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The World Market for Thermal Flowmeters - Coming in September 2009!
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 LimitationsThermal
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 ControllersAnother
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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