Units
| pascal | bar | technical atmosphere | standard atmosphere | torr | pound per square inch | |
|---|---|---|---|---|---|---|
| Pa | bar | at | atm | Torr | psi | |
| 1 Pa | ≡ 1 N/m2 | 10−5 | 1.0197×10−5 | 9.8692×10−6 | 7.5006×10−3 | 1.450377×10−4 |
| 1 bar | 105 | ≡ 106 dyn/cm2 | 1.0197 | 0.98692 | 750.06 | 14.50377 |
| 1 at | 0.980665 ×105 | 0.980665 | ≡ 1 kp/cm2 | 0.9678411 | 735.5592 | 14.22334 |
| 1 atm | 1.01325 ×105 | 1.01325 | 1.0332 | ≡ p0 | ≡ 760 | 14.69595 |
| 1 Torr | 133.3224 | 1.333224×10−3 | 1.359551×10−3 | 1.315789×10−3 | ≈ 1 mmHg | 1.933678×10−2 |
| 1 psi | 6.8948×103 | 6.8948×10−2 | 7.03069×10−2 | 6.8046×10−2 | 51.71493 | ≡ 1 lbF/in2 |
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m−2 or kg·m−1·s−2). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N·m−2. When indicated, the zero reference is stated in parenthesis following the unit, for example 101 kPa (abs). The pound per square inch (psi) is still in widespread use in the US and Canada, for measuring, for instance, tire pressure. A letter is often appended to the psi unit to indicate the measurement's zero reference; psia for absolute, psig for gauge, psid for differential, although this practice is discouraged by the NIST.
Because pressure was once commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g. inches of water). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure.
Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When 'millimetres of mercury' or 'inches of mercury' are quoted today, these units are not based on a physical column of mercury: they have been given precise definitions that can be expressed in terms of SI units. The water-based units usually assume one of the older definitions of the kilogram as the weight of a litre of water.
Although no longer favoured by measurement experts, these manometric units are still encountered in many fields. Blood pressure is measured in millimetres of mercury (see torr) in most of the world, and lung pressures in centimeters of water are still common. Natural gas pipeline pressures are measured in inches of water, expressed as 'inch WC' ('Water Column'). Scuba divers often use a manometric rule of thumb: the pressure exerted by ten meters depth of water is approximately equal to one atmosphere. In vacuum systems, the units torr, micrometre of mercury (micron), and inch of mercury (inHg) are most commonly used. Torr and micron usually indicates an absolute pressure, while inHg usually indicates a gauge pressure.
Atmospheric pressures are usually stated using kilopascal (kPa), or atmospheres (atm), except in American meteorology where the hectopascal (hPa) and millibar (mbar) are preferred. In American and Canadian engineering, stress is often measured in kip. Note that stress is not a true pressure since it is not scalar. In the cgs system the unit of pressure was the barye (ba), equal to 1 dyn·cm−2. In the mts system, the unit of pressure was the pieze, equal to 1 sthene per square metre.
Many other hybrid units are used such as mmHg/cm² or grams-force/cm² (sometimes as kg/cm² and g/mol2 without properly identifying the force units). Using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is forbidden in SI; the unit of force in SI is the newton (N).
Read more about this topic: Pressure Measurement
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