nTorr

nTorr is a non-SI unit of pressure used in vacuum science to express extremely low pressures. It denotes one nano-torr, equal to 10^-9 Torr. Since 1 Torr is defined as 133.322 Pa, 1 nTorr is approximately 1.33322 × 10^-7 pascal. The unit is most commonly used in ultra-high vacuum (UHV) and extreme high vacuum (XHV) contexts where pressures are on the order of 10^-9 to 10^-12 Torr. In practice, many instruments report readings in Torr or pascals, but nTorr is used in some gauges and literature to emphasize very low pressures.

Measuring pressures in the nTorr range requires specialized instrumentation. Ionization gauges, such as Bayard-Alpert and hot-cathode types, as well as other advanced vacuum sensors, can operate down to around 10^-12 to 10^-9 Torr, though accuracy, gas dependence, and calibration challenges increase at the lowest levels. Readings are often cross-checked against calibrated standards and may be influenced by gas composition, contamination, and gauge design. Some instruments display results directly in nTorr, while others present values in Torr or pascals with an explicit exponent or SI prefix.

Usage and context. The term nTorr appears primarily in research and industrial settings dealing with very high or extreme vacuum environments, such as surface science experiments, electron microscopy, semiconductor processing, and particle accelerators. It serves as a convenient shorthand for communicating pressures far below the standard Torr ranges, while still tying the values to the familiar Torr scale through consistent conversion to pascals. The Torr, from which nTorr derives, has historical roots in the 17th century and remains a common unit in vacuum science alongside Pa and other SI-derived units.