Both Nm3/h and Sm3/h are units of measurement for gas flow rates, but they are based on different reference conditions.

Nm3/h (normal cubic meters per hour) is a measure of gas flow rate at standard conditions of temperature and pressure (STP). The standard temperature for Nm3/h is typically 0°C (273.15 K) and the standard pressure is usually 1 atmosphere (101.325 kPa or 14.7 psi). Nm3/h represents the gas flow rate at these standard conditions, and it is often used in industries where the gas composition and flow rate need to be measured accurately.

On the other hand, Sm3/h (standard cubic meters per hour) is a measure of gas flow rate at standard conditions of temperature and pressure (STP), but with the gas corrected to a standard composition. The standard composition is usually defined as dry air with 0% water vapor and a specific amount of nitrogen, oxygen, and other components. Sm3/h is often used in industries where gas purity is important, such as in the production of electronics or pharmaceuticals.

## Explanation of the difference

So, the main difference between Nm3/h and Sm3/h is that Nm3/h measures gas flow rate at standard temperature and pressure conditions, while Sm3/h measures gas flow rate at standard temperature and pressure conditions with the gas corrected to a standard composition.

In essence, gas of a certain weight, under normal conditions can have a volume 7.32% smaller than it would under standard conditions. Therefore 1 Nm3 contains 7.32% less gas than 1 Sm3.

To be absolutely clear, Oxywise provides the following comparison by weight below:

Oxygen:

1m3 (Temperature: 0 °C, Pressure: 1.01325 barA) weighs 1.43kg

1m3 (Temperature: 20 °C, Pressure: 1.01325 barA) weighs 1.33kg

Nitrogen:

1m3 (Temperature: 0 °C, Pressure: 1.01325 barA) weighs 1.25kg

1m3 (Temperature: 20 °C, Pressure: 1.01325 barA) weighs 1.16kg

So be careful when considering how much gas you are getting or will need, you may end up with a lot less than you intended, and if you are ever in doubt, ask for your gas amount in kg – a kg is a kg regardless of the conditions.