Carbon Dioxide (CO2) Gas Sensor

Carbon dioxide concentration is the most widely used indicator of indoor air quality and ventilation effectiveness. In occupied spaces, CO2 builds up from human respiration — rising concentrations signal inadequate ventilation, which correlates with occupant discomfort, reduced cognitive performance, and increased risk of airborne pathogen transmission. The Sensorbee CO2 Sensor uses photoacoustic technology to measure carbon dioxide from 0 to 40,000 ppm with 1 ppm resolution. At just 8 grams, it is the smallest and lightest module in the Sensorbee gas sensor range, yet delivers over three years of operational life — three times longer than the electrochemical gas sensors.

How It Works

The CO2 sensor uses photoacoustic detection — a fundamentally different approach from the electrochemical technology used in the other Sensorbee gas modules. An infrared light source emits modulated radiation at the wavelength absorbed by CO2 molecules. When CO2 absorbs this energy, the gas heats and expands, creating a pressure wave (sound) that is detected by a miniature microphone. The amplitude of this acoustic signal is directly proportional to the CO2 concentration.

Photoacoustic sensing offers several advantages over conventional non-dispersive infrared (NDIR) sensors. The measurement path is extremely short, allowing the sensor to be miniaturised to just 20 mm diameter × 24 mm height. There are no optical filters to degrade over time, no reference gas channels to drift, and no moving parts. The result is a sensor that weighs 8 grams, consumes minimal power, and maintains calibration stability over its three-year operational life.

Why CO2 Monitoring Matters

Carbon dioxide is measured in parts per million (ppm), with outdoor ambient concentrations typically around 420 ppm. Indoor concentrations rise when occupied spaces are insufficiently ventilated:

• Below 800 ppm — generally indicates adequate ventilation and good indoor air quality
• 800–1,000 ppm — ventilation may be marginal; sensitive occupants may begin to notice stuffiness
• 1,000–1,500 ppm — ventilation is likely inadequate; associated with complaints of drowsiness and reduced concentration
• Above 1,500 ppm — poor ventilation; significant impact on cognitive performance and comfort; increased risk of airborne pathogen build-up

These thresholds make CO2 a practical proxy for ventilation rate per occupant. Building managers, environmental consultants, and facility operators use continuous CO2 data to optimise HVAC systems, verify ventilation standards compliance, and demonstrate healthy indoor environments.

Measurement Performance

The sensor covers a 0–40,000 ppm measurement range, with specified accuracy across the 400–5,000 ppm range most relevant to indoor environments. Three accuracy bands reflect the sensor's calibration:

• 400–1,000 ppm: ±(50 ppm + 2.5%) — the accuracy needed for ventilation assessment and occupancy detection
• 1,001–2,000 ppm: ±(50 ppm + 3%) — reliable measurement for identifying under-ventilated spaces
• 2,001–5,000 ppm: ±(40 ppm + 5%) — accurate detection of severely compromised ventilation

Response time is under 60 seconds, appropriate for HVAC demand control where ventilation adjustments operate on minute-scale cycles. The 1 ppm resolution provides the granularity needed to track CO2 trends and identify gradual changes in ventilation effectiveness throughout the day.

Ultra-Compact Design

At 20 mm diameter × 24 mm height and 8 grams, the CO2 sensor is significantly smaller and lighter than the electrochemical gas modules (which measure 55 × 40 × 30 mm and weigh 25 grams). This compact form factor reflects the photoacoustic technology's inherently short optical path and miniaturised acoustic detection chamber.

The small size makes the CO2 module easy to integrate into the Air Pro 2 and Air Lite platforms without consuming significant internal volume, leaving space for other sensor modules in multi-parameter configurations.

Three-Year Operational Life

The photoacoustic sensing principle does not rely on a consumable electrochemical cell, so the CO2 sensor maintains accuracy for over three years — compared to the one-year life typical of the electrochemical gas sensors. This extended life reduces replacement frequency and total cost of ownership for long-term monitoring programmes.

For building management applications where sensors are deployed across dozens or hundreds of locations, the three-year replacement cycle significantly reduces maintenance logistics and sensor procurement costs.

Integration With Sensorbee Platforms

The CO2 sensor integrates with the Air Pro 2 and Air Lite as a plug-in module. The base station handles data acquisition, processing, and transmission to the Sensorbee Cloud. No external power supply, signal conditioning, or calibration equipment is required.

In indoor air quality applications, the CO2 sensor is typically deployed alongside the EnviroSense module (temperature, humidity, pressure) to provide a complete indoor environmental profile. For mixed indoor/outdoor monitoring, the CO2 module can operate alongside particulate matter and other gas sensors on the same base station.

CO2 data on the Sensorbee Cloud supports trend analysis, occupancy pattern detection, and ventilation performance benchmarking across multiple locations.

Applications

• Indoor air quality monitoring — continuous CO2 measurement in offices, schools, hospitals, and public buildings to assess ventilation adequacy and occupant comfort
• Ventilation demand control — real-time CO2 data feeds HVAC control systems to optimise ventilation rates, balancing air quality with energy efficiency
• Occupancy estimation — CO2 concentration trends correlate with occupant numbers, supporting space utilisation analysis and building management optimisation
• Greenhouse monitoring — track CO2 enrichment levels in controlled growing environments where elevated CO2 promotes plant growth
• Energy efficiency assessment — identify over-ventilated spaces where HVAC energy can be reduced without compromising air quality, and under-ventilated spaces requiring intervention
• Post-construction building commissioning — verify that installed HVAC systems deliver the ventilation rates specified in building design documents

Frequently Asked Questions

How does photoacoustic differ from NDIR CO2 sensors?

Both technologies measure CO2 using infrared absorption, but the detection mechanism differs. NDIR sensors use a thermopile or pyroelectric detector to measure how much infrared light passes through the gas sample. Photoacoustic sensors detect the sound wave generated when CO2 absorbs modulated infrared light. The photoacoustic approach enables a shorter optical path and smaller sensor with inherently stable calibration.

Can this sensor measure outdoor CO2?

The sensor measures CO2 from 0 to 40,000 ppm and operates in 0–95% RH. While it can detect outdoor ambient CO2 levels (around 420 ppm), the specified accuracy range starts at 400 ppm. For ambient outdoor CO2 monitoring, results at near-background concentrations should be interpreted with the ±(50 ppm + 2.5%) accuracy specification in mind.

Why is the operational life three times longer than other sensors?

Electrochemical gas sensors use a consumable chemical cell that depletes over time as it reacts with the target gas. The photoacoustic CO2 sensor uses light and sound — no chemical reactions consume sensor materials, so the sensing element does not degrade with use.

Does temperature affect CO2 readings?

The sensor operates from -10 to +60°C and includes onboard temperature compensation. For indoor applications (typically 18–28°C), temperature effects on measurement accuracy are minimal.

Can I use this with the Air Lite Modbus?

The CO2 sensor is compatible with the Air Pro 2 and Air Lite cellular platforms. For Modbus integration, consult Sensorbee technical support regarding module compatibility with the Air Lite Modbus variant.