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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.

While all thermal flowmeters use heat to make their flow measurements, there are two different methods for measuring how much heat is dissipated. One method is called the “constant temperature differential” method. Thermal flowmeters using this method have two temperature sensors: a heated sensor, and another sensor that measures the temperature of the gas. Mass flowrate is computed based on the amount of electrical power required to maintain a constant difference in temperature between the two temperature sensors.

A second method is called a “constant current” method. Thermal flowmeters using this method also have a heated sensor and another one that senses the temperature of the flowstream. The power to the heated sensor is kept constant. Mass flow is measured as a function of the difference between the temperature of the heated sensor and the temperature of the flowstream. Both methods rely on the idea that greater cooling results from higher velocity flows. Both measure mass flow based on the measured effects of cooling in the flowstream.

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.