I know that this is a fairly common question but simple answers are rare.
Problem:
I have an RV with a fresh water storage tank. I would like to know how many gallons of water I have on-board.
I don't like the lights that came installed. Tank shows full until it is down to 3/4 full. It shows empty if it is below 1/4. And all other measurement are meaningless.
I drink from this tank so I want to be careful what I put in it. The water could come from anywhere, so the mineral content and conductivity could fluctuate. I would like for it to be a cheap solution so a solution like mount the tank on expensive strain gauges and calculate its weight is not viable.
There are non-contact methods that offer a linear response... but how much are you willing to spend?
If the tank is plastic-walled, you might try a capacitive approach... alternating parallel strips of Al or Cu tape. As the water level rises or falls, the capacitance should change. I doubt this would work on a metal tank even with insulation in between.
Another option is a differential pressure sensor. Mount one inlet to the bottom of the tank (drain valve?) and then measure the pressure. A differential model automatically compensates for any changes in local air pressure (altitude, storms, etc.).
Ultra-sonic sensors work too... but most should not become wet. Can you ensure that?
Another neat approach I saw was a resistor-ladder approach where a bunch of resistors were mounted in series and as the water shorted them out, the overall resistance changed too. (naturally at very low / safe currents).
Google around, many approaches have been tried. With any luck, you might find one that fits your need perfectly.
The current one has copper bolts through the side of the tank. It then uses (i believe) a few op-amps to measure resistance from one bolt to another. If the resistance is low (relatively) then water is covering the bolt and an LED is lit. So column of LEDs is suppose to reflect the level of the water.
I would like to be able to say "the tank is 56% full" and not "I have somewhere between 50% and 75%". Of course that is if it were will designed. Currently the top bolt is covered as long as it 60% to 70% full and the bottom bolt is uncovered below 20% -25%.
If the tank is plastic-walled, you might try a capacitive approach... alternating parallel strips of Al or Cu tape. As the water level rises or falls, the capacitance should change. I doubt this would work on a metal tank even with insulation in between.
It is a plastic tank and I like the idea of using a capacitive sensor. My concerns were things like stray capacitance and rfi. I have heard of this approach but I have not talked to anyone who has used it. How would you measure the capacitance? I was thinking of a timer circuit using a 555 as an oscillator and measure the frequency/period.
Another option is a differential pressure sensor. Mount one inlet to the bottom of the tank (drain valve?) and then measure the pressure. A differential model automatically compensates for any changes in local air pressure (altitude, storms, etc.).
The tank is only about a foot tall so the pressure sensor would need to be very sensitive. Those aren't cheap.
Ultra-sonic sensors work too... but most should not become wet. Can you ensure that?
Not really. I would think that you would need to mount it so it could see the water. The cheap one measure through air and the sloshing and humidity would get to the sensor, or I would think it would.
Another neat approach I saw was a resistor-ladder approach where a bunch of resistors were mounted in series and as the water shorted them out, the overall resistance changed too. (naturally at very low / safe currents).
I thought about an approach like that. In fact a resistive wire like NiCrome could be used and it would be infinitely variable. But I assume that the change would be low and the mineral content of the water would through off the measurements.
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If you know of anyone who has actually done something like this ...
Parallax has a paper on this, see here I think you might be able to get away with just putting the strips on a corner of the tank. A good writeup, including sample 555 circuits can be found here.
I don't like the lights that came installed. Tank shows full until it is down to 3/4 full. It shows empty if it is below 1/4.
The lights do not indicate that the reservoir is full/empty, when the lower one lights up you must start filling, if the upper light up you can stop filling. But it is not a clear design I agree 100%
there are pressure resistive sensors for continous liquid level measurement like Milone etape http://www.milonetech.com/
There are also magnetoresistive sensors like magnetopots (http://www.spectrasymbol.com/) where you put a magnet inside the tank near wall or in an external tube and outside You install the magnetopot.
There are other manufacturers of this kind of sensors with other names.
Are expensive but precise measurement solution.
You have also take into account the shape of your tank to know how much volume of water You have at different water levels.