Use this conversion calculator to convert American/Britis units to the metrical (SI) units.
Definition of pressure:
Pressure (p) is a derived quantity with its own name. The unit is Pascal (Pa), also known as N/m 2. in SI this will be kg·m -1 ·s -2 (kg/m·s 2 ).
The unit of pressure used to be torr, also known as mmHG (millimeter mercury). Some mercury was put in a glass tube. If the pressure changes, the level of the mercury in the tube changes with it. The reason that mercury is used is that the thermal expansion of mercury is big and is mostly homogeneous. Mercury also doesn't stick to the glass. It was calibrated that on sea-level the height of the mercury was 760 mm. At sea-level the pressure is also defined as 1 atmosphere, that's why 1 atmosphere equals 760 torr. If you measure the
pressure in Pascal at sea-level, you will find 101325 Pa. This also equals one atmosphere.
The pressure of a gas is related to temperature and volume. If the volume is constant and the temperature increases, the pressure also increases. This is because the molecules get more energy and move faster.
The relation is given by:
p * V = c * T :
* = *
p in Pa
V in m 3
T in K
c is a constant, proportional to the number of gas molecules
This formula is called the ideal gas law.It is valid if the temperature (in kelvin) is at least 50% higher than the temperature at the critical point and the pressure does not exceed the critical pressure. These conditions are often fullfilled, for example for air at atmospheric pressure and standard temperature.