The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc. (now Eastman Chemical). Greg will be posting questions and responses from the ISA Mentor Program, with contributions from program participants.

In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Adrian Taylor.

Thermal mass flowmeters can be a relatively inexpensive flowmeter that can handle extremely small flows (e.g., inline 1/16-inch meters) and large flows (e.g., multipoint insertion type in 60 inch ducts). Thermal flowmeters introduce heat into the flow stream and measure how much heat is absorbed using one or more temperature sensors.

However, the meter requires a fixed specific heat capacity, no heat loss or gain from ambient conditions, a fixed composition, predictable heat distribution, no change in phase and no change in heat transfer coefficient unrelated to velocity (e.g., surfaces must be clean and dry). Thermal mass flowmeters are most frequently used for gas flow since heat absorption in liquids and solids can be problematic. The greatest success is seen with properly installed inline meters measuring single component gas flows in a very controlled environment.

The flow measurement uses two temperature sensors, one being heated by an electrical current. Flow is inferred either from the temperature rise for a constant current or from the amount of current needed to maintain a constant temperature differential. Total shipments of thermal mass flowmeters have been increasing by about $5 million per year to become about 2 percent of the total worldwide market for all types of flowmeters.

Adrian Taylor's Questions

1. We have some installed on nitrogen lines to reactor inserts, these give poor agreement and we have a current project to change these out for Coriolis meters (although I suspect we are suffering the effects insufficient upstream piping diameters on these particular meters).
2. We also have some thermal mass flow switches on suction lines from some sample cabinets up to our hygiene vacuum system. The duty is air most of the time from a vent to the sample cabinet but following a sample there are also HCl vapors. The switch is used in an interlock to prevent the cabinet door being opened with insufficient vacuum for fumes from the sample. At the time of installation the switches worked perfectly but we’ve found over time there is a shift in the flow/voltage curve which has to be adjusted for. The elements are Hastelloy, which I think should be fine for the occasional ambient temperature HCl vapors. All I can think is the shift is due to either some kind of coating forming, or due to droplets forming on the element from the occasional vapors.
3. Finally as part of a turnkey burner replacement project we have just installed an array of thermal mass elements in a square combustion air duct on our site. This is intended to work in a similar way to a pitot array but for average mass flow rather than volume. The system has only recently been commissioned so I can’t comment on performance yet.

Hunter Vegas' Answers

Rule #1 (and it is a huge one): If you are trying to use a thermal flow meter on air/gas/vapors never install it in a service where it can ever see a gas/vapor approaching dew point.  Even a tiny bit of liquid will create erratic flow readings.

Rule #2: Most thermal flow meters do not promise high accuracy particularly if they are the insertion type. If you need highly accurate flow readings you should probably investigate a different meter type. However they do work well as a flow switch or a go/no go type of flow application like purges and the like.

1. I’m a little surprised you had trouble on nitrogen. Normally dry nitrogen is a good application, but you do need to have a decent meter run to get accurate results.
2. Can’t really speak to the sample cabinet meters, but if you are getting vapors that condense the reading will be off.  However it should recover if you are just flowing air unless you are getting some kind of a coating as you say. In that case it would definitely impact the meter. Realize too that different gasses will require different calibrations so if you have a mix of gases your accuracy will be impacted.
3. The array meter will only work if you are downstream of the economizers and have hot air coming through that is well above the dew point.  If the meters are in the suction duct where they’ll see outside air at ambient temperature they’ll work great until it is foggy or rainy and then the readings will likely turn unstable.

ISA Mentor Program

The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about the ISA Mentor Program.

Greg McMillan’s Comments

I have seen thermal mass flowmeters extensively used in laboratories and pilot plants to measure air, oxygen and carbon dioxide flows for bioreactors. As you can imagine, the gas and ambient conditions are exceptionally controlled, which may explain their success. For more on the physical principles and practical considerations as to selection and installation of thermal mass flowmeters and all other types of measurements see my ISA book Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance.

Additional Mentor Program Resources

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine feature article Enabling new automation engineers for candid comments from some of the original program participants. See the Control Talk column How to effectively get engineering knowledge with the ISA Mentor Program protégée Keneisha Williams on the challenges faced by young engineers today, and the column How to succeed at career and project migration with protégé Bill Thomas on how to make the most out of yourself and your project. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Mark Darby (principal consultant at CMiD Solutions), Brian Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (director of global project engineering at the Midwest Engineering Center of Emerson Automation Solutions), Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont), Bart Propst (process control leader for the Ascend Performance Materials Chocolate Bayou plant), Angela Valdes (automation manager of the Toronto office for SNC-Lavalin), and Daniel Warren (senior instrumentation/electrical specialist at D.M.W. Instrumentation Consulting Services, Ltd.).

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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