Volumetric flow, mass flow and normal litres explained
What is the difference between a litre per minute and a standard litre per minute? And why do we almost always choose mass flow for gases?
⏱ approx. 6 minutes reading time
When ordering or configuring a gas flow meter, sooner or later you will come across units such as slpm, Nl/min or sccm. They look like ordinary volumetric flows, but they are not. This article explains step by step why that difference matters and how to choose the right unit.
1. Volumetric flow versus mass flow
Volumetric flow (l/min, m³/h)
- Indicates how much space the gas occupies
- Varies strongly with pressure and temperature
- Easy to measure, difficult to interpret
- Example: 10 l/min at 1 bar ≠ 10 l/min at 5 bar
Mass flow (kg/h, g/min)
- Indicates how much substance (molecules) is flowing
- Independent of pressure and temperature
- Directly relevant to processes
- Example: 10 g/min is always 10 g/min
Why gases behave so differently from liquids
Gas is compressible. The same number of molecules occupies less space at higher pressure. A balloon filled with 2 litres of air at sea level shrinks to less than 1 litre at a depth of 10 metres in water, while the number of air molecules remains exactly the same. This is precisely the phenomenon described by the Boyle-Gay-Lussac law:
Boyle-Gay-Lussac law
P₁ · V₁ / T₁ = P₂ · V₂ / T₂
P = absolute pressure | V = volume | T = temperature in Kelvin (°C + 273.15)
This law makes clear why a volumetric measurement of gases is ambiguous: for the same quantity of molecules, the measured volume in litres can vary enormously depending on the process conditions.
Molecules do the work, not the space between them
In most industrial gas processes, it is the number of molecules that counts, not the volume. Whether it concerns chemical reactions, combustion processes, pneumatic systems or respiration: the molecules are the active substance. The space between the molecules is irrelevant. That is why mass flow, or a correct approximation of it, is the right quantity to measure and control.
Key point
A volumetric flow in l/min without stating pressure and temperature provides insufficient information to know how much gas is actually flowing. Mass flow or standardised flow solves this.
2 · Standard and normal litres: slpm versus Nl/min
Because true mass flow (kg/h) is impractical in everyday use, the industry works with a clever intermediate solution: the standardised or normalised volume unit. The volume is converted back to a fixed reference temperature and pressure. Because those reference conditions are fixed, you always know exactly how many molecules are flowing.
You can recognise these units by the prefix S (Standard) or N (Normal/Norm):
The difference between slpm and Nl/min
Both slpm and Nl/min measure the same thing, the number of molecules per unit of time, but they use a different reference temperature. This has a direct effect on the numerical value:
Conversion slpm ↔ Nl/min (at 1013.25 mbar)
1 slpm (20 °C) ≈ 0.928 Nl/min (0 °C)
A meter set to slpm shows a 7.2% higher value than the same meter set to Nl/min, for exactly the same gas flow.
Take care when specifying and orderingAre you passing on a setpoint in slpm while the meter is configured for Nl/min (or vice versa)? Then you will systematically supply 7.2% too much or too little gas. Always check which reference your instrument uses and make sure it matches your process specification.
Practical example
A manufacturing company specifies a gas flow of 100 slpm for a reactor process, based on American datasheet values. The flow meter on the installation is configured for Nl/min (0 °C). The setpoint is set to 100, but the actual gas flow is only ~92.8 Nl/min = ~100 slpm. In this case, the values happen to match. Had the operator read the setpoint as 100 Nl/min and compared it with the 100 slpm specification, the conclusion would have been that the meter was correct, while in fact 7.2% too little gas was being supplied.
3 · Practical considerations in flow measurement
Pressure and temperature strongly affect volumetric meters
Variable area meters (VA meters, rotameters) are fundamentally volumetric meters. They are read and calibrated at atmospheric conditions. As soon as the process pressure deviates from the calibration pressure, the reading deviates significantly from the actual flow. With a pressure change from 1 bar to 7 bar, deviations of more than 150% on the volumetric scale are possible, while the actual mass flow is completely different.
Mass flow meters and controllers compensate automatically
A thermal mass flow meter (MFC or MFM) directly measures heat transfer, which is proportional to the mass flow. Pressure and temperature correction is built into the operating principle. The result is always a corrected, stable flow reading, regardless of fluctuations in line pressure or ambient temperature.
Setting units in the software
Modern flow controllers (such as the Vögtlin red-y series) let you set the reference conditions yourself via software. You can choose between standard conditions (20 °C) and normal conditions (0 °C). Always make sure this setting matches the unit in your process specification to avoid measurement errors.
In summary: what you need to remember
- Volumetric flow (l/min, m³/h) indicates how much space the gas occupies, depending on pressure and temperature.
- Mass flow (kg/h, g/min) indicates how much substance is flowing, independent of process conditions, and is the relevant quantity in most applications.
- slpm and Nl/min are both normalised volume units that approximate mass flow, but they use a different reference temperature (20 °C vs. 0 °C). The difference is 7.2%.
- Always check which reference conditions your flow meter uses and align them with your process specification to avoid systematic errors.
- With variable process pressure: choose a thermal mass flow meter instead of a VA meter for reliable measurements.
