"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
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.