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.
about New Technology Flowmeters:
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 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.
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.
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
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
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
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
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.
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.
further information on thermal meters, including detailed market reports,
please see www.flowthermal.com.