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4.3 Selection and Use of OAMT

The limited performance data on Outdoor Air Measurement Technologies (OAMTs) indicates that measurement accuracy depends on the characteristics of the outdoor air intake louvers and hoods. Measurement accuracy also depends highly on the average air speeds. 

Measurement of outdoor air intake rates are most needed to determine compliance with the minimum ventilation requirements of codes when minimum rates of outdoor air are being supplied. However, with the low air speeds encountered when minimum outdoor air supply is being provided by AHUs with economizers, all of the evaluated OAMTs are expected to have poor accuracy, e.g., errors greater than 50%, under at least some test conditions. Low air speeds and high spatial variability in air speed and direction are sources of measurement errors. 

Fisk et al. [18] suggest the following approaches for reducing measurement errors. 
  • Select a louver with high value of maximum recommended air velocity.
  • Determine OA flow rates from measurements of air velocities at locations between the blades of louvers, which can exceed the velocities downstream of louvers by a factor of three. 
  • Select a highly sensitive and accurate pressure transducer for use in conjunction with the OAMT. To maximize accuracy, the full-scale pressure range of the transducer should be no greater than necessary. Regular calibrations of the pressure transducer may be needed to assure accurate measurements of OA flow rates. 
  • Consider electronic velocity sensors in place of pressure-based velocity sensors because of their potential to remain accurate at lower air velocities. 
  • Consider a redesign of OA intake louvers and dampers for HVAC systems with economizer. For example, the AHU can have two sets of OA intake louvers and dampers installed in parallel. One louver for the minimum outdoor air supply may have a damper that remains open at all times. This louver should be sized to have the maximum recommended air velocity in the louver when the AHU is providing minimum outdoor air. An OAMT is used in this portion of the OA intake system. The second louver and damper is closed when minimum OA is supplied by the AHU. This second damper opens as needed when the economizer controls call for more than the minimum OA. A second OAMT may be used in this second section if desired to measure the total OA. This approach is not new - separate OA intake sections are already often used in large AHUs, but they do increase costs. 
Prototype OAMTs designed taking these findings int account have had good accuracy [22, 23]. However, these prototypes are not available commercially.