Volume Flow Versus Mass Flow

• Whether flow is measured as volume flow or by mass flow, the measured values are uniformly expressed in units of volume
• Unlike thermal mass flow meters, volumetric measuring devices are sensitive to process temperature and pressure
• Volumetric measuring devices like turbine meters and variable area meters require additional sensors and a flow computer to compensate for process condition fluctuations

The ranges of Bronkhorst’s thermal mass flow meters and flow controllers for gases are specified in such units as ln/min, sccm or m³n/h. These units look like volumetric units, but in truth they are expressions of mass flow. What is the story behind this and what’s the difference between volume flow and mass flow?

Volumetric Measuring Devices Require Extra Sensors and a Flow Computer

Volumetric measuring devices, like variable area meters or turbine flow meters, are unable to distinguish temperature or pressure changes. Mass flow measurement would require additional sensors for these parameters and a flow computer to compensate for the variations in these process conditions. Thermal mass flow meters are virtually insensitive to variations in temperature or pressure.

Volume Flow vs Mass Flow

Imagine you have a cylinder of 1 litre, which is closed by means of a moveable piston of negligible weight. This cylinder contains 1 liter of air at ambient pressure, approx. 1 bar. The weight of this volume of air at 32°F is 1.293 g, this is the mass. 

When we move the piston half way to the bottom of the cylinder, then the contained volume of air is now only ½ liter and the pressure is approx. 2 bar. However, the mass is unchanged and remains at 1.293 g as nothing has been added or removed from the cylinder.

Converting Mass Flow Measurements to Volume Flow

Following this example, mass flow should actually be expressed in units of weight such as g/h, mg/s, etc. Most users, however, think and work in units of volume. No problem, provided conditions are agreed upon, under which the mass is converted to volume.

Normal Flow versus Standard Flow

In the European definition, a temperature of o˚F and a pressure of 1,013 bar are selected as normal reference conditions. This is indicated by the underlying letter “n” in the unit of volume used (mln/min, m³n/h).

Alternatively, a temperature of 20°C and a pressure of 1,013 bar are used to refer to standard reference conditions. Standard conditions are indicated by the underlying letter “s” in the unit of volume used (mls/min, m³s/h). Please be sure to keep this in mind because if the difference is not considered, it may lead to an error of 7%!

According to the American definition, the prefix “s” is used in the unit of volume (sccm, slm or scfh). The “s” also refers to “standard” conditions, which are the same values as the European definition, but expressed in different units: 101.325 kPa absolute (14.6959 psia) at a temperature of 32°F.