Low-cost Ultrasonic Anemoter for Use indoors and in HVAC Ducts Prototype Development and Testing

Air systems that provide heating, cooling, and fresh air consume a third or more of the total energy used for heating, ventilation, and air-conditioning (HVAC) in commercial buildings. Much of this energy is wasted due to lack of accurate air flow measurement. Measurement uncertainty causes operators to maintain higher-than-necessary minimum flow rates that overcool and overheat buildings, as well as increasing the use of fan energy. 

To reduce energy consumption in buildings, the project developed low cost anemometers (airflow sensors) to measure air flow. The team successfully developed and tested two different prototypes of the air flow sensors: an anemometer that measures air flow and direction in rooms, and an anemometer that measures the air flow within HVAC systems, such as ducts. The anemometers use Chirp Microsystems’ tiny ultrasonic CH-101 sensors, which are disruptively inexpensive and sophisticated. A series of sound pulses is sent in both directions between each pair of sensors. The difference in the transit time, or time-of-flight, between each pulse determines the air flow velocity, and also its temperature. The research team tried several approaches to measuring the time-of-flight accurately and reliably. The current algorithm tracks the phase angle within a single defined wave cycle within the sonic pulse, using a simultaneous independent temperature measurement to keep its measurements within the wave cycle. 

The current room anemometer measures air speeds from less than 0.05 meters per second (10 feet per minute) to a maximum speed of 7 meters per second (1400 feet per minute). It measures wind direction within an accuracy of ±5 degrees. 

An application where the duct anemometer could greatly reduce energy consumption is measuring the airflow and temperature in variable air volume boxes and terminal units, especially directly downstream of reheat coils. Another application is in testing and balancing. The anemometer has potential to optimize cooling performance in data centers and controlling airborne diseases in hospitals.