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How to measure air flow on air ducts

To ensure good indoor air quality and ideal air distribution within a residential ventilation system, it is important that the volume flows at air vents are set correctly. The growing awareness of good indoor air quality is manifested, for example, in the IDPH Guidelines for Indoor Air Quality. From time to time, it is therefore advisable to check the air flows by means of measurement. There are various methods that can be used here, such as funnel measurement with a large vane manometer or hot wire anemometer.



With a large vane anemometer, the vanes are set in rotation by the air flow. The speed of rotation allows conclusions to be drawn about air velocity and flow volume.

The hot wire anemometer, on the other hand, determines the air velocity by measuring the temperature of the hot wires in the air stream. The volume flow of air is then determined from this. Here, the advantage is that the air jams less in the funnel than in a large vane anemometer. The air can flow more freely and a more accurate result is obtained.

When using a measuring funnel, it is important to ensure that the probe is exactly in the center of the funnel when performing the measurement. This has a great influence on the measurement accuracy. It is also important to cover the complete air register with the funnel, otherwise an accurate result is not guaranteed and the adjustment work on the ventilation is more difficult.

To perform the volume flow measurement, the funnel is placed exactly above the air diffuser. The instrument now determines the air flow volume depending on the area of the measuring funnel and the flow velocity. The volume flow is then indicated in CFM (cubic feet per minute) and shown on the display of the anemometer. In addition, most instruments have the option of displaying the flow velocity and air temperature. These results can now be compared with the design data of the ventilation system and optimized by adjustment work. If needed, a total volume flow measurement should also be carried out at the ventilation main unit. This can easily be done with the Wohler DC 410 FLOW, for example.

While measuring the air volume flow, it makes sense to determine the Indoor Air Quality at the same time. With the Wohler FA 430, the CO2 content, temperature and relative humidity of the outflowing air can also be determined simultaneously within the funnel measurement. This guarantees an optimal indoor climate and healthy ambient air.

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