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Why Accurate Outdoor Air Flow Measurement Can Be Difficult

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HVAC systems use limited amounts of outside air to refresh the air inside commercial high-performance buildings. Accurately measuring the amount of outdoor air being pumped into a building is critical for many reasons, including:

  • Maintaining the proper pressurization throughout the building
  • Avoiding excess costs due to waste of conditioned air 
  • Preventing overuse of fans which increases maintenance and replacement cost
  • Preventing ingress of humidity, dust, debris, and pollutants that can degrade the IAQ
  • Meeting environmental regulations for energy efficiency & LEED certification
  • ASHRAE Standards such as 62.1 for ventilation rates and 90.1 for high performance DOAS units.
accurate outdoor hvac airflow measurement is essential for high-performance buildings

However, accurate outdoor HVAC airflow measurement can be difficult due to weather and local climate conditions, as well as the design of the air inlets. 

Climate and Weather Conditions

Outside air pumped into an HVAC system from the rooftop of a building can vary widely in temperature, moisture level, and particulate load. On top of that, the speed and direction of the wind and the barometric pressure can fluctuate from one hour to the next. All these factors can affect the accuracy of an outdoor air flow measurement system.

Duct Design Difficulties

Accurate measurement is made more difficult by the design of some outdoor air inlets, which usually have limited straight runs available to condition the airflow prior to measurement. Some systems require a certain number of upstream straight runs of the duct to reduce flow turbulence before taking measurements.

In addition to the lack of straight duct runs, HVAC inlets are often retrofitted with an economizer inlet to help meet the building’s energy efficiency goals. The addition of an economizer means that the inlet is oversized for maintaining indoor air quality. The result is very low flow velocities at the inlet most of the time, which directly affects measurement accuracy and reliability.

Rooftop packaged units are a worst-case example, as the minimum outside air and the 100% outside air economizer functions are forced into a single inlet. To make matters worse, some manufacturers combine several unit sizes into a single box/frame/control damper/hood size.

Outdoor Airflow Measurement Technologies 

The three most often used technologies for measuring outside air are thermal dispersion, Pitot array, and pressure difference across a known resistance. In this post, we will briefly look at each technology to determine the best fit for measuring outdoor air flow.

Thermal dispersion 

To get accurate air flow measurements, thermal dispersion systems typically require 2 to 8 diameters of straight duct runs both upstream and downstream – something that’s rarely available in outside air intake duct configurations. Also, thermal sensors do not tolerate any form of moisture in the air and they perform poorly in dynamic and turbulent airflow conditions, making them a poor choice for accurate outdoor hvac airflow measurement.

Pitot array technology

In this type of air flow measurement system, dynamic pressure and static pressure are measured at many points throughout the duct, and the results are averaged. The velocity of airflow is proportional to the difference in pressure, so a differential pressure (DP) transmitter is used to convert the difference into an average airflow velocity. The transmitter then converts average velocity into average airflow volume using the cross-sectional area of the duct. 

The performance and price level of airflow measurement systems that use the Pitot measurement principle can vary widely. These systems have two types of components: Pitot probes and the DP transmitter. System accuracy depends on the number of Pitot tubes required, the quality of the transmitter, and to some degree, the local air quality. Air inlets with limited straight runs available to condition the airflow require large numbers of Pitot tubes to achieve accurate flow measurements, which could make them cost-prohibitive. 

Pressure difference across a known resistance

Yet a third technology uses small differences in static field pressures created by air moving through a fixed inlet to accurately measure airflow without the need for straight ducts to condition flow. The concept is based on measuring static pressure fields that develop when air flows through any screen or louver with a known velocity vs. pressure drop curve without a significant change in the pressure drop. 

These static pressure measurements used in concert with temperature, barometric pressure measurements, and inlet size can be used to calculate the flow rate at any atmospheric condition. This type of system is nearly impervious to dust and moisture, has a high tolerance for dynamic variations in air flow velocity and direction, and requires zero straight runs of duct. 

An Accurate Solution for Airflow Measurement in Outside Air Applications 

Air Monitor’s OAM II Outdoor Airflow Measurement System uses static pressure field measurements to provide accurate airflow monitoring in this challenging application. The system consists of the OAM II Transmitter with a built-in barometric pressure sensor, one or more uni-sensors or OAM II airflow measuring stations, and one or two temperature sensors. The system has been designed to measure the actual airflow rate to within ±5% by measuring differential pressure across a fixed inlet, such as a louver, screen, or layer of expanded metal.

Precision in outside airflow measurement is essential to ensure that ASHRAE ventilation standards are met, energy consumption calculations are accurate, and proper indoor air quality is maintained for occupant health and safety. The OAM II System allows you to fine-tune the amount of outside air flow to maintain pressurization, meet energy efficiency goals, and keep operating costs low. 

Click below to learn more.

OAM II Outdoor Airflow Measurement Systems

Air Monitor to Exhibit at Upcoming IDEA Annual Conference

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We are looking forward to exhibiting alongside ONICON Incorporated at IDEA 2023, the International District Energy Association’s Annual Conference and Trade Show taking place this June with the theme of “Empowering the Next Generation.” As a leading provider of innovative airflow measurement solutions, we’re excited to showcase our products and services to decision-makers, engineers, and other industry professionals.

At our booth, you’ll have the opportunity to learn about our product offerings, including our OAM II Outdoor Airflow Measurement System, the ideal solution for tough outside airflow applications. Our team of experts will be available throughout the show to answer your questions and help you explore how our offerings can be tailored to meet your specific needs. But IDEA2023 is not just about us. It’s also about empowering the next generation of leaders and innovators in the district energy industry. We’re committed to supporting the growth and development of the industry by investing in education and training programs, and by providing opportunities for young professionals to gain experience and expertise.

We believe that empowering the next generation is critical to the success and sustainability of the district energy industry. That’s why we’re looking forward to engaging in meaningful conversations and collaborations with other industry leaders to drive innovation, progress, and positive change. So, if you’re attending IDEA2023, be sure to visit us at booth 152. We’re confident that our innovative solutions and commitment to empowering the next generation will make a positive impact on your operations and on the future of the industry. See you there!

Air Monitor Exhibits at CxEnergy 2023

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Sales Application Engineer Trang LeTran and HVAC Marketing Manager Emilie Brun in Our Booth at the Show.

We had a great time last week sponsoring and exhibiting alongside ONICON Incorporated at the Annual CxEnergy show in Fort Worth, TX where we showcased our airflow measurement solutions to building commissioning, energy management, and testing professionals from all over the country.

Our team of experts was on hand to answer questions, provide demonstrations, and help attendees explore how our solutions can be customized to meet their specific needs. We had the chance to engage in meaningful conversations and collaborations with other professionals, sharing insights and ideas about the latest trends and developments in the industry. The agenda also included 30+ educational sessions

The CxEnergy show is an important event for us as it allows us to demonstrate our commitment to accuracy, reliability, and customer satisfaction. We were proud to be a bronze sponsor of the event this year and thank Associated Air Balance Council, AABC Commissioning Group, and Energy Management Association for putting on a great show.

Thank you to everyone who stopped by our booth at the show and if you missed us at CxEnergy, don’t worry! You can still learn more about our solutions and services here on our website or by contacting our team directly. We’re always happy to answer your questions and help you find the right solution for your needs.

A Comparison of Outside Airflow Measurement Technologies

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Are you using the right product for this challenging application?

The Need for Accurate Outside Airflow Measurement in HVAC

Outside air is used to refresh the air inside modern buildings. Measuring outside air entering the building is a key factor in maintaining proper indoor air quality for occupant health and safety. It is also essential for meeting energy-savings goals and ASHRAE air quality standards required by today’s high-performance buildings. As a result, building automation systems (BAS) must take outside airflow data into account to meet ventilation standards, accurately measure energy consumption, and maintain proper indoor air quality. When building automation systems rely on technologies not specifically developed for this application, it becomes a challenge to reliably measure airflow. This can lead to increased operating costs, inadequate building pressure control, and uncomfortable or unsafe living and working conditions.

HVAC equipment maintaining proper indoor air quality inside the building.

When building automation systems rely on technologies not specifically developed for this application, it becomes a challenge to reliably measure airflow.

Measurement of Outside Airflow is Difficult

Outside airflow is difficult to measure. Conditions on rooftops can be extremely turbulent, with constantly changing wind velocities and directions. This is in addition to temperature, humidity, and barometric pressure variations that can affect density. Outside air is also unfiltered and may contain moisture (fog, rain, sleet, or snow) and particulate matter. All this is made more difficult by the design of outside air inlets. These inlets do not usually have any straight duct runs to condition airflow prior to measurement.

The lack of straight duct runs to condition flow is not the only inlet design feature that makes accurate airflow measurement difficult. Inlets are often designed for multiple uses. Typically, they are used to bring air into the building to maintain indoor air quality, and at the same time, they are used as an outside air economizer inlet. This means that the inlet is oversized for maintaining indoor air quality. The result is very low flow velocities at the inlet most of the time. Low velocities directly affect measurement accuracy and reliability. Rooftop packaged units are a worst-case example, as the minimum outside air and the 100% outside air economizer functions are forced into a single inlet. To make matters worse, some manufacturers combine several unit sizes into a single box/frame/control damper/hood size. Any product chosen to provide critical airflow measurements for this tough application must overcome these issues.

The three most often used technologies in outside airflow measurement are thermal dispersion, pitot array, and the pressure difference across a known resistance. As a manufacturer of all three, Air Monitor has decades of real-world experience utilizing these solutions in various outside airflow applications. This in-the-field experience has allowed us to evaluate and compare the performance characteristics of each technology without a built-in bias. The following information outlines the pros and cons of all three approaches and Air Monitor’s recommendation for measuring outdoor airflow.

Technology #1: Thermal Dispersion

Thermal dispersion is a velocity-based technology. Velocities are measured at several distinct points and the readings are averaged. The flow rate is calculated using the average velocity and the cross-sectional area of the intake measured at the plane where the instrument is inserted (this can be difficult in hoods, with tapered inlets). 

Each point of measurement utilizes two precision-matched thermistors for temperature measurement. One thermistor measures ambient airflow temperature, and the other is heated to maintain a fixed differential above the ambient temperature. Heat is transferred from the heated thermistor to the air stream, and as airflow velocity increases, the rate of heat dispersion increases. The relationship between the airflow rate and the power needed to maintain the temperature difference is used to calculate velocity.

Thermal dispersion airflow measurement works best when used with upstream straight ducts to condition flow.

Pros:

  • Measurement accounts for temperature and elevation; output can be either actual flow rate (ACFM) or standard flow rate (SCFM)
  • High turndown – a wide range, from low to moderately-high velocities
  • Mid-price range
  • Sold as a system, not as component parts

Cons:

  • Thermal sensors do not tolerate any form of moisture in the air
    • Moisture causes sudden quenching of the heated sensor resulting in random measurement spikes and instability
  • They perform poorly in dynamic and turbulent airflow conditions.
    • Measurement apertures are not tolerant of airflow that is off-axis (angular flow through open wall intakes, intake hoods, and duct transitions)
    • Flow elements cannot distinguish between forward and reverse direction of flow
    • There is a tradeoff between the cost and the number of measurement points. This often results in fewer used than industry recommendations for accurate average velocity measurement. To make up for this, thermal dispersion systems typically require 2 to 8 diameters of straight duct runs both upstream and downstream which are rarely available in outside air intake duct configurations
  • Velocity measurement is susceptible to wind gusts resulting in a very noisy signal making dampers difficult to control
  • Requires regular, preventative maintenance
    • The dirt and debris found in the unfiltered outside air can coat and insulate the thermal sensors causing thermal coefficients to change, resulting in inaccurate flow measurement

Technology #2: Pitot Array

Pitot-based measurement (Delta P) has been used in HVAC applications for nearly half a century. It is a velocity-based technology. Dynamic pressure and static pressure are measured at many points, and the results are averaged. The velocity of airflow is proportional to the difference in pressure. A differential pressure (DP) transmitter is used to convert the difference into an average airflow velocity. The transmitter then converts average velocity into average airflow volume using the cross-sectional area of the duct.

There are substantial differences in the performance and price level of products that use this measurement principle. Systems have two components: Pitot probes and the DP transmitter. System accuracy depends on the number of sensing points and the selection of a DP transmitter. Transmitters that offer an excellent turndown (velocity range) are much more expensive. At the low price point, we see mismatched components and systems that use fewer than the required number of probes, and a generic, low accuracy, limited turndown, DP transmitter. At the high end, we see systems with well-matched probes, and transmitters that are used as one flow measurement system. Systems of this type are performance tested and recognized by an independent test laboratory like Air Movement & Control Association International, Inc. (AMCA).

Air inlets lacking straight duct runs to condition flow require large numbers of Pitot tubes to achieve accurate flow measurements.

Pros:

  • Measurement systems with correction for temperature and elevation via secondary measurements can output either actual flow rate (ACFM) or standard flow rate (SCFM)
  • Low-cost systems may be acceptable depending on performance requirements
  • Velocity can be measured at a vast number of points across the duct resulting in accurate average velocities
  • Tolerates moisture better than thermal technologies – moderately tolerant of dust in the air
  • Robust construction, no active components in the air stream improves long-term reliability

Cons:

  • Over a long period of time, sensing holes can plug with accumulated dirt, requiring periodic cleaning
  • Limited turndown for systems with poor transmitter selection, however with a better selection of transmitters, this can be eliminated
  • These systems do not perform as well in dynamic and turbulent airflow conditions
    • Measurement apertures are not tolerant of flow that is off-axis (angular airflow), some Pitot probes are designed to compensate for limited angular velocity vectors, but not all systems offer this
  • Low-cost systems sacrifice performance
    • If velocity is measured with far fewer measurement points than recommended by industry standard
    • Can only output actual flow rate (ACFM) if secondary temperature compensation is not performed
    • Periodic Calibration is required (due to transducer zero drift if the DP transmitter selected does not have AUTO-zero capability)

Technology #3: Pressure Difference Across a Known Resistance

In the late 1700s Giovanni B. Venturi observed the pressure drop that occurs when a fluid accelerates to pass through a constriction. He correlated this with Bernoulli’s work and published the Venturi Effect in 1797. Those mathematical relationships have been used in measurement technologies for over a century in pipes. Still, they are not practical for measuring outside airflow in large intake ducts primarily because they produce very high-pressure drops.

The Air Monitor OAM II Outside Airflow Measurement System utilizes these principles to measure static pressure fields that develop when air flows through any screen or louver with a known velocity vs. pressure drop curve without a significant change in the pressure drop. These static pressure measurements used in concert with temperature, barometric pressure measurements, and inlet size can be used to calculate the flow rate at any atmospheric condition.

Small differences in static field pressures created by air moving through a fixed inlet allow the OAM II to accurately measure airflow without the need for straight ducts to condition flow.

Pros:

  • High tolerance for dynamic variations in velocity and direction
  • The system accounts for temperature, elevation, and barometric pressure; it can output either actual flow rate (ACFM) or standard flow rate (SCFM)
  • Zero straight run of duct required
  • High turndown capability with stacked transducer arrangements
  • Reliable accuracy in real-world conditions
  • Nearly impervious to dust and moisture
  • Robust construction
  • Versatility – can easily be applied with louvered openings, hoods, dampers, ducts, plenums, etc.
  • Can be retrofitted to existing systems

Cons:

  • None for outside airflow applications

The Informed Decision

 Precision in outside airflow measurement is essential to ensure that ASHRAE ventilation standards are met, energy consumption calculations are accurate, and that proper indoor air quality is maintained for occupant health and safety. Each of the technologies discussed in this guide has their strengths and can be a viable solution for some airflow measurement applications. As an experienced manufacturer of all three systems, we at Air Monitor can confidently say that the pressure difference across a known resistance technology found in our OAM II product is the best airflow measurement solution for outside air applications.

Click here for more information on the Air Monitor OAM II Outdoor airflow measurement system.

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Air Monitor at the 2023 AHR Expo

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We had the privilege of exhibiting at the AHR Expo in Atlanta earlier this month, and we’re pleased to report that it was an incredibly successful experience for our team! 

As exhibitors, we had the opportunity to showcase our HVAC Flow and Energy Measurement products alongside ONICON Incorporated to a wide range of HVACR industry professionals.

 We were thrilled to showcase our latest innovations and demonstrate how they can help our customers improve their operations and achieve their goals.

Check out our AHR 2023 Recap Video!

What Were the Highlights of Our Experience at the Expo?

Throughout the expo, we also had the privilege of engaging with our local representatives who journeyed from all over the world to attend. The Expo provided us with ample opportunities to connect with them, including at our sales meeting held in conjunction with the event, and on the expo hall floor.

Our booth at the Expo was a hit with visitors, who were drawn in by demo units showcasing our technologies in action, and new products table. Our newest product, the System-1000 Multi-Channel Flow and Energy Measurement System, generated a lot of interest and was certainly one of our most popular displays. 

Aside from exhibiting, our team also took advantage of the educational sessions, which were invaluable for learning about the latest trends and best practices in the industry. We’re eager to integrate these insights into our future products, ensuring that we continue to provide innovative solutions that meet the needs of our customers. 

Outside of the Expo Hall, the highlight of the show was our annual Rep Appreciation dinner where we got to spend time with and honor our reps for their tremendous efforts over the past year.

Air Monitor's Booth at the AHR Expo

Air Monitor & ONICON Booth

AHR Expo in Atlanta 2023: A Highly Rewarding Event for Air Monitor

Overall, the AHR Expo in Atlanta was a highly rewarding event for us and we thank everyone who participated with us. We’re eagerly anticipating next year’s expo in Chicago, in the meantime be sure to check this year’s event Recap video on ONICON’s YouTube Channel!

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Air Monitor is Heading to Atlanta for the AHR Expo!

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We are excited to exhibit at the upcoming AHR Expo taking place February 6-8 at the Georgia World Congress Center in Atlanta, GA. We will be alongside ONICON Incorporated showcasing our HVAC airflow, flow, and energy measurement solutions in Booth #C5519.

The AHR Expo is the world’s largest HVACR event and attracts the most comprehensive gathering of industry professionals from all over the world. The expo provides hundreds of educational sessions, networking opportunities, and a space for attendees to share ideas all under one roof. We look forward to gathering with our sales reps, customers, and movers and shakers in the HVACR industry at the event.

“We have new and exciting products and cannot wait to share them with the attendees at the upcoming expo.” – said Ashley Bence, Senior Product Manager. “It’s always a pleasure to be able to showcase our products in action on the show floor.”

Registration is free through February 4th and once you have registered, be sure to visit our Map Your Show page to see what we will be featuring and to make sure you don’t miss out on visiting our booth while you’re there. See you in Atlanta!