Mass flow vs Volume flow

What is mass flow rate?

Essentially, mass flow rate is the amount of mass of a gas or liquid that flows in a certain amount of time, for instance expressed in kilograms per hour (kg/h) or grams per second (g/s). In an analogous way, a volumetric flow rate is the volume of a gas or liquid that flows in a certain amount of time, expressed in such units as litres per hour (l/h) or cubic centimetres per second (cm3/s).

How are mass flow rate and volumetric flow rate related?

There is a difference in behaviour when dealing with masses or volumes of fluids. Volumes are influenced by changes in process conditions such as temperature and pressure, while masses are not affected. The same holds for ‘flowing’ masses and volumes.

The density of a gas or liquid, expressed in kg/dm3, relates the mass flow rate to the volumetric flow rate. This density is temperature and pressure dependent: high temperatures generally result in low densities, and high pressures result in high densities for these fluids - although the effect for gases is greater than for liquids. The volumetric flow rate is obtained by dividing the mass flow rate by the fluid density. A volumetric flow rate varies with temperature and pressure, while a mass flow rate remains constant when temperature or pressure changes.

Why use volumetric units for mass flow rates?

Following the logic above, a mass flow rate should be expressed in units of mass such as g/h, mg/s, etc. Most users, however, think and work in units of volume. That’s fine, and to use density in converting mass flow to volume flow, we must pick a set of specific pressure and temperature conditions at which we use the density value for the gas. Worldwide, there are quite a lot of these standard reference conditions for conversion. 

The following reference conditions are used by Bronkhorst:

• When the mass flow rate is expressed with subscript n as in ml_n/min or m3_n/h, this means that a fluid density at a temperature of 0 °C and a pressure of 1 atm (1013 mbar) are selected for conversion from mass flow rate to volumetric flow rate. The subscript n represents normal reference conditions in European style.
• This corresponds to the prefix "s" in sccm (standard cubic centimetres per minute) or slm (standard litre per minute), which refers to American standard conditions at a temperature of 0°C (32°F) and an absolute pressure of 1 atm (1013 mbar, 14.69 psia).
• As an alternative, a temperature of 20°C and a pressure of 1 atm (1013 mbar) are used to refer to European standard reference conditions, indicated by the subscript s in the volumetric units (ml_s/min, m³_s/h). These values resemble average temperature and pressure conditions at sea level. 

Examples of mass flow & volume flow instruments

• The EL-FLOW Select mass flow controllers are used to supply air into the production of ice cream, called aeration. Mass flow rates are typically expressed in ml_n/min and l_n/min.
• Also the new FLEXI-FLOW instruments work according to the thermal mass flow measurement principle. With very short response times - due to TCS (chip sensor) technology for the manufacturing of highly compact instruments that can measure and control gas pressure in addition to mass flow rate. 
• Bronkhorst mini CORI-FLOW instruments are applied in, for example, mRNA vaccine production for accurately and reproducibly measuring liquid vaccine ingredients. Mass flow rates for liquids are expressed directly in mass units like grams per hour (g/h), virtually independent of temperature and pressure fluctuations.
• The ES-FLOW instruments measure and control volumetric liquid flow rates using ultrasound. They effectively measure the flow velocity, which multiplied by the tube cross-section inside the device results in volumetric liquid flow rates. These instruments are used, for example, to measure the amount of colouring agent, flavouring agent and acid that is supplied to a candy production process. 

In case mass flow rates are expressed in volumetric flow units, the reference temperature and pressure conditions of the instruments flow unit is always mentioned on the calibration certificate of Bronkhorst.

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