David W. Spitzer

Which of the following flowmeters cannot be used to measure the flow of gas in a stack?

A. Differential pressure
B. Magnetic
C. Thermal
D. Ultrasonic

Commentary
Differential-pressure flowmeters, such as averaging Pitot tubes, can be used to measure stack flow and are often used for official stack testing. Thermal flowmeters and ultrasonic flowmeters have also been developed for stack flow measurements. Magnetic flowmeters are for liquids only and cannot be used in this application. Therefore, Answer B is the correct answer.

Additional Complicating Factors
Selecting the best stack gas flowmeter is dependent upon a number of factors to include the gas temperature, ambient temperature, flowmeter location, available straight run, amount of particulates in the gas, moisture content, and the chemical composition of the gas.

For example, high levels of particulate in the gas can cause the ports in a differential-pressure flowmeter to plug, coat thermal sensors, and obstruct the beam in an ultrasonic flowmeter. High moisture content in conjunction with some particulates can similarly coat sensors and plug ports. Locating an averaging Pitot tube flow transmitter outdoors with exposure to sun, rain and snow can cause it to be difficult to calibrate and subject to drift as ambient conditions change from day-to-night and summer-to-winter. Single-point stack flowmeters can be affected by poor velocity profile when sufficient straight run is not available.

In the end, flowmeter selection for this application is not as straightforward as it may appear.

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.