Measuring water level in tank.......??

How to measure water level in tank in the cheapest way????
The first thing that came to my mind is pressure sensor..... but it's ruled out because it's expensive.
Can somebody suggest a cheaper mechanism??????
And it is preferred to have a low power mechanism.....

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 Recognitions: Science Advisor http://www.rainharvesting.com.au/rain_alert.asp That one looks ideal but I guess the price might be too high. Looks like they use ultrasonics, although they don't say so. If you just wanted to know if the tank was full or not, you could use a float switch. If the water is conductive, you can have wires of different lengths coming from the top of the tank and as they get immersed, they conduct to a common wire which is always submerged. You can have a tube in the water with an opening at each end and you apply a range of audio frequencies to the tube and see where it resonates. The air column above the water resonates at a higher frequency as the water height rises. Using the same tube, but with a floating radio transmitter in it. The strength of the signal at the top of the tube is a measure of how deep the water is. It only has to pulse once an hour or so, so power consumption can be minimal. You can have a transparent tube coming from the bottom of the tank. The water riises in this tube and you can use a lot of different methods to work out how high the water is. You can have a floating magnet which operates Hall switches as it passes them. Or you can have an opaque float which interrupts a light beam. Or you can have conductance between electrodes embedded in the tube. As you have an interest in Microcontrollers, some of these methods may lend themselves to micro control.
 I used to fill a 100 gallon drum for making chemicals. It took about 15 minutes to fill and I needed to know when the water was reaching the top. I built an ultra-cheap alarm with a 9V battery, a micro-buzzer and two long probes to connect the circuit when the water level reached them. The battery was the most expensive component.

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Measuring water level in tank.......??

You can measure the change in pressure of air being bubbled down a tube inserted in the tank from top to bottom. Cheapest way I can think of.
 Bubbler systems are used to measure water level by detecting the pressure required to force air through a submerged tube. The tube is mounted with the end of the tube below the water surface being measured, and the air emerges from the bottom of the tube as a stream of bubbles. The air flow rate is relatively small--just enough to prevent water from backing up into the tube--so the pressure required to push air through the tube is equal to the pressure at the tube's outlet. This pressure is proportional to the water depth above the bottom of the tube.
http://www.usbr.gov/pmts/hydraulics_...bler/index.htm

 You could also use a capacitive water level sensor where two insulated coaxial conductors are immersed in the tank. The total measured capacitance will be the sum of the water-filled portion and the water-free portion of the sensor, thus making the capacitance a function of the water level in the tank. You'd also have to correct for the temperature dependency of the water's dielectric constant -- with a thermistor perhaps? I have no idea about the cost of such a setup. It's probably not as cheap as opening the lid and peaking inside.
 Quick googling led me to this article http://www.siliconchip.com.au/cms/A_30607/article.html Conversely if you don't want something complex and electric, how about putting a floating ball in the tank attached to a string. The string then pulls a little indicator attached to a spring, and with some calibration you have a fairly accurate measurement.
 Mentor Google sump pum float buy: http://www.google.com/products?q=sum...w=dd&scoring=p Easy way, a $22 tethered float switch. http://www.google.com/products/catal...461&sa=title#p Recognitions: Gold Member Science Advisor  Quote by russ_watters Google sump pum float buy: http://www.google.com/products?q=sum...w=dd&scoring=p Easy way, a$22 tethered float switch. http://www.google.com/products/catal...461&sa=title#p
Isn't that only for determining there is water present? I think the OP wants to measure how much water there is in the tank.

 i need a help in understanding the working of water level indicator
 plzzzzzzzzzzzz ...........help me in my project of water level indicator
 Sorry, we do not spoonfeed homework here. Show us what you've got and we'll help you along.
 0 down vote Another solution (no pun intended); Use a potentiometer . The normal rotation range is 270 degrees. Attach a float to the potentiometer using a boom arm. (length=1 unit) Between full and empty the boom arm will move 90 degrees. The ADC on a PIC is either 256 or 1024 steps (yes, zero is a step). I'll use 256 steps for clarity. 270 degrees = 256 ADC steps. 270/90 = 3 (one third of potentiometer range) 255/3 = 85 ADC steps Program some code to mark the 0 degree point (tank empty) when a button is pushed. This stores an offset point in the PIC eeprom. Now the potentiometer does not have to be exactly at zero because this calibration point can be set. Using trigometry calculate a lookup table (hint: php script) to correspond to each step of the ADC. Hint: each ADC step corresponds to 90/85 = 1.0588 degrees. Yes, you should have paid more attention in maths class. dumb waste of time back then, indespensible now. engage brain. learn trigonometry. teach others. Acronym : The Old Arab Carried A Heavy Sack Of Hay. The hypontenuse is the length of the boom arm. Make it 1 unit long. Lookup table then provides percentage of tank depth. (multiply by 100 of course)
 Recognitions: Gold Member 1.Dipstick 2.Completely or partially transparent tank/container arrangement with scale inscribed. 3.Tank sitting on suitably calibrated scales
 With a ruler
 USound can be used. Here is a concept. Might be helpful for future searches... http://www.open-electronics.org/wate...-with-arduino/