STP - Standard Temperature and Pressure & NTP - Normal Temperature and Pressure

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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 - Standard Temperature and Pressure

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:

• STP - Standard Temperature and Pressure - is defined by IUPAC (International Union of Pure and Applied Chemistry) as air at 0^oC (273.15 K, 32 ^oF) and 10⁵ pascals  
  
• STP - commonly used in the Imperial and USA system of units - as air at 60 ^oF (520 ^oR) and 14.696 psia (15.6^oC, 1 atm)

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 - Normal Temperature and Pressure

NTP is commonly used  as a standard condition  for testing and documentation of fan capacities:

• NTP - Normal Temperature and Pressure - is defined as air at 20^oC (293.15 K, 68^oF) and 1 atm (101.325 kN/m², 101.325 kPa, 14.7 psia, 0 psig, 29.92 in Hg, 760 torr). Density 1.204 kg/m³ (0.075 pounds per cubic foot)

SATP - Standard Ambient Temperature and Pressure

SATP - Standard Ambient Temperature and Pressure is also used in chemistry as a reference:

• SATP - Standard Ambient Temperature and Pressure is a reference with temperature of 25 degC (298.15 K) and pressure of 101 kPa.

ISA - International Standard Atmosphere

ISA - International Standard Atmosphere is used as a reference to aircraft performance:

• ISA - International Standard Atmosphere is defined to 101.325 kPa, 15 degC and 0% humidity.

ICAO Standard Atmosphere

Standard model of the atmosphere adopted by the International Civil Aviation Organization (ICAO):

• Atmospheric pressure: 760 mmHg = 14.7 lbs-force/sq inch
• Temperature: 15^oC = 288.15 K = 59^oF

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