Pascals to Torr (Pa to Torr)

One pascal (Pa) is defined as one newton per square metre (1 Pa = 1 N/m²), or equivalently in SI base units one kilogram per metre per second squared (1 Pa = 1 kg·m⁻¹·s⁻²). The pascal is the SI derived unit of pressure and stress, and unlike the bar, the standard atmosphere, the millimetre of mercury and the pound per square inch, it follows directly from the SI base units with no conversion factor. Conversions to other commonly-encountered pressure units are: 1 standard atmosphere = 101,325 Pa exactly (by the 1954 BIPM definition), 1 bar = 100,000 Pa exactly, 1 psi = 6,894.757 Pa, 1 torr (mmHg) = 133.322 Pa, and 1 inch of mercury (inHg) = 3,386.39 Pa. Because the pascal is inconveniently small for most practical pressures, the SI prefix multiples carry most of the practical work: 1 hectopascal (hPa) = 100 Pa = 1 millibar exactly; 1 kilopascal (kPa) = 1,000 Pa, used for tyre pressure (220–250 kPa typical) and atmospheric pressure variations; 1 megapascal (MPa) = 10⁶ Pa = 10 bar, used for hydraulic systems and material yield strengths; 1 gigapascal (GPa) = 10⁹ Pa = 10,000 bar, used for material elastic moduli and geophysical pressures inside the Earth.

The pascal is named after Blaise Pascal (1623–1662), the French mathematician, physicist and philosopher whose mid-seventeenth-century work on hydrostatics established the foundations of the modern theory of pressure. Pascal's clearest contribution to the unit that bears his name is the law that carries it as well: a pressure applied to a confined fluid is transmitted undiminished in every direction throughout the fluid, the principle on which every hydraulic system since has operated. He arrived at it through a programme of barometric and hydrostatic experiments in the 1640s, including the famous 1648 expedition in which his brother-in-law Florin Périer carried a Torricelli mercury barometer up the Puy de Dôme volcano in central France and measured the column's fall with altitude — direct experimental confirmation that air has weight and that atmospheric pressure decreases with elevation. The dramatic historical anchor for the unit is Pascal's barrel experiment, performed at Rouen around 1646: a long, narrow vertical tube attached to a sealed wooden barrel was filled with water from above, and the small added column generated enough hydrostatic pressure to burst the barrel apart — a vivid demonstration that pressure depends on column height rather than on quantity of fluid. Pascal himself died nearly three centuries before the unit was formally named for him. The pascal was adopted as the SI derived unit of pressure by the 14th General Conference on Weights and Measures (CGPM) in 1971, defined cleanly as one newton per square metre with no conversion factor — the "official" SI pressure unit, in contrast to the bar, psi, atmosphere and millimetre of mercury that already dominated their respective industries. The unit symbol "Pa" is upper-case (because it derives from a person's name) while the spelled-out unit "pascal" is lower-case in running text per SI convention.

Materials science and mechanical engineering are the centerpiece of the pascal's active practical use, almost entirely through its multiples MPa and GPa. Young's modulus (the elastic stiffness of a material under tension) is reported in GPa across every materials-science textbook, ASTM and ISO standard, and engineering data sheet worldwide: structural steel at about 200 GPa, aluminium alloys at 70 GPa, titanium at 110 GPa, copper at 120 GPa, ordinary concrete at 25–30 GPa in compression, polymers at 1–10 GPa, and rubber at 0.01–0.1 GPa. Tensile strength, yield strength, ultimate strength and fatigue limits are reported in MPa: structural carbon steel yield in the 250–500 MPa range, prestressing-tendon high-strength steel at 1,860 MPa, ordinary Portland-cement concrete compressive strength at 20–40 MPa, and engineering ceramics at 200–700 MPa. The ASTM E8 tensile-testing standard, the ISO 6892 metallic-materials tensile testing standard, and every Eurocode structural-engineering reference report results in MPa with no parallel imperial-unit column. Geophysics and high-pressure physics: lithospheric and mantle pressures inside the Earth are denominated in GPa (the Earth's mantle spans about 1 GPa at the lithosphere–asthenosphere boundary to 140 GPa at the core–mantle boundary, with the inner-core boundary at about 330 GPa and Earth's centre at 360 GPa). Diamond anvil cell experiments — the laboratory technique that compresses a sub-millimetre sample between two gem-quality diamond tips — routinely reach 100–300 GPa for mineral-physics work simulating mantle conditions, and have set static-pressure records above 770 GPa for inner-core simulation experiments. Shock-compression experiments at facilities like the National Ignition Facility reach into the TPa (10¹² Pa) range for fusion-relevant studies. Vacuum technology and high-vacuum metrology: pressures below atmospheric are reported in pascal across the SEMI semiconductor-industry standards, the AVS (American Vacuum Society) literature and the European EN 1330 vacuum-technology vocabulary. High vacuum spans roughly 10⁻¹ Pa down to 10⁻⁷ Pa, ultra-high vacuum 10⁻⁷ to 10⁻¹⁰ Pa, and extreme high vacuum below 10⁻¹⁰ Pa. Semiconductor lithography process chambers, surface-science analytical instruments (XPS, AES, STM), particle accelerator beam pipes and space-environment simulation chambers all spec working pressures in pascal — the older convention of torr (1 Torr = 133.322 Pa) survives in some US laboratory practice but is being progressively displaced by Pa in international publications. Acoustics: the standard reference sound pressure for airborne sound is 20 micropascal (20 µPa = 2 × 10⁻⁵ Pa), the nominal threshold of human hearing at 1 kHz, used as the 0 dB SPL reference in IEC 61672 sound-level-meter standards and ISO 226 equal-loudness-contour standards. A normal speaking voice at one metre is around 0.02 Pa (about 60 dB SPL); a jet engine at 30 metres reaches roughly 200 Pa (140 dB SPL); the threshold of pain sits near 20 Pa (120 dB SPL). The micropascal reference makes sound pressure the only common application where the bare unit Pa appears with sub-multiple prefixes. Meteorology retains the hectopascal (1 hPa = 100 Pa) for surface-pressure reporting on synoptic charts and aviation METAR/TAF reports, with standard sea-level pressure 1013.25 hPa — but because 1 hPa = 1 mbar exactly, the printed values are identical and the choice of label is mostly a style decision (the WMO transition from millibar is detailed in the bar entry). The everyday-life paradox: standard atmospheric pressure is 101,325 Pa, a passenger-car tyre is at about 220,000–250,000 Pa, a domestic water pipe runs at 200,000–550,000 Pa. Numbers of this size are unwieldy in conversation, and the prefix multiples or the convenient bar (100,000 Pa) carry the practical work. The pascal is the formally correct SI unit for pressure and the unit used in essentially every scientific publication, but the bare unit "Pa" appears in everyday human practice almost only in acoustics (as 20 µPa) and in vacuum work (as 10⁻⁶ Pa).

One Torr is defined as exactly 1/760 of a standard atmosphere, which equals exactly 101,325/760 = 133.322368... pascals. The relationship to the millimetre of mercury (mmHg) is very close but technically distinct: 1 mmHg = 133.322387415 Pa per the BIPM definition based on standard gravity acting on a column of mercury, while 1 Torr = 133.322368... Pa per the 1/760-atmosphere definition. The two values differ by about 0.000015% and are interchangeable for all practical purposes outside high-precision metrology. The recognised symbol is "Torr" (capitalised, in honour of Torricelli), occasionally seen as "torr" in casual writing but BIPM convention preserves the capital. Vacuum-technology pressure ranges span from ultra-high vacuum (10⁻⁹ Torr and below) through high vacuum (10⁻³ to 10⁻⁹ Torr), medium vacuum (10⁻³ to 1 Torr), low vacuum (1 to 760 Torr) up to atmospheric (760 Torr). Above atmospheric the unit is rarely used; pressure scales transition to bar or kPa.

The Torr is named after Evangelista Torricelli (1608-1647), the Italian physicist and mathematician who in 1643 demonstrated atmospheric pressure with the first mercury barometer — a sealed glass tube inverted in a dish of mercury, with the mercury column standing at the height proportional to atmospheric pressure. Torricelli's barometer experiment, originally proposed by Galileo and executed by Torricelli in his role as Galileo's secretary, established that air has weight and that a column of mercury about 760 mm tall at sea level is equal in weight to the column of air above it. The unit named in Torricelli's honour was formally adopted at the BIPM in 1958 as exactly 1/760 of a standard atmosphere, making it numerically very close to but legally distinct from the millimetre of mercury (mmHg). Both units are used in vacuum technology, with Torr being the dominant convention in physics laboratories and high-vacuum technology and mmHg dominating in clinical medicine and meteorology. The Torr is not part of the SI but is recognised by NIST and BIPM as a non-SI unit accepted for limited use; ISO 80000-4 deprecates it in favour of pascals for new technical writing.

Vacuum technology and high-vacuum laboratory work: physics-laboratory vacuum chambers, semiconductor-fabrication chambers, mass-spectrometry source pressures, electron-microscopy vacuum levels and surface-science laboratory work all denominate working pressures in Torr. Ultra-high-vacuum chambers operate at 10⁻⁹ to 10⁻¹² Torr; high-vacuum lithography systems at 10⁻⁶ to 10⁻⁸ Torr; mass-spectrometer source regions at 10⁻⁵ to 10⁻⁷ Torr; electron-microscope columns at 10⁻⁴ to 10⁻⁶ Torr. Edwards, Pfeiffer, Leybold and Agilent vacuum-equipment manufacturers all preserve Torr alongside Pa and mbar on their product specs and pressure-gauge displays. Semiconductor manufacturing: photolithography, plasma etching, chemical-vapour deposition (CVD) and physical-vapour deposition (PVD) chambers all run at sub-atmospheric pressures denominated in Torr in industry-standard tooling specifications. ASML, Lam Research, Applied Materials and Tokyo Electron all use Torr alongside Pa on their semiconductor process-tool spec sheets. Pressure-swing adsorption gas separation: industrial gas-purification systems (oxygen concentrators, hydrogen purifiers) cycle between low-Torr and atmospheric pressures with Torr as the conventional low-side pressure unit. Medical vacuum: chest-tube drainage and surgical-suction equipment operates at Torr-scale pressures (50-200 mmHg/Torr below atmospheric), with the Torr-vs-mmHg distinction immaterial at the precision required for clinical use.