'POT' issues with long cable

[aratti]

how did you manage to connect the resistor(s) to the stainless rods

You thread one end of the rod and use 2 nuts and two washers to hold the resistors wires.

did you find the readings highly non linear over the range? At times it is difficult to resolve the levels, especially at the lower end

Linearity depends on the liquid in use (how many ions you have in suspention) but don't expect very high linearity from it.

You can always correct the reading via software knowing the real values taken on the bench shorting the electrodes one by one.

Al.

[Archangel]

Hi ardhuru,
All that stuff you mentioned in Post 1 sounds pretty expensive, and Al's SS Rods do too. So using Al's Idea a little differently, suspend 4 copper stiff wires or rods vertically at different lengths, which you can adjust easily to fit your needs. You can Solder the resistors or Potentiometers to those wires, and install a PIC circuit there, powered by a couple of solar cells easily removed from 2 cheapie Yard Lights, and yes use the nicads that come in those too, then send some form of slow serial data using relatively inexpensive wire, TV lead in, sprinkler wire, twisted doorbell wire . . . to another PIC . Compared to shielded wire and fancy Stainless steel rods this should be cheap. You could put the wire array in a piece of plastic pipe and insert it vertically into the Well, sump, or whatever it is you are checking.

[ardhuru]

Hi ardhuru,
All that stuff you mentioned in Post 1 sounds pretty expensive, and Al's SS Rods do too.

Actually, its not. I just use 10 0.1 uf capacitors strung (stringed?) together in series. A bit of labour involved, but the material cost is nothing.

Hi ardhuru,
and install a PIC circuit there, powered by a couple of solar cells easily removed from 2 cheapie Yard Lights, and yes use the nicads that come in those too, then send some form of slow serial data using relatively inexpensive wire, TV lead in, sprinkler wire, twisted doorbell wire . . .

I was also toying with the separate-pic-for-the sensor idea. In fact, I thought I would use Melanie's trick of data-over-power from an earlier thread to be able to do this over just 2 wires. Wasnt sure how reliable the serial comms would be with my already laid twisted, non-shielded cable.

Will give this a try.

[Melanie]

I did have an earlier version with the resistance being a variable, using the ADC, but this tends to get corroded within a few weeks.

The secret is to have AC excitation at the probes... electrolyis is then minimal if not completely eradicated. 1kHz HPWM through say a 100nF Capacitor is just perfect for the source probe.

[mackrackit]

Go to your local welding shop and talk them out of a piece or two of SS tig wire.
And with a little practice and some silver solder you can make the connections that way.

[ardhuru]

The secret is to have AC excitation at the probes... electrolyis is then minimal if not completely eradicated. 1kHz HPWM through say a 100nF Capacitor is just perfect for the source probe.

This would be the best, easiest to adapt/incorporate in my earlier design, with a minimal of hardware/code change if I can get it to work for my app. I didnt realise I could use a PWM'ed signal in conjunction with ADC.

I just hope I am able to resolve the 10 steps I'd like to have in the sensor reliably.

Anand

[Archangel]

Hi Adhuru,
Curiosity has got me, what are you trying to keep track of the water level in ? Have you considered using a float and some switching method like hall effect or optical ? How deep of a cistern, sump, creek, pool . . . , 1 meter, 10 meter . . . does this have anything to do with fresh potable drinking water for people or animals, or vegetables to be eaten by people or animals? I have to ask that question because the way we might measure waste water is somewhat less critical than how one must treat fresh water in terms of chemical and metal exposure. Is there power available in the area where the sensor goes? Is radio telemetry an option? This thread has many useful possibilities for people around the globe, and I am betting many are interested. So I just HAVE to say . . . GOOD SUBJECT !

[ardhuru]

Hi Adhuru,
Curiosity has got me, what are you trying to keep track of the water level in ? Have you considered using a float and some switching method like hall effect or optical ?

Basically 3 tanks 2K to 5K litre capacity, non-metallic, heights 6 to 8 feet. Having got a quantitative reading from all 3 tanks, there's logic that also controls a pump to keep the levels at a required level.

Unfortunately, yes, I did look at the alternatives you mention and had to discard them for various reasons:

Floats (I assume you mean the kind that is anchored, and has a float connected to a pot): would be difficult to get movement thruought the height of the tank. Also, the ambient humidity would junk it in a short time.

Hall effect or optical would be good, but I need to sense 10 levels in each tank; would be complex/expensive. I even considered an IR scheme with the technique Bruce describes on his site for distance measurement. Not to mention optical encoders with a float.

Is there power available in the area where the sensor goes? Is radio telemetry an option?

Yes, power is available, but I think if i could get away with data-over-the-Vcc-line, that would be more elegant. And cheaper than RF.

But first, I'm going to try out Melanie's suggestion about PWM and ADC;

This thread has many useful possibilities for people around the globe, and I am betting many are interested. So I just HAVE to say . . . GOOD SUBJECT !

Thanks; I've been trying to achieve this for a long time. If there's any interest on this subject, I'll keep posting my observations on this thread.

[Melanie]

I've done heaps on water-level sensing over the years... and my conclusions are that anything you put in the water will corrode and die eventually if it's not encapsulated and sealed or it's Stainless Steel.

A Capacitive solution is fine - providing those Capacitors do not touch the liquid... If you use the water-level as a dielectric, the Capacitance between two immersed probes will change - but you MUST use AC excitation for your probe circuitry. (Ardhuru - you MUST use shielded cable where the cores are individually sheilded, because across 50m of cable the Excitation Core will start inducing a signal into the Sensor Core - and you MUST capacitively couple BOTH the outgoing and the return core. Remember also that the return is high impedance). Everyone uses Resistance measurement, but that can change so much depending on the water purity. Go up into the mountains of Scotland, and some of the water coming out of the ground there is so pure that it's in the tens of Megohms with probes only a centimetre apart! Never mind bottling drinking water for humans (which is what the Scots are doing), I'd be bottling 'distilled' water for lead-acid cells and selling it at ten times the price! Hmmm great idea, time to Google for properties for sale in Scotland...

Another method is to use a low level RF tuned circuit with your coil as the probe (this is actually a Melanie invention). The water level rising up the encapsulated coil changes the tuned frequency (it would do since you are adding external Capacitance)... kinda like putting your fingers close to the LO of a Superhet Receiver changes the frequency. This is best in steel Tanks, but you have to watch the RF field level isn't too great otherwise you'll have your local FCC on your case!

Honeywell do some lovely optical Sensors... but you will need to mount ten of them and they are quite pricey each (around $15) - but they are favourites of mine as they have no moving parts the way floats or reed switches do. I'm not sure, but I do recall a multi-level magnetic reed switch (the reeds are mounted in the central column which is fully encapsulated, and there is a ring magnet on a float that travells up and down the central shaft). Danfoss is the manufacturer that spings to mind - but I may be mistaken.

Mind you, even with seven years playing with liquids there's still some level-sensing applications I can't get my head around to solving properly...

[Archangel]

I've done heaps on water-level sensing over the years... and my conclusions are that anything you put in the water will corrode and die eventually if it's not encapsulated and sealed or it's Stainless Steel.

That's it in a nutshell, either or. The key here is eventually. Copper pipes last 40 + years in water, but Eventually fail. Stainless steel is a pain in the . . . eye to work with without a machine shop, and cobalt or carbide tooling. Plastic on the other hand is very user friendly (encapsulated). The reed switch idea is good, and cheap, and could be easily encapsulated in plastic or a glass tube, a magnet on a float ring on the outside. Again it would be helpful to know the range we are measuring, how long it needs to last, 5 months, 5 years, 500 years . . . when selecting a method.

[Byte_Butcher]

Basically 3 tanks 2K to 5K litre capacity, non-metallic, heights 6 to 8 feet. Having got a quantitative reading from all 3 tanks, there's logic that also controls a pump to keep the levels at a required level..

I have a similar project I need to build, but haven't had the time yet.
Same type of deal... need to read the water level in two 11,000+ liter (3000 gallon) tanks and one 1200 gallon (4600 liter) tank.

I was thinking of using a 16F726 at each tank and using the built in capacitive sensing module. I would use small strips or squares of copper tape stuck to the outside of the tank at 10 different levels. That would sense the water level directly, with each square of foil being one capacitor plate, the water being the other, and the plastic tank wall the dielectric.

Water level data from each PIC would be transmitted down the hill (about 500 feet) through Cat5 cable to a "master controller" PIC located here at the house.

I'll be curious to see what solution you come up with and how well it works.


steve

steve

[Darrel Taylor]

How about an ultrasonic transducer like the parallax Ping.

Put a vertical PVC pipe in the tank, open at both ends, with a float in the middle.
At the top is the Ping measuring the distance down the pipe to the float.

Then you have a continuous level sensor.
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[Archangel]

Basically 3 tanks 2K to 5K litre capacity, non-metallic, heights 6 to 8 feet. Having got a quantitative reading from all 3 tanks, there's logic that also controls a pump to keep the levels at a required level.

Unfortunately, yes, I did look at the alternatives you mention and had to discard them for various reasons:

Floats (I assume you mean the kind that is anchored, and has a float connected to a pot): would be difficult to get movement thruought the height of the tank. Also, the ambient humidity would junk it in a short time.

Hall effect or optical would be good, but I need to sense 10 levels in each tank; would be complex/expensive. I even considered an IR scheme with the technique Bruce describes on his site for distance measurement. Not to mention optical encoders with a float.


Yes, power is available, but I think if i could get away with data-over-the-Vcc-line, that would be more elegant. And cheaper than RF.

But first, I'm going to try out Melanie's suggestion about PWM and ADC;



Thanks; I've been trying to achieve this for a long time. If there's any interest on this subject, I'll keep posting my observations on this thread.

I do not know how I missed this post .. . Here is an optical method. Water is a pretty good difuser of light, way better than air, if you make up a vertical pipe with clear lucite pins penetrating the pipe and covered on the pipes inside with black tubing so light will not penetrate from their sides, when they are covered with water the light should be able to enter through the ends, or just use fiber optics. Please, pay no heed to the fact I misspelled TUBES as Tubs in the picture.

[mackrackit]

Hey Joe,
That is a good idea, I am going to give it a try.
For years I have had a similar problem but with hot sulfuric acid under pressure. Sonar and radar sensors work for a time, but sooner or later everything fails in that environment.
I never thought to use fiber optics.
Thanks!!!

[ardhuru]

.. . Here is an optical method. Water is a pretty good difuser of light, way better than air, if you make up a vertical pipe with clear lucite pins penetrating the pipe and covered on the pipes inside with black tubing so light will not penetrate from their sides, when they are covered with water the light should be able to enter through the ends, or just use fiber optics.

Looks good. In fact, I guess one could use IR detectors directly in place of the lucite pins, of course covered with black tubing so that only a small aperture at the tip is open.

Need to try this approach.