Those of you with rainwater tanks at home or in rural situations would understand the difficulty of accurately determining how much water is left. You could have a trained ear and be able to knock the side of the tank for a rough estimate, however doing so is not for everyone. A member of our Freetronics forum has instead devised and published details of a portable ultrasonic water tank gauge that can quickly show the amount of water left in a tank using a simple 'fuel gauge' style readout:
Full details including the Arduino sketch are available to download. Although the author used a Freetronics Eleven board:
perhaps consider using a Leostick to reduce the total size and increase portability of the gauge.