im doing a water level controller project and This circuit works off a 12V battery or 12V AC mains The sensors are connected to the circuit at appropriate terminals. negative terminal of a power supply gnd is at the bottom of the tank, sensor terminal L is just above the bottom of the tank and sensor terminal H is at the top of the tank.

Since gnd terminal is at the bottom of the tank, when the water level falls below sensor L, and micro controller is triggered and its output goes high. As a result, the Relay energises and the motor starts filling water in the tank. The motor remains ‘on’ even when the water level crosses sensor L.

As water in the tank rises to touch sensor H, the micro controller is retriggered as a result, its output goes low. The relay de-energises and the motor stops filling water in the tank. The motor remains ‘off’ even when the water level falls below sensor H.

As water is consumed and its level falls below sensor L, the motor restarts. Thereafter, the cycle repeats.

You can also manually start and stop the motor using switch this is the concept

but im facing some of the problems here i.e due to rusting of sensors when it is drop into water
1) DC voltage when the sensor is powered with the dc voltage the circuit is working fine according to the logic but the rusting of sensor is much faster, i.e the salt like white material is accumulated on the sensor here is the ckt and the pics of the demonstrations


sensors before connecting

sensors after connecting


so this is the result which i got from the above ckt, for dc voltage

2) AC voltage when the sensor is powered with the AC voltage the circuit is working fine according to the logic and there is no rusting across the sensor and here is the pics of the demonstrations for AC source

sensors before connecting


sensors after connecting

this is the result which i got for AC voltage


1) so my question is why the sensors are rusting in DC voltage and why not it is rusting in AC voltage and if i want to use in the dc voltage which sensors i should use so that it should not rust i want to use only metal type sensors not floating type sensor or else how i can prevent the rusting of sensors
2) if i go for the AC source i need to drop AC 12 to the waer from the transformer if the transformer fails due to derating so the high vlt flows to the water which is very serious but the sensor quality will be good


so if you have any idea then please suggest to me how i can over come the above problem

THANK YOU

 

Hello,

When using DC voltage for the sensors, electrolysis can happen.
https://en.wikipedia.org/wiki/Electrolysis
The electrolysis will "eat" the probes.
When using AC, the material is take off and putted back to the probe.
When you want to use DC, the probes must be made of noble material.
The wiki shows platinum and titanium are fairly corrosion resistant:
https://en.wikipedia.org/wiki/Noble_metal

Bertus

 

With DC you're creating a sort of electroplating solution. Material from one probe will migrate to the other, typically the positive lead. When using AC you have current going in both directions; for a moment it's moving toward one probe then it reverses direction and migrates back to the other probe. With AC you don't get the deterioration nearly as fast as you do with DC.

As for your probes, why steel? Why not copper? Or brass? And since I don't know for sure, why salt? Why not vinegar or baking soda? ANY dissolvable medium that can create an electrical path through the water should do. Personally, I'd opt for DC and use just two probes, the high limit and the low limit. If for some reason you wish to have some kind of level indicator then I can see using the 3/4, 1/2 and 1/4 sensors.

This isn't for some sort of drinking system I hope. You're changing the PH of the water as you pass current through it, especially with chemicals added. I would be interested to know what your purpose is for this tank.

[edit] beaten by 3 minutes.

 

If you must use DC (personally I wouldn't), then you could reduce the electrolysis by using much lower currents through the probes and/or only energising the +5V probe occasionally. Since the probes will be subject to interference pickup their inputs to the MCU should have capacitors to shunt AC to ground.

 

You will get electrolysis with DC voltage,
Replace the 5VDC ref bias with 3VAC signal, on the tank electrode alone.

 

With DC you're creating a sort of electroplating solution. Material from one probe will migrate to the other, typically the positive lead. When using AC you have current going in both directions; for a moment it's moving toward one probe then it reverses direction and migrates back to the other probe. With AC you don't get the deterioration nearly as fast as you do with DC.

As for your probes, why steel? Why not copper? Or brass? And since I don't know for sure, why salt? Why not vinegar or baking soda? ANY dissolvable medium that can create an electrical path through the water should do. Personally, I'd opt for DC and use just two probes, the high limit and the low limit. If for some reason you wish to have some kind of level indicator then I can see using the 3/4, 1/2 and 1/4 sensors.

This isn't for some sort of drinking system I hope. You're changing the PH of the water as you pass current through it, especially with chemicals added. I would be interested to know what your purpose is for this tank.

[edit] beaten by 3 minutes.

i want to check the water present in the sump and depending on that i want to turn on the motor and feel the upper tank

 

You will get electrolysis with DC voltage,
Replace the 5VDC ref bias with 3VAC signal, on the tank electrode alone.

sir i change the above ckt and also tried with by putting gnd into water and using pnp transistor the rusting is same.

 

sir i change the above ckt and also tried with by putting gnd into water and using pnp transistor the rusting is same.

Why did you change to PNP transistor?
Why did you put GND in the water?

 

Why did you change to PNP transistor?
Why did you put GND in the water?

i taught the gnd the is a 0V potential so it will not rust more

 

Apply DC to how ever you want and still it will rust.

Why not use a float switch or ACV as suggested.

 

I would try leaving your circuit as shown except change the signal going to the ref electrode.
I would find a very small low voltage transformer that outputs 3VAC. Connect one wire from the transformer to the body of the tank (assuming it is a metal tank).
Connect a 10kΩ resistor in series with the other wire from the transformer. Connect the resistor to the ref electrode.

 

Hello there,

Either type of current, AC or DC, you may run into problems because both types of current cause a movement of atoms of material. There's a formula for how many atoms move with each electron; you could look that up if you wanted to. DC moves the material in one direction, and AC moves material in both directions. DC causes the material to move away from the source never to return, AC moves the material away from the source then back to the source, DC causes deep etching, but because AC does not have a 'brain' when it takes material away it may not return it to exactly the same place where it came from, and so we get an effect there too. The effect may look like a blackening of the surface of the metal.

Back in the 1970's (yes a long time ago) i etched an ID number into some of my tools using DC current. I dont remember how much current i used (100 ma, 1 amp, or more maybe) but i could get some very deep etching into the tool metal with DC current. I would cover the tool with wax and scrape away the area i wanted etched, then immerse it into a salt water solution, then apply current to the tool and water with a carbon electrode. The etching starts fast and completes in a fairly short time, but again i dont remember the current level i used. You can get deep etching though, at least 1/32 inch and even 1/16 inch deep which is pretty deep in a tool.

As mentioned, AC is better but you could still get very undesirable effects. This means you have to find a different way to do this or else turn to something that does not get bothered as much like carbon.

Did you happen to mention exactly what kind of sensors you are using? That might help figure out an alternative.
They look like they might just be pieces of metal. In that case, try using a short piece of carbon rod. You can get this from old carbon zinc batteries that are taken apart very carefully (the materials inside a battery can be dangerous so be careful). The wires probably have to be friction fit, so you may have to wrap turns of copper wire around the electrode, then coat it with an insulating material so the wire end does not get exposed to the water either. There are also conductive epoxies that might help.

Lead is also pretty good at resisting rust, but that's not the greatest idea either as lead can be dangerous too. A kind of funny solution might be to get an old fishing sinker and tie the copper wire though the loop on the sinker, then coat that end with some waterproof material. A big sinker should last a long time

 

Hello there,

Either type of current, AC or DC, you may run into problems because both types of current cause a movement of atoms of material. There's a formula for how many atoms move with each electron; you could look that up if you wanted to. DC moves the material in one direction, and AC moves material in both directions. DC causes the material to move away from the source never to return, AC moves the material away from the source then back to the source, DC causes deep etching, but because AC does not have a 'brain' when it takes material away it may not return it to exactly the same place where it came from, and so we get an effect there too. The effect may look like a blackening of the surface of the metal.

Back in the 1970's (yes a long time ago) i etched an ID number into some of my tools using DC current. I dont remember how much current i used (100 ma, 1 amp, or more maybe) but i could get some very deep etching into the tool metal with DC current. I would cover the tool with wax and scrape away the area i wanted etched, then immerse it into a salt water solution, then apply current to the tool and water with a carbon electrode. The etching starts fast and completes in a fairly short time, but again i dont remember the current level i used. You can get deep etching though, at least 1/32 inch and even 1/16 inch deep which is pretty deep in a tool.

As mentioned, AC is better but you could still get very undesirable effects. This means you have to find a different way to do this or else turn to something that does not get bothered as much like carbon.

Did you happen to mention exactly what kind of sensors you are using? That might help figure out an alternative.
They look like they might just be pieces of metal. In that case, try using a short piece of carbon rod. You can get this from old carbon zinc batteries that are taken apart very carefully (the materials inside a battery can be dangerous so be careful). The wires probably have to be friction fit, so you may have to wrap turns of copper wire around the electrode, then coat it with an insulating material so the wire end does not get exposed to the water either. There are also conductive epoxies that might help.

Lead is also pretty good at resisting rust, but that's not the greatest idea either as lead can be dangerous too.

 

OP might be using OH tank for shower or drinking.
Best option is a float switch

 

will you please tell me in steps how the sensor is getting rusted due to DC voltage and not with the AC voltage with the help of chemical reaction in water

 

Try Google.
Or read this

 

Try Google.
Or read this

ok

 

will you please tell me in steps how the sensor is getting rusted due to DC voltage and not with the AC voltage with the help of chemical reaction in water

Hi,

It's called "electrolysis" and is better explained elsewhere i think. The current causes the material to break down.

Carbon is used for water filtering, but it has to be clean and pure.

There are other techniques too though. If you heat a resistor above the temperature of the bulk of the water and later the water then touches the resistor, the resistor cools and thus the resistance comes down. That can be detected with an electronic circuit. Thermistors would show a more pronounced effect.

 

will you please tell me in steps how the sensor is getting rusted due to DC voltage and not with the AC voltage with the help of chemical reaction in water

This is not rusting. It is called electrolysis. This is a well known electochemical process that is applied to electroplating.

Here is a simple experiment that you can perform. Take two pieces of copper (copper pennies are commonly used in this demonstration) and connect the two to the positive and negative terminals of a battery or DC power supply. Dip the two metals (now called electrodes) into a mild acidic solution. In a short period of time (minutes) you will see the copper on the negative terminal starting to turn a shiny copper colour.

Here is what is happening chemically.

When the copper is dipped into the solution (called the electrolyte), copper atoms will dissociate into +ve copper ions and electrons.

Cu ⇒ Cu++ + 2e-

When you apply a DC voltage to the electrodes, the electrons will drift towards the +ve electrode and the Cu++ ions will drift over and be deposited on the -ve electrode. What you are witnessing is the electroplating process where the -ve electrode is being plated with fresh copper atoms. (This works best if you use a solution of copper sulphate as the electrolyte.)

If AC voltage is used, the process is reversing back and forth leaving no residual effect.

Electrodes made from stainless steel, platinum or carbon are preferred for measuring electrical conductivity.

Rusting is a different kind of electrochemical process where iron is oxidized to iron oxide. The conversion of iron into rust requires the presence of oxygen and water in contact with iron.

(Edit: I was errant in failing to mention that you also get electrolysis of water where water molecules dissociate into hydrogen and oxygen. When bubbles of hydrogen gas form on the -ve electrode and bubbles of oxygen gas form on the +ve electrode, bubbles form an insulating layer of gas that slows or stops the reaction.)

 

Back in the 1970's (yes a long time ago) i etched an ID number into some of my tools using DC current. I dont remember how much current i used (100 ma, 1 amp, or more maybe) but i could get some very deep etching into the tool metal with DC current. I would cover the tool with wax and scrape away the area i wanted etched, then immerse it into a salt water solution, then apply current to the tool and water with a carbon electrode. The etching starts fast and completes in a fairly short time, but again i dont remember the current level i used. You can get deep etching though, at least 1/32 inch and even 1/16 inch deep which is pretty deep in a tool.

That is the exact process I used to etch my cousin's initials into this knife that I made. IIRC I used 1A DC and it took <1hr. It's about 1/16" (1.5mm) deep.

Moral of the story, the more amperage of DC applied to the metal, the more extreme the erosion.