Hg is the chemical symbol for mercury, derived from its ancient Greek name, Hydrargyrum. In pressure measurement, such as in the unit mmHg, it refers to the height of a column of mercury that a specific force can support. This historical method provides a tangible representation of an invisible force like atmospheric pressure. The unit’s continued use, particularly in medicine, highlights its enduring role in science.
Defining the Unit: Millimeters of Mercury
The unit “millimeter of mercury” (mmHg) is a manometric unit of pressure, defined by the vertical distance a fluid is pushed by an external force. One mmHg is the pressure exerted by a column of mercury precisely one millimeter high under standard gravity. Mercury was chosen due to its high density, which is about 13.6 times greater than water. This density allows pressure-measuring instruments to be significantly smaller than if water were used, which would require a column over 10 meters tall to measure standard atmospheric pressure.
Standard atmospheric pressure at sea level is historically defined as 760 mmHg. Although not part of the modern International System of Units (SI), the mmHg unit remains in use due to its historical establishment and practical application in certain fields. Its definition is now standardized as exactly 133.322 Pascals to ensure consistency regardless of local variations in gravity or temperature.
The Physics of Measurement: The Barometer Principle
The concept of measuring pressure using a mercury column originates with Evangelista Torricelli’s 17th-century experiment, which led to the invention of the barometer. Torricelli inverted a long glass tube filled with mercury into a dish containing the liquid metal. The column in the tube dropped until its weight was exactly balanced by the external atmospheric pressure pushing down on the mercury in the open dish.
The space remaining at the top of the sealed tube is known as the Torricellian vacuum. This setup demonstrates that the pressure exerted by the surrounding air is sufficient to support a physical column of mercury against gravity. The height of the mercury column is the direct visual result of the atmospheric force, meaning a higher column height indicates greater atmospheric pressure.
Common Real-World Applications of Hg Pressure
The millimeter of mercury unit is most commonly encountered in medicine and meteorology. In health care, blood pressure is universally measured and reported in mmHg, a practice continued since the invention of the mercury-based sphygmomanometer. A typical reading, such as 120/80 mmHg, represents the systolic and diastolic pressures. These define the maximum pressure during a heartbeat and the minimum pressure while the heart rests between beats.
In meteorology, atmospheric pressure is measured to forecast weather changes, often reported in inches of mercury (inHg), especially in the United States. A rising barometric pressure usually indicates fair weather, while a falling pressure suggests an approaching storm system. This unit remains relevant in aviation, where pilots use it to calibrate altimeters and determine their effective altitude based on local atmospheric conditions.
Converting Hg Pressure to Standard Units
While mmHg is practical in specific fields, conversion to the International System of Units (SI) is often necessary for scientific and engineering contexts. The SI unit for pressure is the Pascal (Pa), defined as one Newton of force per square meter. The conversion factor is approximately 1 mmHg equals 133.322 Pascals.
For industrial or mechanical applications, the Imperial unit Pounds per Square Inch (PSI) is widely used. One standard atmosphere (760 mmHg) is roughly equivalent to 101,325 Pascals or 14.7 PSI. The continued use of different units reflects the varying needs of precision and scale, with mmHg favored for relatively low pressures like those found in the human body or the atmosphere.