connected reservoirs. An actual mercury column reading may be converted to more fundamental units of pressure by multiplying the difference in height between two mercury levels by the density of mercury and the local gravitational acceleration. Because the specific weight of mercury depends on temperature and surface gravity, both of which vary with local conditions, specific standard values for these two parameters were adopted. This resulted in defining a "millimetre of mercury" as the pressure exerted at the base of a column of mercury 1 millimetre high with a precise density of when the acceleration due to gravity is exactly . The density chosen for this definition is the approximate density of mercury at , and is standard gravity. The use of an actual column of mercury to measure pressure normally requires correction for the density of mercury at the actual temperature and the sometimes significant variation of gravity with location, and may be further corrected to take account of the density of the measured air, water or other fluid. Each millimetre of mercury can be divided into 1000 micrometres of mercury, denoted μmHg or simply microns. Relation to the torr The precision of modern transducers is often insufficient to show the difference between the torr and the millimetre of mercury. The difference between these two units is about one part in seven million or . By the same factor, a millitorr is slightly less than a micrometre of mercury. Use in medicine and physiology In medicine, pressure is still generally measured in millimetres of mercury. These measurements are in general given relative to the current atmospheric pressure: for example, a blood pressure of 120 mmHg, when the current atmospheric pressure is 760 mmHg, means 880 mmHg relative to perfect vacuum. Routine pressure measurements in medicine include: Blood pressure, measured with a sphygmomanometer Intraocular pressure, with a tonometer Cerebrospinal fluid pressure Intracranial pressure Intramuscular pressure (compartment syndrome) Central venous pressure Pulmonary artery catheterization Mechanical ventilation In physiology manometric units are used to measure Starling forces. See also Bar (unit) Inch of mercury Inch of water Pound per square inch Torr References Units of pressure Mercury (element)