As a child, you were probably often told not to touch any electric appliances with wet hands, since this can cause an electric shock. It is often said that this is because water can easily conduct electricity, but this is somewhat misleading.
Actually, pure water (H2O) is insulated, and does not conduct electricity at all. However, the water around us contains many impurities. In fact, water can dissolve many things easily, and these impurities are invisible since they are at the micrometer (1/1000 millimeter) level in terms of size. Water appears to conduct electricity since electricity travels on impurities, using them just like stepping-stones.
Ultrapure water is pure H2O with almost no impurities, and is used in the process of pharmaceutical production or semiconductor (IC) production. Especially in the semiconductor industry, ultrapure water is very important when cleaning circuit boards. What if there were invisible dust particles of one-micrometer size in the cleaning water? The particles could fall onto a circuit board, upon which lines as thin as 1 micrometer are implanted horizontally and vertically.
Impurities as small as one micrometer can short out a circuit, and in such cases an IC can be destroyed. Electric appliances and personal computers would be shut down. This is why the semiconductor industry tries to monitor and eliminate as many impurities in the cleaning water as possible. A liquid particle sensor for ultrapure water, which monitors particles using a laser beam, is a great helper. The impurities reflect the laser beam and glimmer in the water, just as dust in a room glimmers in a sunbeam coming through a window. The device counts the number of the reflections, and figures out how many particles are contained in the ultrapure water.
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