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The actual (measured) flow rate is normalized to the flow rate at a certain reference pressure (P) and temperature (T).
For Metric system, the "normal" condition is
Pref = 1 atm = 1.01325 bar = 101325 Pa
Tref = 0 °C = 273.15 k
For Standard (English) system, the "standard" condition is
Pref = 1 atm = 14.6959 psi
Tref = 70 °F = 529.67 °R
Ideal gas assumes that volume is proportional to temperature and the reciprocal of pressure. The normalized volume flow rate becomes
Q = QN × (Pref / Pa) × (T / Tref)
where Pa, the actual pressure, is the sum of surrounding atmospheric pressure and the gage pressure (Pa = Patm + Pg). The unit of flow temperature T must be either Kelvin (k) or Rankine degrees (°R).
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