling going on so I left it briefly.

On checking the progress I found that the soup had turned a brilliant lime green. I mean b r i l l i a n t! Like glow-in-the-dark brilliant! That was AC and certainly something had gone into solution never to return..

And No! I didn't eat it. I may be crazy but I'm stoopid not.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

I'm thinking the shellac and all of that soup's salt had some fun times together...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[itsmoked]

I think the salt and the Iron in the coat hanger?

Keith Cress
kcress -

http://www.flaminsystems.com