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Hygrometer 98mm or 137mm
A mechanical hygrometer indicates relative humidity directly; it is about 5 percent accurate if both temperature and humidity are held constant at room temperature. The sensing element in the hygrometer is usually a bundle of human or horse hair that has been put through some process to remove the oil.The midpoint of several strands of hair is connected, through a lever arrangement in the hygrometer,to an index needle. As the humidity increases, the length of the hairs increases causing the needle to move across the dial on the hygrometer.