Control Engineering Online Update for October 15, 2004

In 2003, hundreds of North Carolina Central University students were forced to evacuate campus housing as the school spent millions of dollars to eradicate toxic mold from new dormitories. Meanwhile, Duke University had to shut down its engineering library to tackle mold growth in its book collections. Where was the mold coming from? And why?

In a word, pressure. The key to mold and fungal growth is moisture, and in both cases improper building pressurization resulted in high internal humidity levels greater than 70% relative humidity that encouraged mold production and replication within the buildings.

Building pressure has a significant role in water transport across the building envelope. Control of a building’s pressurization flow is essential for the reduction of mold and fungal growth (see figure 1), but how can it be done cheaply and efficiently?

Airflow/temperature measuring devices are used to maintain dilution (outside air) flow rates and building pressure essential to acceptable indoor air quality (IAQ). They are also routinely used in laboratory and healthcare institutions for contaminant and infection control.

IAQ can be substantially improved through the proper control of outdoor air intake flow. Dilution airflow control is essential to maintaining acceptable contaminant levels. Indoor pollutants are generated from components within the building as well as its occupants. Minimum outdoor air ventilation levels must be maintained to provide adequate dilution. Therefore, direct measurement of outdoor airflow rates is essential for acceptable IAQ. Verification is a requirement of both ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality and the International Mechanical Code. Proper control requires accurate and reliable airflow measurement.

To maintain such control cheaply and efficiently, Ebtron Inc. offers the Advantage Gold Series multipoint thermal dispersion airflow/temperature measurement devices. These devices use microcontroller technology in transmitters that interface to all host building automation systems using traditional 0-10 V dc or 4-20 mA analog outputs.

The microcontroller technology used in Ebtron’s transmitter is the Freescale 68HC908AP64. It was selected because it holds sufficient RAM and FLASH memory to independently process up to 16 individual airflow and temperature sensors and provide an interface to the user with a pushbutton and LCD display.

According to Mike Urbaniak, Ebtron senior VP, the capabilities of the microcontroller allow the transmitter to output individual sensor airflow and temperature rates, enabling permanently mounted airflow measurement devices to download data via an onboard infrared data association interface to a PDA for instantaneous field measurement.

Ebtron thermal dispersion technology uses two hermetically sealed, bead-in-glass thermistors to determine the airflow rate at each sensing point. The relationship between the power dissipated and the airflow rate is established during the calibration process, where each sensor is tested at multiple airflow rates to standards that are traceable to the National Institute of Standards and Technology. The calibration process is reversed in the field using the microcontroller-based transmitter prior to output to the building automation systems.

Unlike airflow measurement technologies such as pitot arrays and vortex shedding devices, Ebtron airflow measurement accuracy is a percent of reading, not a percent of full scale. An advantage of this approach is the ability to measure airflow at significantly lower rates than is typical for such measurement technologies, a factor paramount for proper control of modern ventilation systems.

Chris Hale is a field applications engineer for Freescale Semiconductor; www.freescale.com