'POT' issues with long cable

[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.

[Melanie]

Joe, Copper is fine if it is 'inert' like a length of pipework... but as soon as you start applying any sort of voltage to it (even if it's micro-volts) it causes electrolysis which speeds up it's demise. I'm not talking about the total destruction of your probe into a heap of debris at the bottom of your tank... think about it... if you are measuring Capacitance or Resistance, the smallest change of just the surface of your probe makes massive changes in your measurements rendering the probe useless - that can happen after only a few days of service!!

[ardhuru]

Some years ago, I had come across a page that advertised a plastic coated tape one could buy in any length; when immersed vertically, the pressure applied on the tape would change its resistance, depending on how much of it was submerged.

Unfortunately, I hadnt saved the URL, and cant seem to locate it now, however much I googled.

[Archangel]

Some years ago, I had come across a page that advertised a plastic coated tape one could buy in any length; when immersed vertically, the pressure applied on the tape would change its resistance, depending on how much of it was submerged.

Unfortunately, I hadnt saved the URL, and cant seem to locate it now, however much I googled.

You can "roll your own" 2 wires, some conductive foam, some heat shrink tubing, seal the end which is immersed . . . HOW DEEP is the water ?

[aratti]

10 stainless still screws (M3) with 20 nuts and 20 washers will cost no more than 1 $. they will be in AISI 304 not 316 but since we are dealing with water, they are ok.

Than he can use a resistive or capacitive solution as he prefer, having the resistors or caps perfectly sealed into a plastic tube.

Where is the problem?

Al.

[Archangel]

Joe, Copper is fine if it is 'inert' like a length of pipework... but as soon as you start applying any sort of voltage to it (even if it's micro-volts) it causes electrolysis which speeds up it's demise.

I am well aware of electrolysis and DC electricity, as you stated:

The secret is to have AC excitation at the probes... electrolyis is then minimal if not completely eradicated. . .

I'm not talking about the total destruction of your probe into a heap of debris at the bottom of your tank... think about it... if you are measuring Capacitance or Resistance, the smallest change of just the surface of your probe makes massive changes in your measurements rendering the probe useless - that can happen after only a few days of service!!

Yeah . . . that's the principle behind electrolytic capacitors, makes them smaller. Again, isn't this a DC problem? I think HIS solution is dependent upon His situation. Is there DC current IN the solution? Is the DC current on constantly, as in a constant monitoring situation? Or is it a check the level once or twice a day situation? How much money is he willing to spend? I have a motor home which has a galvanized steel water tank which has a wire probe to sense the water level, hooked to an analogue meter. this camper is a 1967 model year, and it still works and is not S.S. . The key is it only works when you push the button. I work with stainless steel on an ongoing basis, in a machine shop, and some of it is truly evil to work with. My argument is, unless this is a Life Critical, or very well paid for project which must last for many many years, then there may be alternatives to the highest priced solution. Then as for copper pipe not having electricity you might not be aware in the US the mains are usually grounded to the water supply. Your tank type water heater has a Magnesium sacrificial anode inside to handle electrolysis.

[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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[Byte_Butcher]

The Ping is cute. And its 3 meter range would be just enough for my tallest tank.

The $30 each price tag is a little tough to swallow since I'd need 3 of them.
But it would only require a pipe up the side of the tank with a ping pong ball floating in it.

Hmmm....




steve

[Darrel Taylor]

With the pipe acting as a "wave guide", the range is much further than it would be in open air.

I think even the previous ideas of submerged capcitors etc. would cost at least $30 in man-hours alone building it.
You can mount a pipe in a few minutes.

And you could always "roll your own" with an ultrasonic sensor @ $5.00, if the $30 is too much, but then there's that time vs. money problem again ...
http://www.sparkfun.com/commerce/pro...oducts_id=8505
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[ardhuru]

Sounds good, Darrel.

However, I wonder about 2 things; (a) how would the ambient humidity in a tank (or even that inside the pipe column) affect the transducer's life, and (b) how would the sound of water splashing about, especially when the tank fills up affect the readings?

[comwarrior]

Would it not be far more acurite to use a cheep pic at the tank with 'digital' probes and then comm lines back to main pic?

Other than that, i'm wondering why your using cap/resister solution...
Would two rods running all the way down the tank, one charged with say 5V DC and the other connected to a analogue port... as the water rises the resistance between the two rods changes and therefor the voltage on the analogue port would rise...

Just a thaught...

[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.

[aratti]

Detecting water level with optical system has one big enemy : The algas growth!

Al.

[mackrackit]

Use Ultraviolet light

[Archangel]

Detecting water level with optical system has one big enemy : The algas growth!

Al.

Hi Al,
I was pretty sure You would mention that one, and prepared for it
Illuminate the tube only while checking level. A 2000 liter tank level is not likely to change very much in an hour, so check intervals do not need to occur too often, still EVENTUALLY algae is likely to occur. Perhaps make the sensor rods screw in for cleaning. Dave's UV light would work, but is expensive.

[mackrackit]

A little shot of chlorine down the tube once and a while....

[aratti]

Hi Joe and Dave, going this way it could happen that this thread will come up with the best water level sensor ever invented.

Al.