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Volume Flow versus Mass Flow

The ranges of Bronkhorst's thermal mass flow meters / 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?

Imagine you have a cylinder of 1 litre, which is closed by means of a moveable piston of negligible weight. This cylinder contains 1 litre of air at ambient pressure, approx. 1 bar. The weight of this volume of air at 0°C is 1.293 g, this is the mass. volume flow vs mass flow

When we move the piston half way to the bottom of the cylinder, then the contained volume of air is only ½ litre, the pressure is approx. 2 bar, but the mass is unchanged, 1.293 g; nothing has been added, or left out.

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. Following the 'European' definition, a temperature of 0°C and a pressure of 1,013 bar are selected as "normal" reference conditions, indicated by the underlying letter "n" in the unit of volume used (mln/min, m3n/h). Alternative, a temperature of 20°C and a pressure of 1013.25 hPa(a) are used to refer to "standard" reference conditions, indicated by the underlying letter "s" in the unit of volume used (mls/min, m3s/h). Please be aware of this, because if the difference is not considered, it may lead to an error of 7%!

According to the 'American' definition the prefix "s" in sccm, slm or scfh refers to "standard" conditions 1013.25 hPa absolute (1013 mbara or 14.6959 psia) and temperature of 0°C (32°F).

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.

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