Carbon Dioxide Sensing: PAS vs NDIR vs TVOC

We offer quite a few environmental sensors here at SparkFun, and we thought it’d be cool to give you a look under the hood to see how they actually do their jobs! Starting today, we’re going to look at CO₂ sensors. We have a few different types that use different technologies to sense CO₂ concentration in the air, and today we’re going to dive into Photoacoustic Spectroscopy, Non-Dispersive Infrared, and Total Volatile Organic Compounds. Let’s go!

Photoacoustic Spectroscopy Sensing (PAS)

Photoacoustic Spectroscopy (PAS) for CO₂ sensing is a sophisticated and highly sensitive technique that leverages the principles of sound and light absorption to detect and measure the concentration of carbon dioxide (CO₂) in a given environment. Let’s delve into a more in-depth explanation of how PAS works:

1. Infrared Light Emission: The PAS system starts with an infrared (IR) light source. This light source emits modulated infrared light tuned to specific wavelengths that CO₂ molecules are known to absorb.
2. Interaction with Gas Molecules: The emitted light travels through the gas sample, which may contain CO₂ along with other gases. When the light encounters CO₂ molecules, they absorb specific wavelengths of this light.
3. Photoacoustic Effect: The absorption of IR light by CO₂ molecules causes them to vibrate and increase in energy. As the light is modulated (turned on and off at a specific frequency), these molecules periodically heat and cool, leading to rapid expansions and contractions. This process increases the pressure in the chamber.
4. Detection of Sound Waves: These sound waves are then detected using a sensitive microphone or acoustic detector placed within the gas sample. The pressure is directly proportional to the concentration of CO₂ molecules in the gas.
5. Signal Processing: The detected acoustic signal is processed to determine the concentration of CO₂. There’s a membrane in the sensor that only allows CO₂ through, so other gases don’t need to be accounted for.

Non-Dispersive Infrared (NDIR)

Non-Dispersive Infrared (NDIR) sensors are widely used for detecting and measuring CO₂ concentrations. This technology is based on the principle that CO₂ molecules absorb specific wavelengths of infrared light. Let’s explore how NDIR sensors work and their key characteristics in the context of CO₂ sensing:

1. Infrared Light Source An NDIR sensor starts with an IR light source, which emits light across a range of infrared wavelengths.
2. Gas Chamber The emitted IR light passes through a chamber containing the gas sample. This chamber is typically a light path where the gas sample interacts with the IR light.
3. Optical Filter: An optical filter is placed in front of the detector to allow only the specific wavelengths absorbed by CO₂ to pass through. This ensures that the sensor is selective to CO₂.
4. CO₂ Absorption: CO₂ molecules in the gas sample absorb specific wavelengths of the IR light. Each gas has its unique absorption characteristics, and for CO₂, it primarily absorbs IR light in the wavelength of around 4.26 micrometers.
5. IR Detector: The light that passes through the gas sample and the filter reaches the IR detector. The detector measures the intensity of the light at the specific CO₂ absorption wavelength.
6. Calculating CO₂ Concentration: The decrease in intensity of the received light, due to absorption by CO₂, is used to calculate the concentration of CO₂ in the gas sample. The higher the concentration of CO₂, the less light reaches the detector.

Total Volatile Organic Compounds (TVOC) Sensing

Total Volatile Organic Compounds (TVOC) sensors are used to detect a wide range of gases, including carbon dioxide (CO₂), although it’s important to note that they are not specifically designed for CO₂ detection alone. TVOC sensors measure the total concentration of multiple volatile organic compounds present in the environment. Here’s a detailed look at how they work, especially in relation to CO₂ sensing:

1. Sensitive Material: TVOC sensors typically use a sensitive material, such as a metal oxide semiconductor, that reacts to various gases present in the air, including CO₂.
2. Gas Absorption: When gases come into contact with the sensor’s surface, they are absorbed, leading to a chemical reaction. This reaction is generally a process of oxidation or reduction.
3. Change in Resistance: The chemical reaction on the sensor’s surface causes a change in the electrical resistance of the sensitive material. Different gases cause different changes in resistance.
4. Signal Processing: The sensor measures these changes in resistance to estimate the concentration of gases present. In the case of CO₂, alongside other volatile organic compounds, the sensor provides a combined measurement of total gas concentration.
5. Calibration and Interpretation: Since TVOC sensors are not specific to CO₂, their readings for CO₂ levels are indirect and are part of the total VOC concentration. Accurate interpretation of CO₂ levels specifically requires careful calibration and, often, the use of additional sensors or algorithms.
    

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Comparative Analysis

When comparing PAS, NDIR, and TVOC:

• Sensitivity and Accuracy: PAS sensors, like the XENSIV™, typically offer superior sensitivity and accuracy.
• Power Consumption: PAS sensors are generally more power-efficient.
• Response Time: PAS sensors respond quicker than NDIR sensors.
• Environmental Influence: NDIR sensors can be influenced by atmospheric conditions like humidity and temperature, whereas PAS sensors are most sensitive to atmospheric pressure.
• Longevity and Maintenance: NDIR sensors usually have longer lifespans and require less maintenance. Both NDIR and PAS require calibration about once a week if in continuous use.
• Cost-effectiveness: NDIR sensors are often more cost-effective for applications that do not require the high sensitivity of PAS.
   

Application Scenarios

Each technology has its niche:

• NDIR is often chosen for outdoor environmental monitoring.
• PAS is ideal for indoor air quality and HVAC systems, and work best where there is good air flow.
• TVOC sensors are versatile for broad air quality assessments, not so much for specific CO₂ values. These sensors find a lot of use in industrial environments.
   

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Understanding the distinct features of PAS, NDIR, and TVOC CO₂ sensing technologies is crucial for choosing the right sensor for your application. Each technology has its advantages, and the choice ultimately depends on specific requirements like sensitivity, power consumption, and environmental conditions.
 

Further Resources and References

For those keen on exploring more, here are some useful resources:

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