Gases · flow instrumentationSubscription
Gas flow normalization: Nm³/h, Sm³/h and SCFM
Convert a gas flow between the real line conditions (operating pressure and temperature) and a reference condition, normal or standard. Includes gauge or absolute pressure, altitude, compressibility factor Z and mass flow.
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Where it helps at work
- Compare a compressed air flowmeter reading with the compressor capacity in Nm³/h.
- Check natural gas, nitrogen or oxygen invoices stated at a reference condition.
- Set up pressure and temperature compensation in a flow computer or PLC.
- Size filters, dryers and valves specified in SCFM.
What you enter
What you get
Worked example: compressed air in Nm³/h
A compressed air flowmeter, installed after the dryer, reads 100 m³/h at line conditions: 7 bar gauge and 30 °C, at sea level. The compressor supplier talks about Nm³/h. How much air is flowing, at normal conditions and by mass?
| Flow measured in the line | 100 m³/h |
|---|---|
| Gauge pressure | 7 bar |
| Gas temperature | 30 °C |
| Atmospheric pressure | 101.325 kPa |
| Gas | Dry air |
| Normal flow (0 °C, 101.325 kPa) | 712.6 Nm³/h |
|---|---|
| Factor reference ÷ operating | 7.1258 |
| Mass flow | 920.8 kg/h |
| Same flow at 20 °C and 101.325 kPa | 764.8 m³/h |
How to read it
The 100 m³/h in the line correspond to 712.6 Nm³/h: compressed to 8.013 bar absolute, the air takes about 7.1 times less volume. By mass, it is 920.8 kg/h. If the contract uses the 20 °C reference, the same air is 764.8 m³/h at 20 °C: check which reference the document uses before comparing numbers.
Limits and cautions
- With Z = 1 the gas is treated as ideal. Natural gas and high pressures need Z computed from the composition (AGA 8 or ISO 12213).
- Atmospheric pressure from altitude uses the standard atmosphere; the day’s barometric pressure varies by a few kPa.
- Gas humidity is not considered: the flow is dry gas, or the humidity stays inside the volume entered.
- The right reference is the one in the contract, meter or manufacturer document; each may use a different one.
Technical basis
Gas law with compressibility factor Z, the usual normal and standard conditions and the ISO 2533 standard atmosphere for altitude.
- DIN 1343 — condição normal (0 °C, 101,325 kPa)
- ISO 13443 — condições de referência padrão para gás natural (15 °C, 101,325 kPa)
- ISO 2533 — atmosfera padrão
In the full version, you can
- Convert both ways with gauge or absolute pressure, altitude and Z.
- Choose among four usual references or enter your own.
- Get mass flow and densities for the listed gases or from the molar mass.
- Export the PDF with the conditions, the result and the reference table.
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Log in to my accountFrequently asked questions
Are Nm³ and Sm³ the same thing?
Not necessarily. Nm³ usually means 0 °C and Sm³ 15 °C or 20 °C, always at 101.325 kPa, but each standard and contract defines its own. Between 0 °C and 20 °C the difference exceeds 7 %.
Can I enter gauge pressure?
Yes. The tool adds the local atmospheric pressure, entered in kPa or computed from altitude.
When do I need the Z factor?
For real gases at high pressure, such as natural gas in transmission lines. For plant compressed air, Z = 1 is usually enough.
Does it help set up a flow computer?
It helps to check one: the tool shows the correction factor between operating and reference conditions, which is what the computer applies from the pressure and temperature measurements.