Mercury is used in blood pressure measurement in blood pressure monitors or sphygmomanometers. The device consists of a mercury manometer in which mercury rises to a certain height in response to blood pressure. Mercury has traditionally been used in pressure measurements (in manometers) and in thermometers. Mercury is a stable fluid at room temperature and has a high density. This means that, in comparison to water, mercury will expand less as pressure changes and a column of reasonable and portable height can be used for pressure measurements. Mercury has a specific gravity of about 13.6, i.e. it is 13.6 times denser as compared to water and thus a comparable water column will require 13.6 times the height (as compared to mercury column).
Hence mercury is used as a standard fluid in blood measurement and other pressure measurement devices.
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