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.

Image Credit: Roman Zaiets/Shutterstock.com

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

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.