> Measuring float:

… The float must be robust enough to withstand daily handling, have the ability to have markers inserted into it (which must themselves be light-weight), and not absorb water at the base, at least within a period of around 1 minute. However, it needs to be as light as possible too, so that it does not sink into the water - and by a process of experiment, I found that insulation tubing used to lag domestic water pipes was ideal, with the markers being wooden cocktail sticks (cut in half), and a small, brass screw to act as a reference point.

… Cut a length of approximately 15-20cm (not too long or it becomes unwieldy when inserted into the measuring vessel), and trim / level top and bottom. I found that water did not percolate into the material, at least over a short time, but if thought necessary, the base of the tube could be sealed with a waterproof film, provided it did not add any significant weight to the overall unit. In practice, given the dimensions of the measuring vessel, it is only useful to try and measure 5 mm at a time; multiple measures being needed for heavier falls.

… Calibration: Using the relationship above, place markers to represent 1, 2, 3, mm etc., after deciding where the zero mark is (by placing the float inside the empty measuring vessel - the 'zero' mark will be level with the top of the vessel). I marked this as a special point with a brass screw - this shows that it is different to the other marks. Then measure down from this point according to the relationship found in the previous section (i.e. the first (1 mm) mark would be 16 mm from the 'zero' mark, the next (2 mm) mark 32 mm from the top etc.): the higher out of the tube the float rises, the more water is in the tube, and the more marks are detected. I marked the millimetre divisions with two cocktail sticks (suitably cut down), and the half-millimetre with a single stick. The user locates the 'zero' screw mark, then counts downwards with the finger-tips each mm (or if very sensitive, can try half-cm markers), until the top of the measuring flask is detected.

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