Conversion Factors
The millimeter of mercury by definition is 133.322387415 Pa (13.5951 g/cm3 × 9.80665 m/s2 × 1 mm ), which is approximated with known accuracies of density of mercury and gravitational acceleration.
The torr is defined as 1⁄760 of one atmosphere, while the atmosphere is defined as 101.325 kPa. Therefore, 1 Torr is equal to 101325⁄760 Pa. The decimal form of this fraction (133.322368421...) is an infinitely long, periodically repeating decimal, as is its reciprocal.
The relationship between the torr and the millimetre of mercury is:
- 1 Torr = 0.999999857533699... mmHg
- 1 mmHg = 1.000000142466321... Torr
The difference between one millimetre of mercury and one torr, as well as between one atmosphere (101.325 kPa) and 760 mmHg (101.3250144354 kPa), is less than one part in seven million (or less than 0.000015%). This small difference is negligible for most applications outside metrology.
The millimetre of mercury as used in medicine is in general given relative to the atmospheric pressure. This means that when a doctor tells you you have a blood pressure of 100 mmHg, this is 100 mmHg above atmospheric. So on a day when the barometric pressure is 760 your absolute pressure is actually 860 millimetres of mercury (115 kPa).
The SI unit of pressure is the pascal (symbol: Pa), defined as one newton per square metre. Other units of pressure are defined in terms of SI units. These include:
-
- The bar (symbol: bar), defined as 100 kPa exactly.
- The atmosphere (symbol: atm), defined as 101.325 kPa exactly.
- The torr (symbol: Torr), defined as 1⁄760 atm exactly.
These four pressure units are used in different settings. For example, the bar is used in meteorology to report atmospheric pressures. The torr is used in high-vacuum physics and engineering.
| 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 |
Read more about this topic: Torr
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