Since temperature and air pressure may vary form place to place it is necessary with standard reference conditions for testing and documentation of chemical and physical processes.
Note! There is a variety of alternative definitions for the standard reference conditions of temperature and pressure. STP, NTP and other definitions should therefore be used with care. It is always important to know the reference temperature and reference pressure for the actual definition used.
STP is commonly used to define standard conditions for temperature and pressure which is important for the measurements and documentation of chemical and physical processes:
Note that the earlier IUAPC definition of STP to 273.15 K and 1 atm (1.01325 105 Pa) is discontinued.
1 Pa = 10-6 N/mm2 = 10-5 bar = 0.1020 kp/m2 = 1.02x10-4 m H2O = 9.869x10-6 atm = 1.45x10-4 psi (lbf/in2)
NTP is commonly used as a standard condition for testing and documentation of fan capacities:
SATP - Standard Ambient Temperature and Pressure is also used in chemistry as a reference:
ISA - International Standard Atmosphere is used as a reference to aircraft performance:
Standard model of the atmosphere adopted by the International Civil Aviation Organization (ICAO):