Building Air Flow Measurement, How to measure building air flow

Building air flow measurement speaks to how to measure building air flow and calculate air flow. Air flow detectors measure air in buildings using various detector instrumentation. These detectors also known air flow monitor stations are permanently mounted in the air system ducts. The calibration of these air flow monitors must be checked for accuracy at least once per year.

When a facility makes use of a Building Automation System it is of absolute importance that the signals the automation system receives from the air flow monitor stations are correct. It is these feedback signals that allow the automation system to monitor and control within acceptable tolerances.

Annually portable calibrated test equipment is used to measure air flow close to the various air monitor stations. These measured and calculated values are compared to the automation system indicated values. If the automation indications are incorrect, corrections are made at the automation system to allow it to function correctly and within calibration.

Air Flow Detectors Calculations

Calculations are done using simple tests and simple calculations.

Below listed please find a brief description of the instruments and techniques used.

Duct Traverse Method

To measure air volume in a duct the traverse method is used, using a thermal (hot wire) anemometer velocity measurement instrument.

A straight run of duct must be located with at least 10 ft on each side of the test holes with no turns. If rectangular, 3/8" test holes are drilled usually in the bottom or side. If round, test holes are drilled in two axis, at a right angle to each other.

A number of air velocity test readings are taken over the cross sectional area and an average air velocity in feet per minute (fpm) is determined. This average velocity in fpm is multiplied by the cross sectional area in square feet, which indicates the the cubic ft per minute (cfm) of air.

Results are given in cubic ft per minute, or liters per sec if using the metric system. 2.12 cfm equals 1 L/s air flow and 197 fpm air velocity equals 1 meter per second air velocity.

Thermal anemometers are available in different ranges, but the most useful for air velocity measurement is the 0 to 10,000 fpm range.

Oil filled inclined gauge manometers with pitot tubes were previously used for these measurements but this technology is now considered out of date.

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