Where do these pressure units come from?
Pressure was one of the first physical quantities to be measured accurately — long before its thermodynamic nature was understood. Each unit reflects the instrument or the application that gave birth to it.
Pascal (Pa) — Blaise Pascal, 1647
In the 17th century, the French physicist and mathematician Blaise Pascal showed with his Puy-de-Dôme experiment that atmospheric pressure decreases with altitude — the first proof that air has weight. The International System honoured him in 1971 by naming the pressure unit after him: 1 Pa = 1 N/m² (1 newton of force spread over 1 square metre).
The Pascal is very small: atmospheric pressure is 101 325 Pa. That is why pure Pa is rarely used in industry — we prefer kPa, MPa or bar.
bar — Norwegian Meteorological Institute, 1909
The Norwegian meteorologist Vilhelm Bjerknes proposed in 1909 a simple and readable unit equal to exactly 100 000 Pa, which is roughly atmospheric pressure at sea level (1.01325 atm exactly).
The bar is not an official SI unit, but it is accepted for general use by the International Bureau of Weights and Measures, and it completely dominates European industrial process engineering: people speak of 4 bar compressed air, 16 bar steam, 40 bar nitrogen networks — without having to count zeros.
psi — Imperial, 19th century
Pound per square inch. One pound-force (4.448 N) applied over one square inch (645 mm²) gives 1 psi = 6 894.76 Pa.
psi is the dominant unit in the American oil industry — where it took hold in the very first Pennsylvania oilfields of the 19th century. There you naturally speak of 5 000 psi wellheads, 1 440 psi pipelines (ASME B31.4 class 600) and 32 psi tyres.
atm — standard atmosphere
Defined historically as sea-level pressure at 0 °C at mid-latitude. Standardised in 1954 at exactly 101 325 Pa. Still used in chemistry (gas/liquid equilibria), in physiology (diving) and in older process documentation.
mmHg / Torr — Evangelista Torricelli, 1644
Galileo’s pupil, Evangelista Torricelli, invented the barometer in 1643 and showed the following year that air pushes a column of mercury about 760 mm high at sea level. The height of that column became a natural unit:
- 1 mmHg = 133.322 Pa
- 760 mmHg = 1 atm
- The Torr is exactly the same unit (renamed in honour of Torricelli): 1 Torr = 1 mmHg.
This unit is still everywhere in medicine (blood pressure: 120/80 mmHg) and in vacuum technology (ultra-high vacuum chambers: 10⁻⁹ Torr).
inHg, inH₂O — Imperial
Same principles as mmHg / mmH₂O but in inches of mercury or water column.
- inHg (≈ 3 386 Pa per inch): atmospheric pressure, US instrumentation.
- inH₂O (≈ 249 Pa per inch): HVAC — for the very low differential pressures in ventilation ductwork (typically 0.1 to 10 inH₂O), where Pa would give hundreds of Pa and bar would be absurd.
Why so many units still coexist?
Each discipline picked the unit whose typical values fall in a convenient range — that is, between 1 and 10 000, with no zeros to count.
| Range / domain | Common unit |
|---|---|
| Ventilation, HVAC (very low ΔP) | inH₂O or Pa |
| Industrial vacuum | mbar or Torr |
| Fluid process (Europe) | bar |
| Oil & gas process (US) | psi |
| High pressure (compressors, hydraulics) | bar or MPa |
| Blood pressure (medical) | mmHg |
| Acoustics, general physics | Pa (strict SI) |
Gauge vs absolute: the critical distinction
Industrial instruments almost always show gauge pressure — referenced to local atmosphere. When a gauge reads “4 bar”, the absolute pressure in the line is in fact 4 + 1.013 ≈ 5.013 bar absolute.
- barg / psig: gauge (relative) — what pressure gauges display
- bara / psia: absolute — what thermodynamic calculations use (ideal gas, etc.)
Mixing the two up is a classic source of sizing and flow-calculation errors.