RSES Journal Winter 2025, RETA-RSES

Flow/Correction Factors for a 14 in. x 6 in. duct-mounted supply grille

Tech

Instrument

Grille Size

Vel-ft/min

cfm

Ff-Cf-Kv

Cowden

Rotating Vane

14x6

662

226

0.34

Shoesmith

Rotating Vane

14x6

666

226

0.34

Cowden

Velocity Grid

14x6

647

226

0.35

Fowler

Velocity Grid

14x6

629

226

0.36

Pitot tube duct traverse measured 201 cfm Ff – Cf – Kv = 201 CFM/Hood cfm

Example: Pitot tube duct traverse measured 226 cfm. Ff-Cf-Kv = 226 CFM / Vel – ft/min

Manufacturer’s Data: Nominal Duct Area=0.58 ft² 45° Deflection Area Correction (AC)=0.63 Actual Grill Free Area = 0.37

The Flow Hood The flow hood may require the develop ment and use of correction factors when used on swirl diffusers, or on other types of diffusers with uneven air throw. The flow hood may not be appropriate for use on small supply outlets at high jet velocities or “nozzle” type outlets. These outlets cause an extreme concentration of air velocity on portions of the flow sensing grid. The flow hood readings may be inaccurate under such conditions. Consideration must be given to other system components, such as some single supply air outlet applications, where the flow hood’s slight backpressure may directly affect fan performance. Source: Shortridge Instruments, Inc. Operating Instructions Conclusion Always select the proper instrument. Expe rience, knowledge and “daily use” of the

National Standards 7th Edition Flow/ Correction Factors

instruments allows for the best evaluation of the data obtained. Do not get a number just to report a number. Develop the proper “Ff-Cv-Kv,” and note in the report how it was developed. Jim Hall is the former Owner/President of Systems Management & Balancing, Inc. Though retired, he provides consulting as requested. Copyright Materials This presentation is protected by US and International Copyright laws. Reproduction, distribution, display and use of the presentation without writ ten permission of the speaker is prohibited. © Systems Management & Balancing, Inc. 2018 AMCA International, 30 West University Drive, Arlington Heights, IL 60004, USA. Phone: 847-394-0150; Email: communica tions@amca.org; online: www.amca.org.

1. Take a 60-second reading at the face of the opening. For larger areas, it is recommended to take several readings at various locations along the opening and averaging these reading. 2. Apply the correction factor to your reading from the supplied sticker. 3. Multiply your connected ft/min anemometer reading by the square footage of the opening to obtain your CFM reading. For testing open air spaces, it is recommended to divide the cross section into square foot areas, taking a separate reading for each area. Aver age all these anemometer readings to obtain a single reading for the open space.

WINTER 2025 RSES Journal 17

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