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.