Definition

All liquids have a tendency for evaporation. Evaporation takes place at the surface of the liquid. The gas that is above the liquid is called its vapour and it creates a pressure called vapour pressure. The vapour pressure depends on the temperature.

Units

There are several different units for pressure.

Substance	kPa	Bar	mmHg	Temperature
Methylene Chloride	57.2	0.572	435	25 °C
Ethanol	5.83	0.058	43.7	20 °C
Methyl isobutyl ketone	2.66	0.027	19.9	25 °C
Propanol	2.4	0.024	18.0	20 °C
Water (H2O)	2.3	0.023	17.5	20 °C
Ethylene glycol	0.100	0.001	0.76	20 °C

Reid Vapour Pressure

Reid vapour pressure (RVP) is a common measure of the volatility of gasoline, volatile crude oil, and other volatile petroleum products. It is defined as the absolute vapour pressure exerted by a liquid at 37.8 °C (100 °F). RVP is stated in kilopascals and represents a relative pressure to the atmospheric pressure because RVP is the gauge pressure of the sample in a non-evacuated chamber.