Units used in the converter
Pascal (Pa) is defined as one newton per square meter. Pascal as replacement of newton per square meter (N/m2) was introduced only in 1971 at the 14e Conférence Générale des Poids et Mesures (14th CGPM).
Another unit of pressure is bar, or 100000 pascals. Millibar is 100 pascals, and it is widely used in meteorology. Thus, "standard" atmospheric pressure is 1013.25 millibars.
"Standard" atmospheric pressure, or atmosphere (atm), which is the average pressure of air at sea-level, was considered to be 760 millimeters of mercury, or 760 torrs (see below). However, in 1954, the definition of the atmosphere was revised by the 10e Conférence Générale des Poids et Mesures (10th CGPM) to the currently accepted definition: one atmosphere is equal to 101325 pascals. And, by the way, "101325 pascals" is the average atmospheric pressure at sea-level at Paris latitude.
Technical atmosphere (at) is the pressure of 1 kilogram-force per square centimeter. Kilogram-force is the force produced with 1 kilogram of mass in gravity field with g equals to 9.80665 m/s2, or 9.80665 Newtons. 1 technical atmosphere is 0.96784 of standard atmosphere.
Long time ago, in pre-metric times, there lived an italian those name was Evangelista Torricelli (1608-1647). He was the first who proved that air has pressure, as he famously wrote in a letter: "We live submerged at the bottom of an ocean of air." He also had invented mercury barometer, while making his become-classic experiment with tube approximately one meter long, sealed at the top, filled with mercury and set vertically into a basin of mercury.
Unit of pressure, equals approximately to the pressure of one millimeter of mercury has been named torr after him.
At the 10e Conférence Générale des Poids et Mesures (10th CGPM) the torr was redefined as 1⁄760 of one atmosphere. This was necessary in place of the definition of the torr as one millimeter of mercury, because the height of mercury changes at different temperatures and gravities.
Pounds per square inch
Pounds per square inch (psi) has the same meaning as technical atmosphere. The difference is usage of non-metric (imperial) units.
How to use the converter
Enter value of unit to be converted and use table to find out result. For example, to convert 100 torrs to pascals, enter 100 and lookup value at the crossing of "Millimeter of mercury (torr)" row and "Pascal" column.