Units alone do not reveal the conditions
A reference of 0 °C and 101.325 kPa(a) for dry gas is often used. However, “standard” may refer to a different temperature and pressure. FAD refers to declared free-air conditions, not automatically to 0 °C. Nm³ is also not a unit of specific volume; that is expressed, for example, in m³/kg.
Conversion for the same mass
For an ideal dry gas, Q₂ = Q₁ × (p₁/p₂) × (T₂/T₁). Pressures must be absolute and temperatures in kelvin. If humidity, condensation or non-ideal gas behaviour are significant, this simple relationship is insufficient. Changes in the liquid water fraction require a separate balance.
What to check in practice
- State the reference temperature, absolute pressure and dry or wet basis.
- When comparing meters, check their configured reference conditions.
- Convert delivery to the operating volume before calculating pipe velocity.
Example scenario
For ideal dry air, 1 Nm³/min defined at 0 °C and 1.01325 bar(a) corresponds to approximately 1.088 m³/min at 20 °C and 1.00 bar(a). This is the same amount of dry air, not an increase in production.
What to watch out for
- Treating Nm³ and m³ as always identical.
- Using degrees Celsius in a temperature ratio.
- Changing a flow meter's reference without updating the subsequent assessment.
Can I compare two Nm³/h figures without further information?
Only if their normal conditions and humidity basis are defined identically. For commercial or energy assessments, it is better to state the reference explicitly than rely on the letter N alone.