The Many Useful Benefits of Airflow Measurement

Jeff Yirakby Jeff Yirak, P.E. CPMP LEED AP BD+C, O+M

Summer is a great time to take a vacation. I recently had the pleasure of visiting Jewel Cave National Monument in South Dakota. My first point of advice is to arrive early; we waited about two hours for the next available tour. Once inside, we had a pleasant walk with an informative guide. The cave formations were interesting; the cave continues to evolve every day, although obviously very slowly. The soda straws pictured here are evidence of this.

01The guide mostly discussed the formations we were walking through, but one thing she said caught my attention. Only about 5% of the cave has been mapped! Jewel cave is technically considered to be “big” by experts.

02How do these “experts” know that only 5% of the cave has been mapped? How can they simultaneously know and not know how big it is? The answer is: science. Since 2003, cave climatologists have been measuring the airflow in and out of Jewel Cave (and the cave next door, Wind Cave). Specialized devices called ultrasonic anemometers measure the volume of air that passes in and out of the cave as it “breathes,” in response to changing air temperature and atmospheric pressure conditions. Using these measurements, the scientists can calculate the volume of the cave chamber, leading them to believe we’ve only mapped about 5% of the space. You can hear more here as cave specialist Mike Wiles discusses the airflow study.

Airflow Measurement: Not Just for Caves

You, however, probably don’t live in a cave. We use similar instrumentation to measure the airflow into and out of buildings. The HVAC industry uses hot-wire anemometers instead of ultrasonic anemometers. These are similar to what’s found in most modern automobiles. Ultrasonic anemometers have an advantage in an outdoor environment, where dirt or other contaminates may damage the elements of the hot-wire anemometer. Hot-wire anemometers may have an advantage in sensitive measurement applications or in ductwork where elbows or other flow disturbances are present.

Tamco Ebtron’s airflow measurement device is an example of hotwire “thermal dispersion” technology.

Tamco Ebtron’s airflow measurement device is an example of hotwire “thermal dispersion” technology.

Measuring the volume of air entering a building is very useful. For example, engineers can program the system to maintain an outdoor airflow set point based on real-time factors, such as indoor population or measured indoor air quality. This has huge benefits to energy consumption versus a static (non-variable) minimum outdoor air quantity based on some assumed worst-case scenario.

Imagine that it’s 20 degrees F outside in the middle of a winter blizzard, and an office building is only half-populated because nobody can get to work. On-line measurement of occupancy or indoor air quality will minimize the amount of frigid outdoor air being brought into the building, which must then be conditioned (heated and humidified), to serve the actual needs of the building occupants. This is much more sophisticated than simply bringing in the same amount of outdoor air regardless of indoor or outdoor conditions.

Airflow measurement, whether for cave exploration or building indoor air quality control, relies on accurate measurement. I hope this discussion serves as a “tour guide” to anemometers for you.

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