Since temperature and air pressure varies from place to place a standard reference is necessary for comparing the testing and the documentation of chemical and physical processes.
Note! There is a variety of alternative definitions for 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 (1 bar).
- STP - commonly used in the Imperial and USA system of units - as air at 60oF (520oR, 15.6oC) and 14.696 psia (1 atm, 1.01325 bara)
- also named "1 Standard Atmosphere"
- At these conditions, the volume of 1 mol of a gas is 23.6442 liters.
- These conditions are the most commonly used to define the volume term Sm3 (Standard cubic meter)
Note! The earlier IUAPC definition of STP to 273.15 K and 1 atm (1.01325 105 Pa) is discontinued. However,
- These conditions are still the most commonly used to define the volume term Nm3 (Normal cubic meter)
- At these conditions, the volume of 1 mol of a gas is 22.4136 liters.
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 20oC (293.15 K, 68oF) and 1 atm (101.325 kN/m2, 101.325 kPa, 14.7 psia, 0 psig, 29.92 in Hg, 407 in H2O, 760 torr). Density 1.204 kg/m3 (0.075 pounds per cubic foot)
- At these conditions, the volume of 1 mol of a gas is 24.0548 liters.
Example - Fan Pressure Increase
A fan that produce a static pressure of 3 in H2O (a good average value) - will increase the absolute air pressure by
((3 in H2O) / (407 in H2O)) (100 %) = 0.74 %
SATP - Standard Ambient Temperature and Pressure
SATP - Standard Ambient Temperature and Pressure is also used in chemistry (International Union of Pure and Applied Chemistry - IUPAC) as a reference:
- SATP - Standard Ambient Temperature and Pressure is a reference with temperature of 25oC (298.15 K) and pressure of 100.000 kPa.
- At these conditions, the volume of 1 mol of a gas is 24.4651 liters.
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, 15oC 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: 15oC = 288.15 K = 59oF
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