In chemistry, physics and engineering, the term "standard temperature and pressure" (STP) refers to a standard set of conditions for a gas as a basis for calculations. There are several definitions for standard temperatures and pressures, and the values of these references will depend on the desired applications, industries or academic settings. Historically, the most common set of standards was a temperature of 32 degrees Fahrenheit (0 degrees Celsius) and a pressure of 1 atmosphere, or 101.325 kilopascals (kPa), although a pressure of 1 bar, or 100 kPa, is currently accepted by the International Union of Pure and Applied Chemistry (IUPAC). The standard pressure reflects the accepted average pressure of the atmosphere at sea level, and the standard temperature reflects the freezing point of pure water at that pressure.
Standard conditions are most often used for gas calculations in chemistry and engineering. For example, it is possible to calculate the molar volume of an ideal gas at the STP via the ideal gas law. In engineering, standard conditions might be used to calculate mass flow rates of gases based on standard volumetric flow rates from flow meters.
The ideal gas law shows the relationships between pressure (P), volume (V) and temperature (T) for an ideal gas. It can be expressed as PV=nRT, with n referring to the number of moles of gas and R referring to the ideal gas constant. Using the current standard temperature and pressure as defined by the IUPAC, 1 mole of an ideal gas will have a volume of 1,385.9 cubic inches (22.711 liters).
Standard temperature and pressure might actually refer to many different conditions used by various industries. For example, an engineering firm might decide to use 1 atmosphere and 77 degrees Fahrenheit (25 degrees Celsius) as its STP to better reflect ambient conditions of the area. The Society of Petroleum Engineers, for example, defines standard conditions as 59 degrees Fahrenheit (15 degrees Celsius) and 1 bar. When working with engineering designs, it is important for a person to understand the designer's intended standards.
Differences in the accepted values for standard temperature and pressure might depend on regional conditions. Higher pressures and temperatures make more sense for laboratories and industries situated in areas subject to long periods of hot weather. Lower pressures might be used for laboratories at high elevations. Of course, differences in the accepted values might also be because of the personal preferences of chemists, laboratory technicians and engineers.