'POT' issues with long cable

[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]

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.

I like that better than submerged rods, and it will not require a machine shop, wires are in the pipe (encapsulated) great! See, that's some of that great engineering and design found in Ferrari, Alfa Romeo, I like it.

[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.