Comparing Single- and Dual-Channel NDIR CO2 Sensors in HVAC Systems

Carbon dioxide (CO2) is a constant factor and potential risk in daily life, making indoor air quality (IAQ) a top priority for all types of buildings. Ensuring CO2 levels remain safe is now an essential aspect of HVAC system functionality.

 

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When it comes to CO2 detection and mitigation processes via HVAC systems, NDIR (non-dispersive infrared) sensors offer several performance characteristics that are important for HVAC manufacturers or system designers: accuracy, durability, reliability, and adaptability.

Understanding NDIR CO2 Sensors

An NDIR CO2 sensor is a category of gas sensor that utilizes infrared (IR) technology to detect the concentration of CO2 in the air. The sensor consists of an infrared light source, an infrared detector, an optical cavity or light tube, and a wavelength filter.

An NDIR sensor works on the principle that molecules of specific gasses, such as CO2, absorb specific wavelengths of infrared light. Where infrared light passes through an air sample in the optical cavity, the CO2 molecules in the sample will absorb those specific wavelengths of light based on their unique absorption spectrum.

The wavelength filter ensures that only the light wavelengths absorbed by CO2 reach the detector. As the amount of light absorbed is proportional to the concentration of CO2 in the air, any detected decrease in light intensity measured by the detector can be utilized to calculate the CO2 concentration.

When employing HVAC systems to enact countermeasures in instances where CO2 levels get too high, NDIR sensors provide a greater level of performance than other sensor types. There are a number of reasons for this.

Specificity in CO2 Measurement

The high specificity of NDIR sensors in measuring CO2 minimizes the risk of interference from other gases, ensuring accurate and reliable readings. In contrast, other sensor types may respond to a wider range of gases, which can lead to cross-sensitivity issues that affect the accuracy of CO2 measurements.

Performance at Low Concentrations

Due to their functionality, NDIR sensors are especially effective at detecting CO2 at low concentrations in the 400 to 2000 parts per million (ppm) range. This ability is vital in applications that require precise monitoring and management of CO2 levels, such as indoor agriculture.

Long-Term Stability and Accuracy

Not only are they built to last anywhere between ten to fifteen years, but NDIR sensors are also engineered to provide consistent and accurate readings throughout their useful lives, ruling out the need to factor in drift.

Breaking Down NDIR CO2 Sensors

NDIR CO2 sensors can be broken down into two categories: single-channel and dual-channel.

Single-channel NDIR sensors use a single wavelength detection design in combination with sophisticated firmware algorithms to preserve sensor accuracy over the sensor's lifetime.

Dual-channel NDIR sensors incorporate two independent wavelength detection measurements as a method of sensor drift compensation.

Single-Channel Versus Dual-Channel NDIR CO2 Sensors

Single-channel and dual-channel NDIR CO2 sensors are not interchangeable elements of an HVAC system design. Integrating either into a system depends on the application in question, and this holds true across all HVAC sensor types.

For example, single-channel NDIR sensors are ideally suited to environments that periodically return to baseline CO2 levels, including:

  • Conference rooms
  • Classrooms
  • Arenas
  • Gymnasiums
  • Auditoriums

In contrast, dual-channel NDIR CO2 sensors are ideally suited to settings that are occupied continually or are unlikely to experience regular CO2 level fluctuations, including:

  • Correctional facilities
  • 24-hour operations
  • Hospitals
  • Call centres
  • Greenhouses
  • Controlled environment farming

Source: Amphenol Advanced Sensors

Feature Single-Channel NDIR Sensor Dual-Channel NDIR Sensor
Functionality Utilizes one infrared wavelength to measure CO2 levels. Uses two wavelengths: one for measuring CO2 and a reference wavelength for calibration and accuracy.
Accuracy Offers high accuracy without field recalibration, provided baseline exposures can be assured. Retains accuracy over time via secondary reference compensation, independent of field exposure.
Cost Typically less expensive than dual-channel sensors. More expensive due to additional complexity and technology for improved accuracy.
Long-Term Stability Stable over the sensor's life provided periodic exposures to baseline CO2 concentrations. Stable over the life of the sensor, with periodic recalibration.
Application Suitable for environments where baseline CO2 concentrations occur regularly Ideal for critical applications where accuracy is essential and exposure to baseline CO2 concentrations can not be assured.
 
Maintenance No maintenance required.  Periodic recalibration required, dependent on application.

 

NDIR Sensors for CO2 and Managing Drift

Everyday wear and tear of NDIR sensors creates drift. This drift is caused by changes in a range of factors:

  • Detector and/or filter selectivity and sensitivity
  • Waveguide reflectivity due to surface oxidation and/or cleanliness
  • Waveguide geometric optical paths due to thermal and/or mechanical stress and strain
  • IR source output; for example, incandescent lamps, LED, or MEMs technologies

Potential nominal changes in performance through everyday use can lead to a noticeable difference in performance over time, and while sensors may need periodic recalibration, frequent recalibration is not always possible or practical.

Amphenol’s advanced NDIR CO2 sensors are designed to minimize drift over the sensor's lifespan, maintaining their accuracy and reliability. The company’s single-channel NDIR CO2 sensors feature proprietary Automatic Background Calibration (ABC) Logic firmware, which continuously and automatically adjusts the sensor's set-point.

ABC Logic firmware operates on a simple premise whereby the sensor intelligently gathers data on background CO2 concentrations while simultaneously and continuously monitoring the environment.

The resulting data is subsequently utilized to compensate for any sensor drift, allowing it to effectively perform as an ongoing recalibration process. Employing ABC Logic brings a new level of functionality to an HVAC system and its CO2 sensors.

Adapting to Environmental Variations

Background CO2 levels will typically be in the 400 – 450 ppm range, subject to slight variations influenced by temporary or cyclic fluctuations caused by factors like human activity or the presence of vegetation. Analyzing these patterns allows ABC Logic to utilize the gathered background concentration data as a statistically derived reference point.

Recognizing and Responding to Daily Patterns

A notable feature of ABC Logic is its ability to recognize diurnal patterns in CO2 concentrations. This is especially relevant in commercial buildings and other settings where occupancy and CO2 levels fluctuate throughout the day.

As the level of occupancy changes, CO2 concentrations will naturally revert to background levels. ABC Logic can identify these patterns and utilize them to establish offset correction data that will keep the sensor attuned to real conditions.

NDIR Sensors’ Hardware-Based Approach

Each dual-channel sensor possesses two infrared detectors, each equipped with narrow bandpass optical filters. One filter is aligned with the CO2 absorption peak at approximately 4.2 microns, and the other is at 3.9 microns, unaffected by CO2 concentration.

The second channel serves as a reference point unaffected by CO2 levels, which allows it to detect any drift in sensor performance. Adjustments are then made to the active channel's readings to compensate for any detected drift, ensuring accurate CO2 measurements over time.

Whether an HVAC system uses a single-channel or dual-channel NDIR CO2 sensor, it will operate more effectively and efficiently in maintaining CO2 levels within safe limits.

Maintaining Safe CO2 Levels via Advanced NDIR Sensor Technology

While occupant comfort is a major and probably the most noticeable performance metric for any HVAC system, maintaining safe IAQ levels is just as important.

Whether CO2 in a space is produced via respiration or a product of machinery, leveraging the right sensor technology in an HVAC system to detect this potentially dangerous gas is vital.

NDIR sensors, either single—or dual-channel, provide the most reliable way to continuously monitor and manage CO2 levels through HVAC systems, regardless of the setting or the source of the gas.

Acknowledgments

Produced from materials originally authored by Amphenol Sensors.

This information has been sourced, reviewed and adapted from materials provided by Amphenol Advanced Sensors.

For more information on this source, please visit Amphenol Advanced Sensors.

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