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4.2 OAMT Performance

The designs of commercially available outdoor airflow measurement technologies (OAMT) are highly variable. Most OAMTs are marketed as products that the user adds to their existing HVAC but some are provided only as an add-on component of a packaged unit. 
Some OAMTs consist of electronic velocity probes with multiple sensors that are installed, as specified by the manufacturer, downstream of an outdoor air intake louver or inside an outdoor air intake hood. One OAMT consists of an outdoor air intake louver with integrated air velocity probes installed at the outlet of louver blades. 

Other OAMT measures the pressure drop as air flows through a louver. One OAMT uses air speed probes installed on the outdoor air damper and tracks the damper position.
The data available on accuracy of commercially available OAMTs, independent of the performance specifications of manufacturers, are highly limited [18-21]. The available data indicate that measurement accuracy can vary highly depending on how the system is used. For example, accuracy can depend on the type of upstream outdoor air intake louver or hood. Accuracy can also depend on the average air velocity at the measurement location.

The following summarizes test results of OAMTs:
  • An OAMT that used a tubing manifold to channel air through an electronic velocity sensor downstream had errors exceeding 30% with an average air speed exceeding 0.8 m/s (160 fpm) [21]. When the same OAMT was installed downstream of a horizontal blade louver, the measurement error was approximately 5% with an average air speed greater than 0.8 m/s (160 fpm). But the error increased to 40% as the average air speed was decreased to 0.5 m/s (100 fpm). 
  • An OAMT that was consisted of probes with multiple electronic velocity sensors was evaluated when installed in air intake hoods. Measurement errors varied from a few % to greater than 100%. Measurement errors varied with the design of the air intake hood and the average air speed. With average air speed less than 0.5 m/s (100 fpm), measurements errors were approximately 50% or higher. 
  • An OAMT with air velocity probes integrated with vertical-blade louver was generally accurate within +/-15% with high air speeds when a research grade transducer was used to measure the systems pressure signal. However, measurement errors were as high as 100% at the lower air speeds that would be encountered when minimum outdoor air is supplied by an AHU with economizer. These large errors are results of the very small pressure signals measured when minimum outdoor air is being provided.
  • An OAMT consisting of pressure probes installed upstream and downstream of outdoor air intake louvers was evaluated with three louver designs. Airflow rates were based on the measured pressure drops across the louvers and the louver manufacturer's data on pressure drop versus airflow rate. At the maximum recommended airflow rates of the louvers, errors ranged between 20% and 30%. However, measurement errors were as high as 200% with the low air speeds that would be encountered when minimum outdoor air is supplied by an AHU with economizer. These errors varied with the louver type. 
  • An OAMT was tested using honeycomb airflow straighteners placed upstream of a set of air velocity probes that are connected to a pressure transducer. When installed downstream of a vertical blade louver, measurement errors were less than 10% at moderate to high air speeds. However, measurement errors were again greater than 100% when minimum outdoor air is supplied by an AHU with economizer. 
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