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All Mass Flow Measurements Are NOT Created Equal

  I was perusing Control Global's website this morning, (an aside – they have GREAT stuff on their site, check it out if you haven't been there before) and ran across a new white paper discussing thermal mass flow meters. It reminded me of an important distinction we commonly discuss with customers: inferred mass flow measurement vs. direct mass flow measurement.   Inferred Mass Flow Measurement vs. Direct Mass Flow Measurement An inferred mass flow measurement is made when one or more characteristics of the media flowing through a pipe are measured or known, and then combined into an equation to calculate the mass flow rate. To get an inferred mass flow rate out of a turbine or orifice plate meter, users commonly multiply the volumetric flow rate that these meters provide by the media's density at the operating conditions – a mass flow calculation based on the law of conservation of mass. As the white paper points out, a common method to find operating density is to measure the media's temperature and pressure and use those values to calculate the density at the operating conditions from known media properties. A direct mass flow measurement occurs when an instrument directly measures the mass flow rate of the media without requiring the user to know additional media properties. A coriolis flow meter is the only type of flow meter currently available that provides a direct mass flow measurement. Despite having "mass" in the title, a thermal mass flow meter can only provide an inferred mass flow measurement; it cannot provide a direct mass flow measurement. If we take the example of an insertion thermal mass flow meter, it is operated by heating two or more probes that are inserted into a pipe with the media flowing through it. The media flow pulls heat away from one or more of the probes, the difference in probe temperature is measured by the meter, and the difference is typically used in an equation derived from King's law to determine the media's mass flow rate. A derivation of King's law is the mass flow calculation method used by other types of thermal mass flow meters as well. When using this calculation method to derive mass flow with a thermal meter, the equations typically require:
  • Difference in temperature between probes/sensors
  • Amount of power supplied for heating
  • Cross-sectional area of the flow path
  • Specific information about the media – such as the specific heat. 
  Inferred Mass Flow Measurement – An Economical Choice in Certain Applications Regardless of the mass flow calculation method, an inferred mass flow measurement is an economical choice for applications where the other parameters used in the mass flow equation are fixed or easy to measure.  If we use the example application from the white paper that measures the mass flow of air and fuel gas to improve burner efficiency, thermal mass flow meters are a good choice for an inferred mass flow rate because the pipe's internal area and the specific heat value of each gas would be fixed. Fixing these values greatly simplifies the mass flow equation so that the mass flow rate is proportional to the power supplied for heating – as stated in the white paper. However, it's important to recognize that this inferred mass flow measurement is only accurate while the fixed values remain fixed – thus additional error would be introduced in applications where the gas composition (and thus the specific heat) changes.   When to Invest in Direct Mass Flow Measurement Applications where media parameters change that would be used in an inferred mass flow calculation are a great place to invest in direct mass flow measurement. If the mass flow rate of a gas mixture is needed, changes in the mixture's composition can change the density, viscosity, and specific heat of the gas. This complicates an inferred mass flow calculation, and may require the purchase of additional instrumentation to monitor the parameters that change. Such applications are where direct mass flow measurement shines; as a coriolis flow meter is a single instrument that provides an accurate mass flow measurement while these parameters change. You're welcome to contact my colleagues and me to discuss your mass flow applications in more detail.

About The Author

Brandon Kulp
Applications Engineer

Brandon is applications engineer at Brooks Instrument.

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