¿Qué es un/una Torr (Torr)?
Formal Definition
The torr (symbol: Torr) is a unit of pressure defined as exactly 1/760 of a standard atmosphere. Since one standard atmosphere equals 101,325 pascals, one torr equals exactly 101,325/760 pascals, or approximately 133.322 Pa. The torr is named after Evangelista Torricelli, the Italian physicist who invented the mercury barometer in 1643.
The torr is very nearly equal to one millimeter of mercury (mmHg), but the two units are not identical. One mmHg is defined as the pressure exerted by a 1 mm column of mercury at 0°C under standard gravitational acceleration, which equals approximately 133.322 387 415 Pa. One torr equals exactly 133.322 368 421 Pa. The difference is less than 0.000015% — negligible for all practical purposes — but exists because the torr is defined algebraically (1/760 atm) while the mmHg is defined physically.
Primary Domain
The torr is used primarily in vacuum science, where it serves as a convenient unit for pressures well below atmospheric. Atmospheric pressure equals 760 Torr, and vacuum systems typically operate in the range from hundreds of torr down to 10⁻¹⁰ Torr or lower. The torr's wide dynamic range and historical association with vacuum technology have made it the dominant unit in this field.
Etymology
Named After Torricelli
The unit is named after Evangelista Torricelli (1608-1647), an Italian mathematician and physicist who studied under Galileo Galilei in the last months of Galileo's life. Torricelli's greatest contribution was the invention of the mercury barometer in 1643, which demonstrated that the atmosphere exerts measurable pressure. The unit name "torr" was proposed by the International Organization for Standardization (ISO) in 1950 and has been in common use since.
Pronunciation and Plural
The torr is pronounced to rhyme with "bore" or "more." The plural of torr is "torr" — not "torrs." This follows the convention for units named after people: one torr, two torr, 760 torr. The symbol is "Torr" with a capital T, following the convention that unit symbols derived from proper names begin with a capital letter.
Precise Definition
Exact Definition
The torr is defined as exactly 1/760 of a standard atmosphere: 1 Torr = 101,325/760 Pa ≈ 133.322 Pa. This definition is exact because both the numerator (101,325) and denominator (760) are exact integers. The resulting decimal expansion, however, is infinitely repeating, making the torr an inconvenient unit for exact calculations in the SI system.
Relationship to mmHg
The torr and mmHg are often used interchangeably, and for all practical purposes they are equal. The formal distinction is that 1 Torr = 1/760 atm exactly, while 1 mmHg = the pressure of 1 mm of mercury at 0°C and 9.80665 m/s². Due to the precise density of mercury (13,595.1 kg/m³ at 0°C), 1 mmHg = 133.322 387 415 Pa, compared to 1 Torr = 133.322 368 421 Pa. The difference of 0.000019 Pa is irrelevant in any practical measurement.
Vacuum Ranges in Torr
The vacuum industry classifies vacuum levels using torr: Rough vacuum: 760-1 Torr. Medium vacuum: 1-10⁻³ Torr. High vacuum: 10⁻³-10⁻⁹ Torr. Ultra-high vacuum (UHV): 10⁻⁹-10⁻¹² Torr. Extreme high vacuum (XHV): below 10⁻¹² Torr. Outer space has a pressure of approximately 10⁻¹⁷ Torr.
Historia
Torricelli's Barometer
In 1643, Evangelista Torricelli filled a glass tube (approximately 1 meter long, sealed at one end) with mercury, inverted it in a dish of mercury, and observed that the mercury column dropped to approximately 760 mm. The space above the mercury — now called Torricelli's vacuum — was one of the first artificial vacuums ever created. Torricelli correctly concluded that the atmosphere pushes on the mercury in the dish, supporting the column, and that the height of the column measures the atmospheric pressure.
Torricelli wrote to his friend Michelangelo Ricci: "We live at the bottom of an ocean of air." This poetic description captured a revolutionary insight — the atmosphere has weight, and its pressure can be measured. Torricelli's experiment resolved decades of debate about why suction pumps could not raise water more than approximately 10 meters, a limitation that had puzzled engineers since antiquity.
Development of Vacuum Science
The torr became the natural unit for vacuum science as the field developed. Early vacuum pumps, developed by Otto von Guericke (1654) and Robert Boyle (1659), were crude by modern standards but could reduce pressure to a few torr. By the late 19th century, Heinrich Geissler's mercury displacement pumps reached pressures below 0.01 Torr, enabling the discovery of cathode rays and X-rays. Modern turbomolecular pumps and ion pumps can achieve pressures below 10⁻¹¹ Torr.
Formalization
The torr was formally adopted as a pressure unit by ISO in 1950, defined as 1/760 of a standard atmosphere. This definition was chosen to make 760 Torr equal exactly 1 atm, simplifying the long-standing convention of measuring atmospheric pressure as "760 mm of mercury." The unit has been endorsed by scientific organizations worldwide and remains the dominant pressure unit in vacuum science.
Uso actual
Vacuum Systems
The torr is the standard unit in vacuum technology across most of the world (with the millibar used as an alternative in some European applications). Vacuum pumps are rated by their ultimate pressure in torr: rotary vane pumps reach 10⁻³ Torr, turbomolecular pumps reach 10⁻¹⁰ Torr, and cryopumps reach 10⁻¹² Torr. Vacuum gauges — Pirani, Penning, ion gauge, and capacitance manometer — all display pressure in torr.
Semiconductor Manufacturing
The semiconductor industry relies heavily on vacuum systems and uses the torr extensively. Chemical vapor deposition (CVD) processes operate at 0.1-10 Torr. Physical vapor deposition (PVD/sputtering) requires 10⁻³-10⁻² Torr. Ion implantation chambers operate at 10⁻⁶-10⁻⁵ Torr. Process recipes in semiconductor fabs specify pressures in torr or millitorr (mTorr).
Thin Film Deposition
Coating technologies — from anti-reflective coatings on eyeglasses to decorative coatings on watches — use vacuum deposition at pressures measured in torr. Thermal evaporation operates at 10⁻⁵-10⁻⁶ Torr. Electron beam evaporation requires similar pressures. Magnetron sputtering operates at 1-10 mTorr.
Scientific Research
Particle accelerators, surface science, and mass spectrometry all operate in vacuum conditions measured in torr. The Large Hadron Collider at CERN operates at pressures below 10⁻¹⁰ Torr in its beam pipe. Surface analysis instruments (XPS, AES, STM) require ultra-high vacuum below 10⁻⁹ Torr to keep surfaces clean during analysis.
Everyday Use
Light Bulbs and Vacuum Tubes
Incandescent light bulbs contain inert gas at reduced pressure, typically 500-700 Torr (slightly below atmospheric). The reduced pressure decreases convective heat loss from the filament. Fluorescent tubes contain mercury vapor at approximately 0.003-0.01 Torr. Neon signs operate at 1-20 Torr — the exact pressure affects the brightness and color of the glow.
Food Packaging
Vacuum-sealed food packaging removes air to pressures of approximately 10-50 Torr, extending shelf life by reducing oxidation and microbial growth. Freeze-drying (lyophilization) of foods like instant coffee and astronaut ice cream involves reducing pressure to approximately 0.1-1 Torr while the product is frozen, causing ice to sublime directly to vapor.
Blood Pressure
Although typically expressed in mmHg rather than torr, blood pressure measurements are numerically equivalent in both units. Normal blood pressure of 120/80 mmHg is essentially 120/80 Torr. Sphygmomanometers (blood pressure cuffs) historically used mercury columns, directly measuring pressure in mmHg, though modern digital devices calculate the same values electronically.
Thermos Flasks
Vacuum-insulated containers (Dewar flasks, thermos bottles) maintain a vacuum of approximately 10⁻³-10⁻⁴ Torr between their double walls. This vacuum dramatically reduces heat transfer by conduction and convection, keeping hot drinks hot and cold drinks cold for hours. The silvered inner surface minimizes radiative heat transfer.
In Science & Industry
Surface Science
Surface science requires ultra-high vacuum (UHV) conditions below 10⁻⁹ Torr to study clean surfaces. At atmospheric pressure (760 Torr), a clean metal surface becomes covered with a monolayer of gas molecules in approximately 3 nanoseconds. At 10⁻⁶ Torr, this takes about 1 second. At 10⁻¹⁰ Torr, it takes about 3 hours — long enough to perform detailed surface analysis. This relationship between pressure and surface contamination rate drives the need for extreme vacuum in surface science.
Mass Spectrometry
Mass spectrometers require vacuum conditions for ion beams to travel without colliding with background gas molecules. Quadrupole mass spectrometers operate at 10⁻⁵-10⁻⁶ Torr. Time-of-flight (TOF) instruments require 10⁻⁶-10⁻⁸ Torr. FT-ICR mass spectrometers, which trap ions for extended periods, require pressures below 10⁻⁹ Torr.
Plasma Physics
Plasma processing — used in semiconductor etching, plasma cleaning, and fusion research — specifies conditions in torr. Plasma etching operates at 0.01-1 Torr. Plasma-enhanced CVD operates at 0.1-10 Torr. Fusion reactors require initial vacuum below 10⁻⁸ Torr before plasma ignition.
Space Simulation
Space simulation chambers replicate the vacuum of space for testing spacecraft and instruments. Low Earth orbit pressure is approximately 10⁻⁷ Torr. Interplanetary space is about 10⁻¹⁴ Torr. Space simulation chambers achieve 10⁻⁷-10⁻¹⁰ Torr, combined with thermal cycling and solar radiation simulation, to test equipment before launch.
Interesting Facts
Torricelli died in 1647 at age 39 — just four years after his famous barometer experiment. The unit bearing his name was not formally adopted until 1950, over 300 years after his death.
The best vacuum achievable on Earth (approximately 10⁻¹³ Torr) is still far worse than the vacuum of interstellar space (approximately 10⁻¹⁷ Torr) or intergalactic space (approximately 10⁻²¹ Torr).
A vacuum of 10⁻⁶ Torr contains roughly 3.2 × 10¹⁰ molecules per cubic centimeter. While this sounds like a lot, it is about 10 trillion times fewer than at atmospheric pressure (2.7 × 10¹⁹ molecules/cm³).
At atmospheric pressure (760 Torr), the mean free path of an air molecule — the average distance it travels before hitting another molecule — is about 68 nanometers. At 10⁻⁶ Torr, this increases to about 50 meters.
Thomas Edison's light bulb (1879) relied on vacuum technology. His early bulbs were evacuated to about 0.01 Torr using hand-operated mercury pumps — a process that took hours per bulb.
The LHC beam pipe at CERN maintains a pressure of about 10⁻¹⁰ Torr — comparable to the vacuum on the Moon's surface. This ultra-high vacuum is necessary to prevent proton beams from scattering off residual gas molecules.