In a similar way to the question "[**best way to determine if a 5 gallon water jug is getting empty**](https://arduino.stackexchange.com/questions/488/best-way-to-determine-if-a-5-gallon-water-jug-is-getting-empty)", I'm looking for a way to measure water in a container. This is so it can be connected to an Arduino for logging and reporting. But it's a very different container.

I have a home with the household water supplied from an underground concrete tank (around 3000 US gallons, 11000 litres). Although there is a float switch cutout when it gets empty (to protect the pump), I'd like to know it's gettting low long before that (as it takes a while before a tanker delivers a fresh load). Ideally something that measures exactly or in steps (at least in eights of a load), so software can predict when it'll run out based on average usage, and it can be monitored online when away.

The tank has a heavy concrete hatch, and a small hole that has an existing primitive dipstick, so access is limited. It's in a harsh desert environment, but the pump room and power are only a few feet away.

Some ideas I had:

* suspend an array of float switches with strings of different lengths from a bar inside the tank, and wire them all back to a controller. Hard to install, ugly, low tech. Or something similar on a vertical pole.
* contacts (like a US 2 pin plug) on a vertical pole that again are connected back to a controller (probably using a ribbon cable). Corrosion/anodes might be a problem, although power could just be a short pulse at times. Condensation, spiders webs and dead bugs may give false readings.
* run a fully insulated cable vertically down it, and use as an antenna to send a pulse through, and use the difference between how it works as an antenna in air, and in water, (like an SWR meter) to calculate how far down the water is.
* ultrasonic (or light?) distance finder mounted under the hatch. Range needs to be up to 2 metres (6 feet).
* depth finder (sonar) on a float. Biggest challenge here is getting the signal out (long cable might get tangled when the tank fills).

The last two would be even better if they were wireless (to save running a cable that might get damaged) but I don't think I'd get the battery life I'd like (1 year or more).

Any other ideas? Has anybody ever done this?