A Giant Golf Ball in Your Outside Air Opening

"What the heck is that thing"

That's often what we hear when we introduce people to the Tek-Air IAQ-Tek outside air monitor.

Sometimes elegant solutions to difficult problems are a little surprising.

The Problem: Outside Air Measurement

Getting an accurate measurement of outdoor air flow is a vexing problem for HVAC professionals. It's about the most difficult airflow measurement situation around.

The air is often moist, dirty, and at extreme temperatures. Most air inlets, especially on packaged air handling units, are poorly designed for accurate flow measurement. The airflows in the inlets are usually highly turbulent, non-uniform and at very low velocities. Wind impinging on the inlets can cause large flow fluctuations. This is tough duty for any flow measurement system.

To make matters worse, the outdoor air flow is one of the more important measurements in an building HVAC system with big implications to the indoor environmental quality and energy use of the building. Understandably, LEED® guidelines encourage the use of outdoor air monitoring.

Generally, the air flow velocities available at an OA probe need to be slow enough to prevent moisture carryover--this makes traditional pressure measurements with pitot-type sensors very unreliable, because the signal from these probes varies with the square of the velocity. At low velocities, the noise from turbulence, wind and other sources simply drowns out the signal with a very low signal-to-noise ratio.

This has led to the use of hot-wire anemometers (thermistors) in this application. These products provide excellent low velocity air flow measurement, but this application provides challenges unique to this technology. In particular, dirt and moisture build-up on the sensors will cause the calibration to stray and upstream filters are usually recommended. Additionally, since the sensors measure the velocity at a discrete point in the air opening, a large number of sensors are required to adequately provide a representative flow measurement for large openings. And even with a large number of sensors, the turbulence and non-uniformity of the airflow in an outdoor air hood or behind a louver makes it very difficult to get a useful reading, no matter how accurate each sample measurement is.

A Different Way

Tek-Air saw the above difficulties and looked for a new solution. And that's why they developed this unique airflow sensing device.

Most flow sensors are designed to minimize the disturbance they create in the airflow. Tek-Air realized they needed to take a different approach for this difficult challenge:



The IAQ-Tek probe is large--really large. In fact each sensor body is about 8" in diameter and has over a dozen pressure ports in it. It dampens out the effect of localized turbulence on the airflow measurement by forcing a large-scale diversion of the airflow in the inlet. The measured variable is the average pressure difference between the ports on the front of the sensor body and the ports on the back of the sensor body. The 'golf-ball' dimples in the face of the sensor ensure stagnation of the airflow to significantly decrease the effect of localized turbulence and ensure a steady, accurate reading.

The unique design of this probe allows accurate readings at 6-8" behind an oudoor air louver, and directly in front of dampers. No prefilters, air straighteners or sections of straight duct are required.

So what does this give you?

• Accurate and stable low velocity readings from 75 to 750 fpm
• Immunity to signal noise
• Great flexibility in application

These probes can get accurate measurements in places you wouldn't even consider other OA probes:






The units come with a temperature and density compensating transducer (-40º to 120º F), for accurate measurement in all conditions. And each system comes with a Nema 4x monitor with LCD readout for local observation. They are rugged devices that need no significant maintenance requirements and can even be hosed down, if needed, for cleaning.

Can they really be accurate in such tight conditions? A test with the unit installed 4" behind a louver outlet, with 18" between the louver and an OA damper yielded the following results:



That's from -6% to +4% (of full range) error at velocities of 100 to 700 fpm with damper positions from full open to 45º. That's fantastic accuracy in an extremely difficult measurement condition.

So maybe you do need a giant golf ball, after all.

Advanced Airflow Measurement

Airflow measurement is a tricky business. Getting an accurate reading is dependent on a host of factors, not the least of which is the inherent accuracy of the technology you are using to measure the flow velocity. Traditional airflow measurement has usually utilized either pitot-type probes or hot wire anemometers, but each technology has its own drawbacks. Pitots rely on the velocity pressure of the air to develop their signal, so at low speeds the noise-to-signal ratio makes readings unreliable. Hot wires are sensitive to moisture and require complicated signal conditioning to resolve their output. Both are susceptible to fouling and require periodic re-calibration.

Tek-Air has developed an advanced airflow measurement technology that eliminates many of these traditional weaknesses, and delivers highly accurate, robust air velocity measurement.

The VorTek air sensor uses the physical phenomenon known as vortex shedding to accomplish this remarkable performance. Vortex shedding is the creation of alternating spiral eddies off of the back side of a bluff body in a flow of any fluid. Examples are the eddies off of a rock in a stream, the ripples in a flag, or, as seen below, the clouds behind an island in the trade winds:


The VorTek sensor uses a simple trapezoidal bluff body that is positioned in the air flow to create these alternating vortices. Two small pressure ports on the back side of the sensor body measure the local air pressure, and the frequency with which a low pressure (due to the shedding vortex) is switches from one side of the body to the other is determined.

(vortex generation behind VorTek sensor body)

There are several reasons why this technology has advantages over traditional measurement methods. The first is calibration. The principle by which the air velocity is measured is simply a property of the geometry of the bluff body in the airflow. This is not a characteristic that will drift over time, so the calibration of the device when installed will be the same as the calibration of the device years downstream.

The second advantage is noise-to-signal ratio. Unlike other technologies where a analog signal amplitude depends on the velocity of the air, the output from the VorTek sensor is a digital signal that is insensitive to fluctuations in a pressure signal--the device simply counts the number of vortices shed off of the sensor and converts this number into an analog velocity output.

Another advantage is signal conditioning. The VorTek technology depends on a linear relationship between vortex shedding frequency and velocity. Creating a velocity output is as simple as applying a constant to the measured shedding frequency. Note how much simpler this is than either hot-wire or pitot technology.



And lastly, the VorTek sensor is extremely insensitive to particulate fouling, as this demonstration illustrates:

(errata: 25 is not the 'square root of 50', but 25% is the square of 50%)

The VorTek sensor was designed for demanding applications like fume hood service, but is applicable for any airflow measurement job. And with accuracies on the range of +/-2% of signal for its entire range of measurement, it will meet the most stringent specification requirement. Cutsheets for the Tek-Air air flow stations using this sensor can be found here and here.