CO vs. CO₂ Sensors: Types, Differences & How to Choose the Right Detector

Carbon monoxide (CO) and carbon dioxide (CO₂) are two different gases that require different detection technologies. Although both can be measured in parts per million (ppm), the sensors used to detect them often operate on different physical and chemical principles.

An electrochemical sensor is widely used in carbon monoxide safety detectors, while non-dispersive infrared (NDIR) technology is widely used for indoor CO₂ monitoring. Other technologies, including metal oxide semiconductor and photoacoustic sensors, offer alternative approaches for specific applications.

Understanding these differences can help homeowners, campers, RV owners, HVAC technicians and product buyers choose more suitable gas detection equipment.

1. What Is the Difference Between CO and CO₂?

Before comparing sensors, it is important to understand what each gas indicates.

Carbon Monoxide (CO)

Carbon monoxide is a colorless, odorless gas produced by incomplete combustion. Potential sources include fuel-burning furnaces, generators, fireplaces, stoves and vehicle exhaust.

CO can interfere with the body's ability to transport oxygen. Dangerous exposure can occur without any warning from human senses.

A dedicated carbon monoxide detector is therefore an important safety device in homes and other locations where fuel-burning equipment is used.

For practical safety guidance, see the CDC Carbon Monoxide Poisoning Basics.

Carbon Dioxide (CO₂)

Carbon dioxide is a different gas produced by processes such as human respiration and fuel combustion. In occupied indoor spaces, CO₂ measurements are often used to understand ventilation performance and changes in indoor air conditions.

Elevated CO₂ readings can indicate that a room may need more fresh air, depending on occupancy and other conditions. However, CO₂ concentration alone does not measure overall indoor air quality or identify every indoor pollutant.

A CO₂ monitor is useful for environmental awareness, but it is not a substitute for a dedicated CO alarm.

2. What Types of Sensors Are Used in CO Detectors?

Several sensing technologies can detect carbon monoxide. The most relevant types for understanding consumer and professional CO detection are electrochemical, metal oxide semiconductor and biomimetic sensors. Infrared analyzers are also used in specialized measurement applications.

2.1 Electrochemical CO Sensors

Electrochemical sensors detect CO through a chemical reaction at an electrode. This reaction generates an electrical signal that can be related to the gas concentration.

Advantages

• High sensitivity to CO at relatively low concentrations.

• Low power consumption, making the technology suitable for many battery-powered and portable devices.

• Compact design for integration into household and professional detectors.

• Established use in gas safety and industrial monitoring equipment.

Limitations

• Sensor performance can change over time.

• Temperature, humidity and interfering chemicals may affect readings, depending on the sensor design.

• Sensors have a finite service life and may require replacement according to the manufacturer's instructions.

Electrochemical technology is widely used in CO safety detectors because it can combine low-power operation with useful sensitivity.

2.2 Metal Oxide Semiconductor (MOS) CO Sensors

Metal oxide semiconductor sensors detect gas through changes in the electrical resistance of a sensing material. Materials such as tin dioxide are commonly studied and used in this type of sensor.

When the target gas interacts with the sensing surface, the electrical properties change. The device electronics interpret that change as a gas response.

Advantages

• Compact sensing elements.

• Potentially low manufacturing cost.

• Fast response under suitable operating conditions.

• Useful for a range of gas sensing applications.

Limitations

• Many designs require a heater, increasing power consumption.

• Response may be affected by humidity, temperature and other gases.

• Reliable concentration measurement depends on calibration, compensation and the sensor's selectivity.

MOS sensors can be useful in cost-sensitive detection applications, but performance should be evaluated against the requirements of the intended use.

2.3 Biomimetic CO Sensors

Biomimetic sensors imitate aspects of how hemoglobin interacts with carbon monoxide. Traditional designs use a chemically responsive material that changes its optical properties when exposed to CO.

The change is measured and used to trigger an alarm when the device's criteria are met.

Advantages

• Relatively simple sensing concept.

• Can be implemented in low-power alarm designs.

• Does not rely on the same electrochemical mechanism as an electrochemical sensor.

Limitations

• Some designs have longer recovery times after an alarm.

• Environmental conditions can affect performance.

• Availability and practical performance vary by design.

Biomimetic technology is less commonly discussed than electrochemical sensing in many modern consumer CO detector comparisons, but it remains an important example of an alternative sensing principle.

2.4 Infrared CO Measurement

Infrared analyzers detect gases by measuring their absorption of infrared light at characteristic wavelengths.

CO has an infrared absorption band that can be used for optical measurement. Infrared-based CO analyzers are used in certain industrial, laboratory and ambient-air monitoring applications.

However, NDIR technology is more commonly associated with consumer and building CO₂ monitoring than with ordinary household CO alarms.

The application matters: an industrial gas analyzer and a residential safety alarm may use different sensor technologies, measurement ranges and performance requirements.

For a technical overview of CO detector technologies, see the NIST guide to how carbon monoxide detectors work and Honeywell's application guide to system-connected CO detectors.

3. CO Sensor Comparison: Which Technology Fits Which Application?

Sensor technology	Main principle	Typical strengths	Key considerations
Electrochemical	Chemical reaction generates an electrical signal	Low power, good CO sensitivity, compact integration	Service life, environmental effects and interfering chemicals
Metal oxide semiconductor (MOS)	Gas interaction changes electrical resistance	Compact, potentially economical, responsive under suitable conditions	Heater power, cross-sensitivity and environmental compensation
Biomimetic	Chemical change alters optical properties	Alternative low-power alarm approach	Recovery time and environmental sensitivity vary by design
Infrared analyzer	Measures gas-specific infrared absorption	Useful in specialized continuous and analytical measurement	System size, cost and application-specific requirements

There is no single sensor technology that is automatically best for every application. Buyers should evaluate selectivity, detection range, response characteristics, operating conditions, sensor service life and applicable safety standards.

For household CO alarms, also check the complete detector's certification and alarm performance—not just the sensor type listed on the packaging.

4. What Types of Sensors Are Used in CO₂ Monitors?

Carbon dioxide has different sensing requirements from carbon monoxide. For indoor CO₂ monitoring, NDIR technology is one of the most widely used approaches.

Other technologies, including photoacoustic sensing and metal oxide semiconductor research, offer additional options.

4.1 Non-Dispersive Infrared (NDIR) CO₂ Sensors

NDIR sensors measure CO₂ by passing infrared light through a measurement chamber and measuring how much light is absorbed at a wavelength associated with CO₂.

The sensor compares the measured light intensity with a reference or calibrated value to estimate the gas concentration.

Advantages

• Good selectivity for CO₂ when appropriately designed.

• Suitable for continuous indoor monitoring.

• Established use in HVAC, building automation and portable air quality monitors.

• Available in compact sensor modules for consumer devices.

Limitations

• Optical components and the measurement chamber add design complexity.

• Dust, condensation, optical contamination and environmental conditions can affect performance.

• Accuracy and long-term stability depend on sensor design, calibration and compensation.

NDIR is widely used in indoor CO₂ monitoring because CO₂ has a characteristic infrared absorption band near 4.26 micrometers.

Read more in Vaisala's technical explanation of infrared sensor technology for HVAC CO₂ measurement.

4.2 Photoacoustic CO₂ Sensors

Photoacoustic sensing uses modulated infrared light absorbed by CO₂ molecules.

The absorbed energy creates periodic changes in molecular motion and pressure within the measurement chamber. A microphone or acoustic detector measures the resulting signal, which can be related to CO₂ concentration.

Advantages

• Can support compact sensor designs.

• Uses an optical absorption principle to target CO₂.

• Offers an alternative way to measure absorbed infrared energy.

Limitations

• Performance depends on optical design, acoustic detection and signal processing.

• Mechanical noise, environmental conditions and device design can affect measurements.

• Not every photoacoustic design has the same accuracy, response time or service life.

Photoacoustic sensing is often implemented as a form of photoacoustic NDIR technology. It is useful to distinguish the method from conventional transmissive NDIR, where a detector measures the infrared light passing through the gas.

For a detailed explanation, see Sensirion's technical note, Transmissive and Photoacoustic NDIR Sensing.

4.3 Metal Oxide Semiconductor CO₂ Sensors

Metal oxide semiconductor sensors detect gas through changes in the electrical properties of a sensing material.

Researchers have investigated materials such as tin oxide, zinc oxide and other metal oxides for CO₂ detection. These technologies offer potential advantages in cost and miniaturization.

However, CO₂ is relatively difficult to detect selectively using conventional resistive gas sensing materials under ordinary indoor conditions.

Advantages

• Potential for compact, low-cost sensing elements.

• Compatible with electronic sensing systems.

• Active area of research and development.

Limitations

• Some designs require elevated operating temperatures.

• Selectivity and cross-sensitivity can be challenging.

• Humidity, temperature and long-term stability must be considered.

• Performance varies significantly across materials and implementations.

For these reasons, a MOS sensor should not automatically be assumed to offer the same performance as a well-designed NDIR CO₂ sensor. Always check the specific device's measurement method and published specifications.

Further reading: Metal Oxide Resistive Sensors for Carbon Dioxide Detection.

4.4 Other CO₂ Sensing Methods

Specialized applications may use electrochemical, capacitive, optical colorimetric, thermal conductivity or other sensing approaches.

These methods are not interchangeable. Some are intended for particular concentration ranges, controlled gas mixtures, research systems or specialized industrial applications rather than everyday indoor CO₂ monitoring.

When choosing a consumer CO₂ monitor, start with the manufacturer's stated sensing principle, measurement range, accuracy, calibration method and intended application.

5. CO₂ Sensor Comparison: NDIR vs. Photoacoustic vs. MOS

Sensor technology	Main principle	Potential strengths	Key considerations
Conventional NDIR	Measures transmitted infrared light absorbed by CO₂	Established indoor monitoring technology, good gas selectivity	Optical design, chamber size, calibration and contamination
Photoacoustic NDIR	Measures acoustic waves produced by modulated infrared absorption	Can support compact designs and optical CO₂ measurement	Acoustic noise, system design and signal processing
Metal oxide semiconductor	Measures changes in electrical properties	Potentially economical and compact	Selectivity, humidity, operating temperature and stability
Other specialized methods	May use chemical, optical, electrical or thermal properties	Can suit particular applications	Performance and suitability depend heavily on the implementation

The correct comparison is not simply which sensor is cheapest or newest. It is which technology can deliver the required performance in the actual operating environment.

6. CO vs. CO₂: Why One Detector Cannot Replace the Other

Although both gases can be measured in ppm, CO and CO₂ have different chemical properties, sources and safety implications.

Feature	CO detector	CO₂ monitor
Target gas	Carbon monoxide	Carbon dioxide
Main purpose	Detect a potentially life-threatening combustion gas	Monitor CO₂ concentration and support ventilation awareness
Common sensing technology	Electrochemical; MOS and other designs also exist	NDIR; photoacoustic and other technologies also exist
Typical applications	Homes, garages, RVs, fuel-burning appliance areas	Bedrooms, living rooms, classrooms, offices, RVs and other occupied spaces
Key buying consideration	Alarm performance, certification, placement and sensor life	Measurement accuracy, range, calibration and environmental compensation

Important: A CO₂ monitor does not replace a dedicated CO alarm, and a CO detector does not tell you whether indoor CO₂ levels are elevated.

If you need protection against carbon monoxide, choose an appropriate CO alarm and install it according to the manufacturer's instructions and local safety requirements.

The CDC's carbon monoxide safety guidance explains recommended household precautions, including installing CO detectors near sleeping areas and maintaining fuel-burning appliances.

7. How to Choose the Right Gas Detector

The best detector depends on what you need to measure and where you will use it.

For Home Safety

Choose a dedicated CO detector that meets the applicable requirements for your market. Review its alarm behavior, power supply, sensor service life and installation instructions.

For indoor ventilation awareness, consider a separate CO₂ monitor. It can help you understand how CO₂ changes with occupancy and ventilation.

For Camping and RV Use

Enclosed or semi-enclosed spaces require particular attention to combustion safety. Fuel-burning equipment, generators, heaters and cooking devices can create dangerous conditions if used incorrectly.

A suitable CO alarm is an important safety measure. A portable CO₂ monitor can provide additional environmental information, but it should not be treated as a substitute for a safety alarm.

For Offices and HVAC Applications

CO₂ monitors can help building operators observe changes in occupied-space CO₂ levels and evaluate ventilation patterns.

For HVAC professionals, NDIR-based CO₂ sensors are a widely used option. Consider measurement accuracy, long-term stability, response time and whether the sensor is suitable for the building control system.

For Industrial and Professional Applications

Professional users should select instruments based on the target gas, expected concentration range, environmental conditions and relevant standards.

For CO safety monitoring, sensor technology is only one part of the system. Alarm thresholds, calibration, fault detection, maintenance and certification are also important.

8. Explore FABISENSE Gas Monitoring Solutions

FABISENSE develops gas detection and environmental monitoring products for home safety, portable use, camping, RV applications and other suitable environments.

PTH-24D Carbon Monoxide Detector

The FABISENSE PTH-24D combines CO monitoring with temperature and humidity measurement. It is designed for users who want dedicated carbon monoxide monitoring in a plug-in format.

[FABISENSE PTH-24D Product Page — https://www.fabisense.com/products/24-d]

PTH-282B+ Smart CO & CO₂ Monitor

The FABISENSE PTH-282B+ combines CO and CO₂ monitoring in one portable device. Bluetooth app connectivity enables users to view readings and historical trends, alongside temperature and humidity monitoring.

It is designed for environmental awareness at home, during camping trips and in RV applications.

[FABISENSE PTH-282B+ Product Page — https://www.fabisense.com/products/282b]

Explore the FABISENSE product range to compare monitoring solutions for different applications.

When evaluating a particular model, consult its product specifications and user manual to confirm the exact sensor technology, operating range, accuracy, calibration requirements and applicable certifications.

Frequently Asked Questions

What is the most common sensor used in CO detectors?

Electrochemical sensors are widely used in consumer and professional CO detectors because they can provide good CO sensitivity with relatively low power consumption. Other technologies, including MOS and biomimetic sensors, are also used in certain designs.

Which sensor is commonly used in CO₂ monitors?

NDIR is one of the most widely used technologies for indoor CO₂ monitoring. Photoacoustic NDIR is another optical approach, while MOS and other methods are used in specialized designs and research.

Are electrochemical CO sensors and NDIR CO₂ sensors interchangeable?

No. They are designed around different sensing principles and target gases. A CO sensor cannot be assumed to measure CO₂, and a CO₂ sensor cannot replace a CO safety alarm.

Are semiconductor gas sensors always less accurate?

No. Performance depends on the sensing material, design, calibration, compensation and application. However, cross-sensitivity and environmental effects can be important considerations, especially when measuring low concentrations of a specific gas.

Does a CO₂ monitor measure overall indoor air quality?

No. CO₂ is only one indicator of indoor conditions. It does not directly measure all pollutants, such as carbon monoxide, particulate matter, formaldehyde or volatile organic compounds.

What should I check before buying a gas detector?

Check the target gas, sensing technology, measurement range, accuracy, alarm function, calibration requirements, sensor service life, operating conditions and relevant certifications. For safety alarms, follow local requirements and the manufacturer's installation instructions.

Final Thoughts

CO and CO₂ sensors solve different measurement problems. Electrochemical technology is widely used for CO safety detection, while NDIR is a well-established choice for indoor CO₂ monitoring. Photoacoustic and semiconductor technologies offer additional options, each with its own design trade-offs.

Understanding the sensing principle is a useful first step, but the complete detector's performance, intended use, certification and maintenance requirements matter just as much.

FABISENSE — Smarter Sensing. Safer Living.