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Spotlight on Thermal Flowmeters
By Jesse Yoder, Editor
Should thermal flowmeters be considered in the camp of new-tech meters or are they more of a traditional technology? New-technology flowmeters include Coriolis, magnetic, ultrasonic, vortex, and multivariable differential pressure (DP). They meet the following criteria:
1. They have been introduced since 1950.
2. They incorporate technological advances that avoid some of the problems with earlier flowmeters.
3. They are more the focus of new product development efforts by the major flowmeter suppliers than traditional technology meters.
4. Their performance, including criteria such as accuracy, is at a higher level than that of traditional technology flowmeters.
Thermal flowmeters satisfy Criterion 1. While hot wire anemometers go back at least to the 1920s, thermal flowmeters were not developed out of this technology until the 1960s. Since 1950 is the dividing line between new-tech and traditional tech meters, they are more of a new-tech meter in terms of date of introduction.
Thermal flowmeters are on the way towards satisfying Criterion 2. Certainly thermal flowmeter suppliers have introduced product improvements, and they will continue to do so. While thermal flowmeters have not yet been developed to the level of sophistication as ultrasonic and Coriolis meters, they are moving in this direction.
In terms of Criterion 3, thermal flowmeters are offered by some of the major flowmeter suppliers, including Endress+Hauser, ABB, and Yamatake. While Emerson Rosemount does not offer thermal flowmeters, Emerson Brooks offers mass flow controllers. Of the other main flowmeter suppliers, Yokogawa, Foxboro, and Krohne do not offer thermal flowmeters.
While not all the major flowmeter suppliers offer thermal flowmeters, there is a great deal of product development going on among the existing suppliers of thermal meters. This is in fact one of the hottest areas of product development among traditional technology meters. So thermal flowmeters mostly satisfy Criterion 3.
The main area where thermal flowmeters fall short is in Criterion 4: level of performance. The accuracy of many thermal flowmeters is in the 1 – 3 percent range. While this is not explicitly stated in the criterion, new-technology flowmeters typically have accuracies that are better than one percent. Even though thermal flowmeter suppliers are offering products that have improved accuracy, many thermal flowmeters still fall short in this area. Of course, they have compensating advantages, such as low cost and little or no pressure drop, but many thermal flowmeters still do not perform at the same level as new-technology meters.
Multipoint Thermal: A New-Technology Flowmeter?
One reason why thermal flowmeters have difficulty in achieving high accuracy levels is because they typically measure flow at a point. Flowrate in a closed pipe typically is faster in the center of a pipe than at the edges, so it is significant where the flow in a pipe is measured. Some thermal flowmeter suppliers have responded by creating multipoint thermal flowmeters that take measurements at multiple locations in the pipe. This results in a more accurate reading.
Should multipoint thermal flowmeters be considered a type of new-technology flowmeter, with their enhanced accuracies? Consider the parallel case of multivariable differential pressure (DP) meters. Probably no technology is more traditional than DP flow technology. Yet multivariable DP meters are a type of new-technology meter because they have been developed recently, their accuracy levels are higher in many cases, and because they incorporate recent technology advances. Why aren’t multipoint thermal flowmeters just like multivariable DP meters?
Based on this analogy of multivariable DP meters, there is a strong case for considering multipoint thermal flowmeters to be a type of new-tech meter. Some thermal flowmeter suppliers also say that some of their flowmeters achieve accuracy levels that are better than one percent. If these statements can be independently verified, then at some some subset of thermal flowmeters would be in the new-technology camp.
We welcome any comments on this from thermal suppliers or anyone else who is interested. Please forward your comments to jesse@flowresearch.com. And watch for a special article in Control magazine for October 2003, where this issue will be discussed.