Torr

The torr (symbol Torr; sometimes improperly called torricelli) is a unit of pressure, defined as exactly 1/ 760 of a standard atmosphere (101 325 Pa). Thus one torr is exactly 101 325/760 pascals (≈ 133.32 Pa).

Historically, a torr was introduced to be the same as a "millimeter of mercury". Later definitions of the two units make them subtly different (by less than 0.000 015%). The torr is not part of the International System of Units (SI). It is often combined with the metric prefix milli to name a millitor (mTor) or 0.001 Torr.

The unit was named for Evangelista Torricelli, an Italian physicist and mathematician, who discovered the principle of the barometer in 1644.

In the 2006 edition of the SI, the torr has disappeared. Although its use is still frequent in low pressure measurements (although mmHg is used to measure blood pressure in medicine), the pascal is the recommended unit of pressure.

Nomenclature and common errors

The name of the unit torr is written in lowercase, while its symbol ("Torr" is always written with an initial capital letter; including in combinations with prefixes and other unit symbols, as in "mTorr" (millitorr) or "Torr-L/s" (torr-liters per second).The (uppercase) symbol should be used with prefix symbols (so, mTorr and militorr are correct, but mtorr and miliTorr are not).

The torr is sometimes incorrectly denoted by the symbol "T", which is the SI symbol for the tesla, the unit of measurement of magnetic field strength. Although frequently encountered, the alternate pronunciation "Tor" It is incorrect.

History

Torricelli attracted considerable attention when he showed the first mercury barometer to the general public. He is credited with the first modern explanation of atmospheric pressure. Scientists at his time were familiar with small fluctuations in height that occurred in barometers. When these fluctuations were explained as a manifestation of changes in atmospheric pressure, the science of meteorology was born.

Over time, 760 millimeters of mercury at 0 °C came to be considered the standard atmospheric pressure. In honor of Torricelli, the torr was defined as a unit of pressure equal to one millimeter of mercury at 0 °C. However, since the acceleration due to gravity (and thus the weight of the water column) is a function of elevation and latitude (due to the rotation and non-sphericity of the Earth), this definition is imprecise and varies by location.

In 1954, the definition of atmosphere was revised by the "10^e Conférence Générale des Poids et Mesures" (10th CGPM) to the currently accepted definition: one atmosphere is equal to 101,325 Pascals. The torr was then redefined as 1/760 of an atmosphere. This produces a precise definition that is unambiguous and independent of measurements of the density of mercury or the acceleration due to gravity on Earth.

Gauge units of pressure

Gauge units are units such as millimeters of mercury or centimeters of water that depend on an assumed density of a fluid and an acceleration due to assumed gravity. The use of these units is ruled out. Despite this, manometric units are routinely used in medicine and psychology, and continue to be used in areas as diverse as weather reporting and diving.

Conversion factors

The millimeter of mercury by definition is 133.322 387 415 Pa (13.5951 g/cm³ x 9.806 65 m/s² x 1 mm), which is approximated with known accuracies of mercury density and standard gravity.

The torr is defined as 1/760 of a standard atmosphere, while the atmosphere is defined as 101,325 pascals. Thus, 1 Torr is equal to 101 325/760 Pa. The decimal form of this fraction (133.322 368 421 052 631 578 947) is infinitely long, periodically repeating decimal (repeating length: 18).

The relationship between the torr and the millimeter of mercury is:

The difference between one millimeter of mercury and one torr, as well as between one atmosphere (101.325 kPa) and 760 mmHg (101.325 014 435 4 kPa), is less than one part in seven million (or less than 0.000 015%). This small difference is negligible for most applications outside of metrology.

Other units of pressure include:

These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures. The torr is used in physics and high vacuum engineering.