A Comparison of Outside Airflow Measurement Technologies

Are you using the right product for this challenging application?

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.

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

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)

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