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Mass Flow Controller Study

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The roots of thermal flowmeters go back to the hot wire anemometers that were used for airflow measurement in the early 1900s.  Hot wire anemometers were used in velocity profile and turbulence research.  They are very small and fragile, and consist of a heated, thin wire element.  Hot wire anemometers have a quick response time, because they are so small and thin.  However, their fragility makes them unsuitable for industrial environments.

Thermal flowmeters were first introduced for industrial applications in the 1970s.  The story of how they came on the market is a fascinating one that involves Sierra Instruments, Fluid Components International (FCI), and Kurz Instruments.  Sierra Instruments and Kurz approached the subject through hot wire anemometers.  FCI approached the subject through flow switches.  All three companies were pioneers in the development of thermal flowmeters, and all three companies still offer thermal flowmeters today. 

Thermal flowmeters make use of heat in making their flow measurements.  Thermal flowmeters put heat into the flowstream and use one or more temperature sensors to measure how quickly this heat dissipates.  Heat dissipation is measured in several different ways.

One method of measuring heat dissipation keeps a heated sensor at a constant temperature and measures how much current is needed to keep it at that temperature.  Another method measures the temperature difference between the flowstream temperature and a heated sensor.  What is common to both methods is the idea that higher speed flow results in increased cooling.  Both methods measure the effects of this increased cooling, and compute mass flow based on this result.

Advantages and Limitations

Thermal flowmeters have fast response time, and they excel at measuring flow at low flowrates.  They also provide a direct means of measuring mass flow.  They can also handle some difficult-to-measure flows.  Insertion thermal flowmeters are used in Continuous Emissions Monitoring (CEM) applications to help measure the amount of sulfur dioxide and nitrous oxide being released into the environment.  Concentration measurements, along with flowrate measurements, are required.

One limitation of thermal flowmeters is that they are used almost entirely for gas flow measurement.  Thermal flowmeters have difficult in measuring liquid flows because of the slow response time involved in using the thermal principle on liquids.  Some companies have released thermal flowmeters for liquid flow measurement, however. 

A second limitation is in their accuracy.  Thermal flowmeters are not nearly as accurate as Coriolis meters, and typical accuracy levels are in the one percent to three percent range.  However, thermal suppliers are working to improve the accuracy of their flowmeters.  Expect wider use of thermal flowmeters as their accuracy levels increase.

Mass Flow Controllers

Another type of thermal flowmeter is called a mass flow controller.  Most mass flow controllers use thermal principles to determine mass flow, although some use a pressure-based measurement.  Mass flow controllers contain an integrated control valve that is used to control the flow as well as measure it.

The mass flow controller market is far larger than the thermal flowmeter market.  A major portion of mass flow controllers are used in the semiconductor industry for measuring gas flow.  However, a group of companies also sells mass flow controllers for industrial markets.  The mass flow controller market is highly competitive, with a large number of suppliers.  It is one of the most rapidly developing markets in the flowmeter world today.

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