Sensing a Solar Hot Water Tank level

I have a non-pressurized (gravity driven) open solar thermosyphon hot water system on my roof installed like this:

Cold water is fed through a float on the top, dropping the level opens the valve. Hot water is sucked in somewhere in the lower 1/3 of the 220l tank.

Here is my situation: At the end of a cloudy day I have roughly only 45C degrees temperature of the water and if I am not to cut off the cold water filler, it will soon become too cold for a shower. Therefore I turn it off and use 45 degrees C of water until the tank is empty. I'd like to have knowledge though how much water I have in the tank left.

I was thinking of attaching a water pressure sensor at the hot water pipe, but these seem a bit expensive. I have access to the tank (it is open) from the top, where the cold water filler is, so I can insert a wet probe there too.

I am looking for reasonably priced sensor to read the water level in the tank, any advice is welcome.


How about a load sensor - something like this: although I suspect that this particular one doesn't measure high enough weights.

i like capacity (electrical) sensors... but i guess in this case an external electrical field would be swallowed by the metal container...

but maybe it is worth a try? maybe something changes when the tank is filled with water?

i would not put anything inside the tank... it might contaminate the water... :blush:

Ultrasonic distance sensor?


Yes, Ultrasonics for a larger tank and IR for a smaller one, you failed to give a measure of the tank size so... first there are level sensors for 'potable water' and some, that are of the rheostat type (like a gas guage) and there are capacitive types where the dielectric constant changes the effective capacity of a probe (used for poor (pure water) conductors., there are optical level sensors and conductive level sensors. The easiest one though is to make your own. Picture a 10K pot mounted opn an 8 X 10 X .2CM plastic panel centered on the narrow plane and 2 cm from one end of the long (10 CM) plane. an arm about 5 cm long is affixed to the pot (a knob and a 12 mm X 5 CM piece of 2 mm thick plastic glued to it W/Epoxy resin). a stainless steel or nylon or other non reactive string fastened from a 1 - 2 mm hole on the free end of the arm to a float ball (again non toxic). This makes a Potentiometer that changes with fluid level. This assembly will fit into an outdoor (Nema 5?) watertight enclosure... remember to leave a protected hole on the bottom so that air pressure can equalize or moisture WILL condense out on the inside Grainger sells a number of "Level Detectors for Potable Water" and these are certified safe for [u]water for human consumption[/u] and that is what you want. Before I retired Irrigation controllers (radio-operated valve controllers) and pump controls for filling water tanks as well as soil conductivity, PH and moisture measurement tools were all I did professionally. If you only wanted a 1/4 - 1/2 - 3/4... level indicator well that is even easier... 5000 - 10000 gallons in a medium size tank... that's a somewhat difficult issue altogether...


Thanks to all for the replies.

I can't use load (weight) sensor because the tank is mounted on 4 legs on the roof with hydro-isolation already in place I don't think the capacitance method will do for me either, the tank may get up to the top and there is no way an ultrasonic/IR to do as well.

I really could go for 1/4 - 1/2 - 3/4 - 1/1 sensor as suggested by Doc.

I also googled last evening and found this project

The guy recommends using MPX5010DP that is a differential pressure sensor, one end goes open to the atmospheric and the other connects to the hot water pipe. It will then read out 0-5V depending on the pressure difference, that will be ideal for my case.

The capacitor thing is a noncontact probe, fitted into the tank as are the float things the only requirement for any of them is that they are mounted parallel to the plane of the water, this is a requirement for any measurements except flow and that is an area for examination. Flowmeters are used to measure flow in various units and some even have triggerrable outputs when a certain point is reached, if not certainly the Arduino could and there are many with 4 - 20 Ma loop sensors that aren't at all difficult to interface to a Uno, simply accumulate enough data and shut the valve. There are also ones as i remember that will talk RS232 and likely many others as water use and conservation is a very BIG business today. I guess the thought is Don't give up, yet... you haven't found all the rocks to look under, Yet.