1) "In chemistry and other sciences, STP or standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. Internationally, the current STP defined by the IUPAC (International Union of Pure and Applied Chemistry) is an absolute pressure of 100 kPa (1 bar) and a temperature of 273.15 K (0 °C). Other organizations have established a variety of alternative definitions for the standard reference conditions of temperature and pressure. In industry and commerce, it is necessary to define the standard reference conditions of temperature and pressure when expressing a gas volume or a volumetric flow rate because the volume of a gas varies with the temperature and pressure of the gas. The available data on the various definitions of standard reference conditions clearly indicates that there is no universally accepted definition of the standard conditions of temperature and pressure. For that reason, simply stating that a gas flow rate is 10,000 m³/h (i.e. cubic meters per hour) at "standard conditions" or at "STP" has no meaning unless the reference conditions that were applied are clearly stated." Source and further information: http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure 2) "Standard Temperature and Pressure (STP) is defined as 0 degrees Celsius and 1 atmosphere of pressure. STP is used in many thermodynamic calculations and tabulations. Certain properties of matter such as density, viscosity, boiling point etc. will vary with changes in temperature or pressure. Having one common set of conditions ("state") for tabulating these values makes comparisons possible and eases calculations." Source and further information: http://www.ilpi.com/msds/ref/stp.html 3) "Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. These values, however, are very important in many chemical and physical processes, in particular with regard to measurements. Therefore, it is necessary to define standard conditions for temperature and pressure." Source and further information: http://www.biocrawler.com/encyclopedia/Standard_temperature_and_pressure 4) "Temperature and Pressure Ratings These are the two important determinants for the safe and effective working of any industrial fitting. The range of temperature and pressure [...] depends on the final application, the material being used etc. " Source: http://www.pipetubefittings.com/specifications.html