The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly 1/760 of a standard atmosphere (101325 Pa). Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).
Historically, one torr was intended to be the same as one "millimetre of mercury", but subsequent redefinitions of the two units made the torr marginally lower (by less than 0.000015%).
Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure.
Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre (N/m); similarly, the pound-force per square inch (psi, symbol lbf/in) is the traditional unit of pressure in the imperial and US customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the unit atmosphere (atm) is equal to this pressure, and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.
In electromagnetism, an electric discharge is the release and transmission of electricity in an applied electric field through a medium such as a gas (i.e., an outgoing flow of electric current through a non-metal medium).
Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressure lower than about 1×10 torrs (1×10 mbar; 1×10 Pa). UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber.
UHV conditions are integral to scientific research. Surface science experiments often require a chemically clean sample surface with the absence of any unwanted adsorbates. Surface analysis tools such as X-ray photoelectron spectroscopy and low energy ion scattering require UHV conditions for the transmission of electron or ion beams. For the same reason, beam pipes in particle accelerators such as the Large Hadron Collider are kept at UHV.
A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high. Currently, it is defined as exactly 133.322387415 pascals, or approximately 1 torr = 1/760 atmosphere = 101325/760 pascals. It is denoted mmHg or mm Hg.
Although not an SI unit, the millimetre of mercury is still often encountered in some fields; for example, it is still widely used in medicine, as demonstrated for example in the medical literature indexed in PubMed. For example, the U.S. and European guidelines on hypertension, in using millimeters of mercury for blood pressure, are reflecting the fact (common basic knowledge among health care professionals) that this is the usual unit of blood pressure in clinical medicine.
A helium–neon laser or He–Ne laser is a type of gas laser whose high energetic gain medium consists of a mixture of helium and neon (ratio between 5:1 and 10:1) at a total pressure of approximately 1 Torr (133.322 Pa) inside a small electrical discharge. The best-known and most widely used He-Ne laser operates at a center wavelength of 632.81646 nm (in air), 632.99138 nm (vac), and frequency 473.6122 THz, in the red part of the visible spectrum. Because of the mode structure of the laser cavity, the instantaneous output of a laser can be shifted by up to 500 MHz in either direction from the center.