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issue: June 2006 APPLIANCE Magazine

Technology Report
CO2 Sensor


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An IR-based CO2 transmitter module is said to offer engineers a low-cost, reliable gas sensing solution for kitchen fans, air cleaners, ranges, and other appliances.

The CO2 Engine features a metal optical absorption cell, which provides 12 cm of absorption length. According to SenseAir (Delsbo, Sweden), this is the key to the sensor’s high performance.

When creating the CO2 Engine™, SenseAir AB wanted to develop a CO2 gas sensor that was accurate, inexpensive and easily integrated. Existing technologies, however, didn’t offer all of these features, forcing the Sweden-based supplier to create a new solution from the ground up.
“Traditional low-cost gas sensing technology is commonly realized either by metal oxide semiconductor (MOS) technologies, changing its resistance when exposed to a certain target gas, or by electrolytic (EC) technologies, having a chemistry that produces an electrical current when exposed to the target gas,” explains Dr. Hans Martin, R&D manager. “Both of these low-cost techniques have severe limitations constituted by limited lifetimes, limited target gas specificity (they to some degree react on other gases in addition to the target gas), risk of fatal errors from overexposures of high gas concentrations, and output dependency of environmental conditions, among other limitations.”
The alternative is IR-based CO2 gas detection, which is said to be more accurate and overcomes the limitations of MOS and EC technologies. The drawback to this technology, however, is the cost. “IR technology, known for its good performance in medical and industrial equipment, was previously…too expensive to even consider in consumer products,” notes Dr. Martin. “This cost situation is now radically changed by the market introduction of the CO2 Engine sensor.”
To keep costs low, engineers stripped down the standard sensor electronics to only the necessary components, keeping in mind that the end appliance will already have a well-defined sensor interface, including stabilized sensor power and an EMC-protected environment. The remaining electronics were then redesigned to fit into a system-on-a-chip microcontroller platform, reducing the total number of components. Finally, the supplier created a new automated sensor production method, which Dr. Martin says includes convenient logistic solutions in the different production steps. “The CO2 Engine uses innovative IR waveguide technology, based on polymer precision replication technology, to enable low-cost, high-volume production,” he says. “High-volume production of one and the same CO2 sensor variant has not previously been realized to this efficiency and extent.”
The sensor’s accuracy is ensured via a patented gold-metalized optical absorption cell. “In optical and IR absorption measurements, the interaction length between light and gas is important for the sensitivity of the system,” explains Dr. Martin. “The longer path length, the stronger absorption and larger gas detection sensitivity. By design, the SenseAir patented optical absorption cell features a long absorption path length at a high optical throughput and signal stability. These features provide the key for the CO2 Engine’s high performance, as compared to competing CO2 sensors that almost without exceptions have shorter path lengths, higher gains and, consequently, are less accurate.”
The CO2 sensor is intended to be an add-on component to complement other microprocessor-based controls and equipment, and can be customized when necessary. Suitable applications include kitchen fans, kitchen ranges and air cleaners, although Dr. Martin says the technology can be applied as a value-added feature to any appliance. “For instance, a ‘dumb’ wall ventilation exhaust valve, or a window ventilator, can easily become clever by adding a CO2 sensor to automatically control the exhaust or fresh air supply,” he suggests. “Such an improved standard product will then get the added ‘green’ features of reduced energy waste and at the same time, assure healthy indoor air by ventilating on demand as measured by the CO2 indoor levels.”

 

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