A Simple Water Cut-Off Switch We have designed. a simple and reliable water cut-off switch with no moving parts to turn off power supplies using up to 2 kW in the event of interuption in the water supply. Due t o the presence of other sensitive electronic apparatus in our laboratory i t was imperative that the switch did not generate any appreciable spark in operation and consequently a hydrostatic switch previously described' in this Journal was not suitable for our purpose. Our switch (diagram not to scale) was constructed from borosilicate glass. Tube C i s 16 mm bore and tube A is 3 mm bore capillary tubing. The U-tuhe U is 6 mm bore and contains mercury. It has two platinum wires P inserted through pinch seals and secured by a n expoxy resin glue such as Araldite. The upper P t contact is about 1 cm above the mercury surface when the water supply is off. R is a suitable relay energized by the voltage V; we use a Sunvic Hot Wire Vacuum Switch, model 602, operating on 20 mA a t 30 Vac and iscapable of switching 2 kW. When the water supply is on, water enters through B and the switch is closed by the weight of the column of water in C acting an the mercury in the U-tube; if the water supply fails Cempties throughA and the switch opens. de Lima, R. Alves, andLima, J. Ferreira, J. CHEM. EDUC.,49,404, 1974. University College Galway, Ireland
522 / Jourml of Chemical Education
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Brendan Coughlan D a n O'Sullivan
