Thermal flowmeters are used almost entirely for gas flow measurement.
They have fast response times and excel at measuring low flowrates. They can also handle some difficult-to-measure flows and provide a direct means of measuring mass flow.
They have difficulty measuring liquid flows,
however, because of the slow response time involved in using the thermal principle on
liquids -- although some companies have released thermal flowmeters for liquid flow measurement.
Thermal flowmeters are not nearly as accurate as Coriolis meters, and typical accuracy levels are in the
1% to 3% range. However, thermal suppliers are working to improve the accuracy.
who in thermal meters
Thermal flowmeters go back to the hot wire anemometers that were used
for airflow measurement in the early 1900s in velocity profile and turbulence
research. Hot wire anemometers
are very small
and fragile, and consist of a heated, thin wire element. Because they are so
small and thin, they have a quick response time, but their
fragility makes them unsuitable for industrial environments.
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.
After FCI, Sierra, and Kurz got thermal flowmeter technology going in the 1970s, a second generation of suppliers appeared in the 1980s and 1990s. Eldridge Products, founded by Mark Eldridge, introduced its first thermal mass flowmeter in 1988. In 1993, Magnetrol brought out a thermal mass flowmeter. Fox Thermal
Instruments, founded by Brad Lesko, was formed in 1994. Then in 2002, Bob Steinberg formed Sage Metering. All these companies were in the Monterey, California area, with the exception of Magnetrol,
which is headquartered in Illinois.
European and Asian suppliers join
While thermal flowmeters may seem to be primarily an American phenomenon, some European companies have since
begun offering them. Chief among these is Endress+Hauser, which is the only thermal company that offers all five types of new-technology flowmeters. Other European companies include E+E Electronik in Austria and VP Instruments in the Netherlands.
Tokyo Keiso of Japan has joined the ranks of thermal flowmeter suppliers, and the technology has also spread to China. So today, even though this technology originated in the United States, it has become a worldwide phenomenon.
about New Technology flowmeters:
thermal flowmeters measure gas flow
introduce heat into the flowstream and use one or more temperature sensors
to measure the rate of heat dissipation
This method works
better for gas flow than for liquids because the much greater heat absorption capacity of liquids rapidly saturates
the signal, leading to a loss of measurement resolution.
While all thermal flowmeters inject
heat into the flowstream, there are two different methods for measuring the
rate of heat dissipation. Both methods make use of the principle that higher velocity flows produce a greater
cooling effect. And both methods measure mass flow based on measuring the amount of cooling that occurs in the
Thermal flowmeters using
constant temperature differential utilize a heated RTD as a velocity sensor and another sensor that
measures the temperature of the gas. The flowmeter attempts to maintain a constant difference in temperature between the two sensors.
It computes mass flowrate based on the amount of electrical power added to heat the velocity sensor to maintain this constant
difference in temperature.
current method also uses a heated RTD as a velocity sensor and another temperature sensor to measure the temperature of the flowstream.
The power to the heated sensor is kept constant. Mass flow is computed based on the difference in temperature
between the heated velocity sensor and the temperature of the flowstream.
applications drive the market
Thermal flowmeters are used for a wide
variety of applications. However, environmental applications,
especially continuous emissions monitoring (CEM) have directly and clearly boosted sales.
In the early 1990s, when
the need for CEMs required measuring sulfur dioxide (SO2) and nitrous oxides
(NOX). Thermal flowmeters were ideal for this purpose. Combining a measurement of the concentration
of SO2 and NOX with a measurement of flowrate can determine how much of these gases are released into
the atmosphere. This is important since they have been identified as the primary
causes of acid rain. At that time, thermal flowmeters competed with averaging Pitot tubes and ultrasonic flowmeters to make this measurement.
The new age of environmental awareness
later spawned the Kyoto Accord (1997), the Paris Agreement (2016), and other greenhouse gas
initiatives -- and resulted in a rewriting of the rules on measuring greenhouse gas emissions.
The Kyoto Accord
resulted in the creation of several mechanisms for measurement of greenhouses gases internationally. These include Certified Emission
Reductions (CER), a credit system designed to help European countries achieve reduced emission targets. A second program,
the Clean Development Mechanism, allows countries to help reduce emissions in developing countries
by investing in sustainable development programs.
There is now a need and a demand to measure greenhouse gases in applications that formerly may have gone unnoticed.
And this is definitely good news for thermal meters.
further information on thermal meters, including detailed market reports,
please see www.flowthermal.com.