I think I know this unit, if I'm correct it works on the principle of measuring the varying electrical resistance of the water that the sensing rods are immersed in.So it has to be recalibrated as it were for every differing top up of water you put in the tank. unless of course it's exactly the same brew of water. What you have to do is fill tank up, push the push button in and at same time turn the adjuster knob until the indicator needle gauge is on fulll and then release, thats it. If you know you've put in half a tank or any other known quantity of water you can of course adjust the needle to match. Having said all that it's not much of a device in practise.
