I am a proud owner of an underground cistern. It holds about 9000 l of rainwater, enough to water the garden all summer long.
Several years ago I mounted an ultrasonic range sensor inside. And with help of a microcontroller, rs485 transceivers, long cables, water tight boxes, and a bit of perl code I can monitor the height of the water surface. This is actually very helpful! I know that a good rainstorm can fill the cistern within a short time. And I can literally see, when we use water, or worse, when a leak has developed.
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The diagram shows the height of the water surface over the course of approximately 6 weeks. At the start it is raining twice filling the cistern. Water is leaving through the escape (near 2000mm). The slope during roughly two weeks to follow is the result of the leak. The leak is a little above 1200mm, so the curve goes flat below the leak. This is a lot of information just from regular measurements.
So over the course of a year or so this leak has become too big for my peace of mind. Moreover it was time to clean the cistern and remove the sludge lurking at the bottom. So I had ordered someone with a big truck to empty the cistern and remove the sludge. Then I went in there with a high pressure cleaner. It sure looked much nicer in there afterwards.
The leak, well actually two of them, showed themselves by water coming back in from the very wet outside -- a backleak, so to speak! :) This made diagnostics very simple. I put a cover of some cement-type seal over the joints (the cistern is built from four cylindrical segments). A few dry days will allow the seal to cure.
Whether or not the leaks are fixed will show itself after rain has filled the cistern again.
