How to Measure Air Flow

Most of us don’t think much about the airflow inside our offices or homes unless there's a problem. When there is, the issue is typically either too much air flowing through the space or — more likely — too little. However, airflow matters, even when we’re not thinking about it. Proper flow is necessary to help regulate indoor air quality, especially as pollutants and contaminants move through the interior of the building. Insufficient airflow can lead to overheating and discomfort for everyone inside.

Conversely, even if lots of air is blowing, that can be a problem as it moves more contaminants and can make the space feel too cold. Also, if the system is moving too much air, you're likely wasting energy.

For these reasons, it's important to measure airflow with care and precision. Veris makes it easy with monitoring tools and sensors to keep you up to date at all times. Here are a few guiding pointers on how to measure air flow so that you can keep your office or home in prime condition.

What is Air Flow?

Airflow is the movement of air from one location to another. Air naturally moves in a fluid-like state, transferring from an area with high pressure to one with low pressure.

Pressure isn't the only thing that can affect airflow, though. Other factors that can influence the speed and volume of air movement include:

• Humidity: Humid air is heavier and doesn't move as quickly as dry air.
• Temperature: Hot air is denser, which increases the air pressure, forcing the air to move to a cooler area.

There are also two primary types of airflow - laminar and turbulent. Laminar airflow is smooth and fluid and moves in a single direction. Turbulent airflow occurs when there are obstacles or other elements that can affect the direction of movement. When talking about airflow in a building, the goal is to ensure laminar flow throughout each room for maximum energy efficiency.

Methods of Measuring Air Flow

The best method to accurately measure airflow is with an anemometer. This device has been a standard for many years, and it measures air speed and velocity by using cups attached to a spinning device. The air moves the cups, which are connected to a sensor that reports various data points.

In more modern technology, engineers and building designers use a hot-wire or thermal anemometer system. In this case, a heated wire is exposed to the air, which cools it. The speed at which the wire cools can indicate everything from air temperature to airflow. This system is much more precise and doesn't take up as much space as a traditional anemometer.

Some systems use pitot tubes as well, such as in ducts and stacks. A pitot tube is used to measure the flow of liquid, but it can be connected to sensors also to provide (relatively) accurate measurements of airflow.

How to Calculate Air Flow

Airflow is typically measured in cubic feet per minute (CFM). There are a couple of different ways to calculate CFM air flow, depending on the system being used. Here's a breakdown of those equations and how they work:

Anemometer Calculation

The equation used to calculate air flow using a vane anemometer is:

Airflow (m3/h) = vane area (m2) x air velocity (m/s) x 3600

In this case, the vane area refers to the size of the vane and the different cups. Air velocity is the speed of the air passing through the anemometer.

Pitot Tube Calculation

Since ducts and pipes can move air differently, it's important to know how to calculate airflow in these spaces. First, you will need a differential sensor so you can calculate the air velocity as accurately as possible. Then, it's just a matter of multiplying the velocity by the cross-sectional area of the duct.

If the duct has square edges, you find the area by multiplying the height and width. For round ducts, you find this area by using the formula Pi x radius(squared). The radius is the distance from the center of the circle to the outer edge.

So, the entire calculation looks like this:

Air Flow = Air Velocity x Cross-Sectional Area

Common Mistakes to Avoid When Measuring Air Flow

Because airflow measurements are so vital for different purposes, you need to make sure you're as accurate as possible. So, try to avoid these mistakes when making your calculations:

• Incorrect placement of measurement devices: A sensor is only as good as its placement. If you put it in the wrong spot or orient it incorrectly, the readings will be skewed and incorrect.
• Failure to account for air density: Hot and humid air is much denser than cold, dry air. Consider your climate, both inside and outside.
• Improper calibration of measurement devices: Putting a sensor in the right place is just one step of the process. You must also calibrate it for your specific needs. Otherwise, the readings will be inaccurate.
• Inadequate sample size: Air flow can change from one spot to the next, so you must place sensors in different areas to know how the flow adjusts. If you only pay attention to one small location, you won't get the bigger picture.
• Inadequate preparation: Measuring air flow is important, so treat it as such. Don't rush through the calibration and testing process - remember, accuracy is more important than speed.

Get the Best Air Flow Measurement Products From Veris

Fortunately, measuring your building's airflow is easy when you use Veris airflow sensors. We have a wide selection of products to suit various applications and needs. Our sales team can also walk you through each item so you can find the best option for your business. Contact us today to find out more.