In the physical sciences, such as chemistry, the standard temperature and pressure (STP) is a set of conditions for experimental measurements that enables true comparisons between sets of data representing gas volumes. There are many different definitions of standard temperatures and pressures applicable to gas volumes and volumetric flow rates. The differences depend on the organization defining the standard reference conditions.
In industry and commerce, it is necessary to define the standard reference conditions of temperature (T) and pressure (P) when expressing a gas volume or volumetric flow rate because the volume of a gas varies with its temperature and pressure. The various definitions of standard reference conditions by the different organizations clearly indicate that there is not a universally accepted definition. In addition, the many air sampling programs that do not correct rotameter or other analog flow measurement devices to any reference standard does not enable one to compare different sets of air monitoring data among different industry members.