How pressure measurement evolved — from Torricelli to psi
Why psi and kPa and bar are all 1 atm apart by design.
In 1643, Evangelista Torricelli inverted a mercury-filled tube over a dish, and the column stayed at 760 mm. He had invented the barometer, and 760 mmHg became a unit of pressure.
A century later, pascals (Pa) were defined: 1 Pa = 1 N/m². Atmospheric pressure is 101,325 Pa. Pressure units proliferated because every industry picked one:
- Meteorology uses millibars (mbar) or hectopascals (hPa) — they're the same thing, 100 Pa each. - Tire pressure in the US: PSI (pounds per square inch). In Europe: bar, or kPa. - Aviation uses inches of mercury and hectopascals on altimeters. - Diving uses bar. - Refrigeration and HVAC use psi in the US and bar in Europe. - Vacuum physics uses torr (= 1 mmHg, exactly).
The factors relating them all derive from the SI definition, plus a few historical accidents like 14.696 psi = 1 atm.
The site lets you type "30 psi" and see it's ~2.07 bar, or type "1 atm" and see 14.696 psi. The math is independent of which you start with.