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I would like to monitor the level of water in my salt water tank. i dont want to put probes in the water that could corroide. So i was thinking that with 2 insulated probes(screws covered with silicon) I could then wath the capacitance when the water falls so that the probes are in the air the cap value would change and trip a relay. CAn any one point me in the right direction. MAybe freq to voltage then watch voltage level to trip at a given set point.

Smaug

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I am not really following the change in capacitance theory on screws coated with plastic, but you did make me think of some other ways to accomplish this. You can use a pressure sensor with a tube from the transducer to the water. As soon as the water falls below the end of the tube, the pressure will change to be equal with the outside ambient barometric pressure.
Another way is with a temperature bead if the water is a different temperature than the outside air.
And thirdly, how about a laser or IR detector that shines into the tank near the top and is monitored by a receiver on the other side? When the water is low, the amplitude should change since the angle of refraction will change.
...of course, there is also the old standby. A small float connected to a switch that activates a relay. :D

But please tell me more about the capacitance method you are describing. I would like to understand it better.

MP

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Well what i was thinking is that as the water falls the screws will now have a different dielectric water/ air that should result in a change of capacitance. And that would then trip and alarm or pump. I want this to be as simple as possible. so freq to voltage is actualy more than i want. the curcuit would need to be calibrated when installed. Thanks for the promt responce.

Smaug

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My other concern is RF from the ballast and lights that my affect the curcuit. It is just a thought. not sure if it is even possible. I would like this something that could be made from Radio Shack parts(LMAO) I know they dont even have 555 anymore but something that can be made from parts that any hobbiest shop will have. Cheap and easy. ;) Thanks agian.

Smaug

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here is a tip....
the probes don't corrode is you apply an alternative current on them.
the reason why they corrode is because if electricity flows in just one way, then a process of electrolises starts. water gets split in O2 and H2 and these react, especially the O2 with the probes.
also, i suggest using some stainless steel wires for the probes, or better aluminium. it doesn't corode that fast.

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I looked around to see how others have done this. Here are a couple of links:
http://www.parallax.com/dl/docs/cols/nv/vol1/col/27.pdf

http://hawthorn.csse.monash.edu.au/~njh/electronics/watersensor/

These do not seem to be real simple switch methods, but might give you a platform to build upon.

MP

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:) if your looking for a way to measure water levels you might want to try useing a small clean modified car feul level sender unit from a cars feul tank if it has an arm with float on it just varnish that tank and arm unit ., the rest you can use , just wire what ever circiut you have to it. you can modify the arm to tanl float to use for deeper water, with a bit of emagination and ideas you can get what you may seek

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They make float switchs for 5 bucks but they are somewhat bulky. So i was looking for an alternitive. Not wanting to put metal in the tank made me think that capacitance would work. Maybe a simple set up would be a 555 timer and a varible res and the tabk for capacitance the pot would allow me to tweak the freq coming out of the 555. Take the freq ouput into a freq to volatge then when the voltage pass a threshhold the relay would trip. So i would need a 555 or 556 a freq to volt and a soild state relay. Does this sound good, i figure the cost will be more than a float switch, maybe 10 bucks.

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if you do like to try an alternative to the metal float of a feul system sender unit from a car feul tank modified to use as a water level indicater scratch the metal float and try plastic anything like but bigger than a ping pong ball would be a good replacrement for the metal float . christmass tree ball decorations may do, just remove the matierial covering it and whala a float. as for the arm being made of metal scratch that and your left with this thing thats your variable resistor . if any of this can be of use to you, as soon as i get a computer scanner ill email you some ideas in drawing and even throw in some circiuts to help you acomplish waht you seek that would be more satisfying to you [email protected]

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Yes, Like I said on the 18th, "...of course, there is also the old standby. A small float connected to a switch that activates a relay."
This is much simpler to do. And you do not have to use metal parts for the float as Steven has indicated.
Good luck with your project!

MP

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Im still a student and my teacher in industrial electronics suggested that I make a precision water container and make the container like a big capacitor with the varying amount of air and water as the container is filled as the dielectric.
I am just planning it and I am thinking of using a wein-bridge oscillator with one of the capacitors being the container. The oscillator is then connected to a PWM. In theory, as the container is filled, container capacitance decreases, frequency increases and pulse width is shortened.
My problem is that I don't have any idea of converting pulse duration into level indicators.
My teacher suggested that I use an averaging amplifier connected to an ADC. Would anybody be kind enough to explain this as I am not yet familiar with analog to digital conversion.

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Normally, you would use ADC converters for connection to a microprocessor. There are many documents regarding ADC conversion with a search on google. I would recommend that you use a pulse to voltage converter and use the voltage to trigger your relay from a set point. (less circuitry involved).
Look at this article and included schematic:

http://www.discovercircuits.com/per-volt.html

MP

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Take a look at the project I attach you:

Wireless Liquid Level Sensing for Restaurant Applications

It measures capacitance to determinate the liquid level of a glass and send the results wireless on a base station. It's quite interesting and i think you can take much ideas on how to convert it to fit your needs.

Hope this turn you to the right direciton.

Wireless_Liquid_Level_Sensing.pdf

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why not use ultrasonics?
put a senstor at the top of the tank and it can determine the distance between the sensor and the water.
or, i am thinking of another way, with the capaciatance switch. maybe a better rensor would be to run 2 insulated paralel, horizonatal wires in the tank. put them very close together. water cobing between them means an increase in capacitance.
i am thinkting of another basic method.
use a magnet attached to a floating device, and enable the floating device to moove up/down on a vertical wire at the edge of the tanks. on the other side of the tank you could have some reed switches.

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:D

Looking at the circuits that converts pulse width to voltage. I found that it might be very tasking to implement the circuit as well as finding the neccessary parts. Some devices are not readily available as there are not a lot of experimenters from where I come from.
Looking at the catalog of one of our electronics stores, I found that they have an available frequency to voltage IC named the LM2907N.
I wonder if it would be feasible If I could connect oscillator output directly to this device so I could use its output to drive my ADC and display?
By the way, I changed my capacitance tank design. Instead of making the sides of the tank the conducting plates, I plan to place two copper plates wrapped in insulation(I plan to use two PCBs for my prototype), and place them inside the tank at a few centimeters from each other. I'm thinking I could get a more accurate capacitance with this design.
What do you guys think?

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i have a suggestion.
putting the plates closer togther increases the capcity, wich is small when using water as dielectric.
also, i suggest that when insulating the plates, use something wich has a high relative permitivity. this means that you will have a big difference between the 2 capacitances.
why not use thick insulated wire from a transformer?

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Hey Smaug, something come to my mind when i first see this topic, why not use an emittercoupled clapp-oscillator with a frequency around 200 to 600 Khz or so, with a capacitive sensor as part of the tuning capacitor, and using a 4046 ic to detect when the saltwater is reaching the sensor. I can upload a little schematic for you to show what i mean, if you want to. Btw, what is your tank made of?

//Staigen

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