Measuring Carbon Dioxide (CO2) Using Non-Dispersive Infrared (NDIR) Technology

Why Measure Carbon Dioxide?

Carbon Dioxide (CO2) is a colorless, odorless gas that is produced naturally in the environment, and through human actions involving the burning of fossil fuels such a coal, natural gas and oil. According to the United States Environmental Protection Agency (EPA), CO2 is the most common greenhouse gas emitted by humans, accounting for approximately 84% of gas emissions in 2011.1

An increasingly dangerous trend in home and building Energy Audits, currently performed by many companies, is that they focus solely on the “energy efficiency” of the building while ignoring the potential risks of sealing in CO2 emissions and moisture in the home causing serious health and comfort concerns. By sealing homes to be “air tight” for the purpose of decreasing gas and electric bills, we could also be making our buildings too tight. In many cases, the benefits associated with a drop of energy usage carry a heavy price in the form of moisture damage and health risks produced by higher levels of CO2.

When buildings are too tight, pollutants generated inside the house (CO2, CO, smoke, odors, germs, VOCs, etc.) are trapped. Those chemicals, if not dealt with carefully with an intelligent ventilation system, could make the premises very uncomfortable for the residents. Furthermore, people and pets could become drowsy, get sick more frequently, and have potentially fatal consequences in extreme cases.

The Carbon Cycle

How Does Non-Dispersive Infrared Technology (NDIR) Work?

Non-Dispersive Infrared (NDIR) technology is the most common method of measuring CO2 in Indoor Air Quality analyses. The NDIR sensor consists of the following (see image to right): 1) Infrared (IR) Lamp, 2) Gas Sample Chamber, 3) Wavelength Filter, and 4) an IR Detector.

NDIR sensors work using spectroscopy, which pertains to the dispersion of light into its component colors or energies. An example of this is light in the Visible Spectrum (the light humans can see), which is made up of a wide range of wavelengths (~390nm-700nm). When all of these wavelengths are present, the light appears white to the human eye. But, when visible white light is fractured by rain or a prism, the white light is split into each color of the rainbow , each of which have a specific wavelength of energy (e.g. violet-400nm, red –650 nm). When white light passes through a filter such as through a stained-glass window or sunglasses, some wavelengths of light are absorbed or redirected which changes the color of the light that we see.

The Electromagnetic Spectrum

The Electromagnetic Spectrum

Similarly, an NDIR sensor works via spectroscopy in that an IR lamp emits a beam of IR light containing a wide energy spectrum (~900nm-100,000 nm) which is measured by the IR detector at the other end of a chamber. When a sample of gas is pumped into the Gas Sample Chamber, certain gases such as CO2, act as a filter absorbing a specific wavelength of energy within the IR spectrum. Other gases in the sample will not absorb that wavelength of IR light. In an NDIR sensor measuring CO2, a Wavelength Filter placed just before the IR Detector only allows that wavelength of IR energy absorbed by CO2 to pass through. The IR Detector measures the concentration of that wavelength of IR light after it has passed through the sample.

There is an inverse relationship between the amount of CO2 in the gas sample and the amount of IR light detected at the target energy level. When there is no CO2 in the chamber, all of the IR light will reach the detector. The greater the concentration of CO2 in the gas sample, the greater the IR Light absorbed at that energy and the less IR light that is detected by the IR Detector. Using this energy measurement, the sensor calculates the concentration of CO2 in the gas sample in parts per million (ppm).

Spectroscopy

CO2 Monitoring Solution

AQ EXPERT, AQ PRO and AQ COMFORT Indoor Air Quality Monitors

The measurement of the concentration of CO2 can be performed using the E Instruments AQ EXPERT portable IAQ monitor, the AQ PRO Handheld IAQ monitor and the AQ COMFORT IAQ monitor specifically for the HVAC professional. These specialized monitoring instruments utilize NDIR sensors that allow professionals to quickly and accurately monitor the level of CO2 present in the environments of homes, office buildings, laboratories or industrial facilities. These monitors include software with real-time continuous data logging, wireless Bluetooth compatibility, and can be customized to monitor up to 11 different parameters relevant to indoor air quality.

References:
“Overview of Greenhouse Gases”. United States Environmental Protection Agency
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