Torr

Encyclopedia

The

unit of pressure

with the ratio of 760 to 1 standard atmosphere

, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is

, an Italian physicist and mathematician who discovered the principle of the barometer

in 1644.

was born.

Over time, 760 millimetres of mercury (abbreviated mmHg) came to be regarded as the standard atmospheric pressure. In honour of Torricelli, the Torr was defined as a unit of pressure equal to one mmHg.

In 1954, the definition of the

. The Torr was then re-defined as of one atmosphere. This was necessary in place of the definition of a Torr as 1 mmHg, because the height of mercury changes at different temperatures and gravities.

The

There are several things to notice about this definition:

In practice, of course, measurements are made using local values, which vary little enough at the Earth's surface. These assumptions limit both the validity and the precision of the mmHg as a unit of pressure.

According to the UK’s National Physical Laboratory (NPL):

The performance of modern transducers approaches the precision required to distinguish between the Torr and the millimetre of mercury.

The NPL concludes

) is the "gold standard

" for blood pressure

measurement.

In physiology, manometric units are used to measure Starling forces

. Other applications include:

Manometric results in medicine are sometimes given in torr.

This is usually incorrect, since the torr and the millimetre of mercury are not the same thing.

Pressures obtained with a manometer (or its transducer equivalent) should be reported in metres of mercury.

The torr is defined as of one atmosphere, while the atmosphere is defined as 101.325 kPa. Therefore, one torr is equal to Pa. The decimal form of this fraction is, unfortunately for practical use, an infinitely long, periodically repeating decimal, as is its reciprocal.

The relationship between the Torr and the millimetre of mercury is:

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

.

The millimetre of mercury as used in medicine is in general given relative to the atmospheric pressure. This means that when a doctor tells you you have a blood pressure of 100 mmHg, this is 100 mmHg above atmospheric. So on a day when the barometric pressure is 760 your absolute pressure is actually 860 millimetres of mercury (114,657.3 Pa).

The SI

unit of pressure is the

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 high-vacuum physics and engineering.

**torr**is a non-SIInternational System of Units

The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

unit of pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

with the ratio of 760 to 1 standard atmosphere

Atmosphere (unit)

The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is

*approximately*equal to 1 mmHg. Note that the symbol is spelled exactly the same as the unit, but the symbol is capitalized, as is customary in metric units derived from names. It was named after Evangelista TorricelliEvangelista Torricelli

Evangelista Torricelli was an Italian physicist and mathematician, best known for his invention of the barometer.-Biography:Evangelista Torricelli was born in Faenza, part of the Papal States...

, an Italian physicist and mathematician who discovered the principle of the barometer

Barometer

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...

in 1644.

## History

Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. He is credited with giving the first modern explanation of atmospheric pressure. Scientists at the 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 meteorologyMeteorology

Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

was born.

Over time, 760 millimetres of mercury (abbreviated mmHg) came to be regarded as the standard atmospheric pressure. In honour of Torricelli, the Torr was defined as a unit of pressure equal to one mmHg.

In 1954, the definition of the

*atmosphere*was revised by the*10e Conférence Générale des Poids et Mesures*(*10th CGPM*) to the currently accepted definition: one atmosphere is equal to 101,325 pascalsPascal (unit)

The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

. The Torr was then re-defined as of one atmosphere. This was necessary in place of the definition of a Torr as 1 mmHg, because the height of mercury changes at different temperatures and gravities.

## Manometric units of pressure

*Manometric units*are units such as*millimetres of mercury*or*centimetres of water*that depend on an assumed density of a fluid and an assumed acceleration of gravity. The use of these units is discouraged. Nevertheless, manometric units are routinely used in medicine and physiology, and they continue to be used in areas as diverse as weather reporting and scuba diving.The

*millimetre of mercury*(symbol:*mmHg*) is defined as the pressure exerted at the base of a column of fluid exactly 1 mm high, when the density of the fluid is exactly 13.5951 g/cm^{3}, at a place where the acceleration of gravity is exactly 9.80665 m/s^{2}. Under most conditions, 1 mmHg is approximately equal to 1 Torr.There are several things to notice about this definition:

- A fluid density of 13.5951 g/cm
^{3}was chosen for this definition because this is the approximate density of mercury at 0 °C. The definition, therefore, assumes a particular value for the density of mercury. The density can depend on temperature, exogenous pressure, and other similar variables, so those have to assume certain conventional, normal values as well. - The definition assumes a particular value for the acceleration of gravity: the standard gravityStandard gravityStandard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...

*g*= 9.80665 m/s_{0}^{2}. In theory, the precise acceleration would vary, and the measurement would have to be recalibrated against the local value; in weightless conditions, this kind of measurement would not even make sense. - The definition does not address the quality of the vacuum, including the vapor pressure of the mercury, above the column of fluid.

In practice, of course, measurements are made using local values, which vary little enough at the Earth's surface. These assumptions limit both the validity and the precision of the mmHg as a unit of pressure.

According to the UK’s National Physical Laboratory (NPL):

The performance of modern transducers approaches the precision required to distinguish between the Torr and the millimetre of mercury.

The NPL concludes

### Manometric units in medicine and physiology

In medicine, the millimetre of mercury (measured with a sphygmomanometerSphygmomanometer

A sphygmomanometer or blood pressure meter is a device used to measure blood pressure, comprising an inflatable cuff to restrict blood flow, and a mercury or mechanical manometer to measure the pressure. It is always used in conjunction with a means to determine at what pressure blood flow is just...

) is the "gold standard

Gold standard (test)

In medicine and statistics, gold standard test refers to a diagnostic test or benchmark that is the best available under reasonable conditions. It does not have to be necessarily the best possible test for the condition in absolute terms...

" for blood pressure

Blood pressure

Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

measurement.

In physiology, manometric units are used to measure Starling forces

Starling equation

The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces in the movement of fluid across capillary membranes.Capillary fluid movement may occur as a result of three processes:...

. Other applications include:

- Intraocular pressure (tonometry)
- Cerebrospinal fluidCerebrospinal fluidCerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...

pressure - Intracranial pressureIntracranial pressureIntracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...
- Intramuscular pressure (compartment syndromeCompartment syndromeCompartment syndrome is a limb threatening and life threatening condition, defined as the compression of nerves, blood vessels, and muscle inside a closed space within the body . This leads to tissue death from lack of oxygenation due to the blood vessels being compressed by the raised pressure...

) - Central venous pressureCentral venous pressureCentral venous pressure describes the pressure of blood in the thoracic vena cava, near the right atrium of the heart...
- Pulmonary artery catheterPulmonary artery catheterIn medicine pulmonary artery catheterization is the insertion of a catheter into a pulmonary artery. Its purpose is diagnostic; it is used to detect heart failure or sepsis, monitor therapy, and evaluate the effects of drugs...

ization - Mechanical ventilationMechanical ventilationIn medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows...

Manometric results in medicine are sometimes given in torr.

This is usually incorrect, since the torr and the millimetre of mercury are not the same thing.

Pressures obtained with a manometer (or its transducer equivalent) should be reported in metres of mercury.

## Conversion factors

The millimetre of mercury by definition is (13.5951 g/cm^{3}× ), which is approximated with known accuracies of density of mercury and gravitational acceleration.The torr is defined as of one atmosphere, while the atmosphere is defined as 101.325 kPa. Therefore, one torr is equal to Pa. The decimal form of this fraction is, unfortunately for practical use, an infinitely long, periodically repeating decimal, as is its reciprocal.

The relationship between the Torr and the millimetre of mercury is:

- 1 Torr = 0.999 999
*857 533 699*... mmHg - 1 mmHg = 1.000 000
*142 466 321*... Torr

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

Metrology

Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...

.

The millimetre of mercury as used in medicine is in general given relative to the atmospheric pressure. This means that when a doctor tells you you have a blood pressure of 100 mmHg, this is 100 mmHg above atmospheric. So on a day when the barometric pressure is 760 your absolute pressure is actually 860 millimetres of mercury (114,657.3 Pa).

The SI

Si

Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

unit of pressure is the

*pascal*(symbol: Pa), defined as one newton per square metre. Other units of pressure are defined in terms of SI units. These include:- The bar (symbol: bar), defined as 100 kPa exactly.
- The atmosphere (symbol: atm), defined as 101.325 kPa exactly.
- The Torr (symbol: Torr), defined as atm exactly.

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 high-vacuum physics and engineering.

## See also

- Inch of mercuryInch of mercuryInches of mercury, ' is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere....
- PressurePressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
- Pressure headPressure headPressure head is a term used in fluid mechanics to represent the internal energy of a fluid due to the pressure exerted on its container. It may also be called static pressure head or simply static head...
- Atmosphere (unit)Atmosphere (unit)The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
- Conversion of unitsConversion of unitsConversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors.- Process :...
- Centimetre of waterCentimetre of waterA centimetre of water is a less commonly used unit of pressure derived from pressure head calculations using metrology...
- Pascal (unit)Pascal (unit)The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...