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's 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.  

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.   

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 in

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. 

Learn more about New Technology flowmeters:

How thermal flowmeters measure gas flow

Thermal flowmeters 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 flowstream.

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.

The constant 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. 

Environmental 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.

For further information on thermal meters, including detailed market reports, please see www.flowthermal.com.