The Completed Guide to NDIR CO2 Sensor

Introduction

An NDIR CO2 sensor (Non-Dispersive Infrared CO2 Sensor) is a device used to detect and measure the concentration of carbon dioxide (CO2) in the air. It operates based on the principle that CO2 molecules absorb infrared (IR) light at a specific wavelength. This guide will share the NDIR CO2 sensor working principle, structure, components, and Applications.
1. What is an NDIR CO2 Sensor?
2. What is the Structure of the NDIR Gas Sensor?

3. Components of NDIR Gas Sensor

3.1 Infrared Light emitter

3.1.1 LED (Optical type)

3.1.2 Lamp (Thermal type)

3.1.3 MEMS heater (Thermal type)

3.2 Infrared Sensor

3.2.1 Photodiode (Optical type)

3.2.2 Thermopile (Thermal type)

4. What is the working principle of an NDIR CO2 sensor?
5. Applications for NDIR CO2 Sensors
6. Features for NDIR CO2 Sensors
7. FAQ NDIR CO2 Sensors+

1. What is an NDIR CO2 Sensor?

An NDIR CO2 sensor is designed to measure the concentration of carbon dioxide gas in the air. It works by emitting infrared light through a gas sample and detecting how much of that light is absorbed by CO2 molecules. This absorption happens because CO2 has a specific wavelength at which it absorbs infrared light, around 4.2 micrometers. The main components of the sensor include an infrared light source, a sample chamber or optical path, an optical filter, an infrared detector, and electronic circuitry to process the signal. Next, we will explore the structure of the NDIR gas sensor.

NDIR CO2 sensor

Figure1-NDIR CO2 sensor

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2. What is the Structure of the NDIR Gas Sensor?

The components of an NDIR gas sensor include an infrared source, detector, optical filter, gas cell, and signal-processing electronics. A single light source, dual wavelength gas sensor consists of two detectors and two optical filters with distinct wavelengths positioned in front of each detector. To detect the target gas, infrared light absorbed by the gas passes through the active filter within a specific bandwidth. The reference filter allows infrared light not in contact with the target gas to pass through. Gas concentration conversion happens when there is a difference in transmitted light intensities in these two bandwidths. Since the output signals at the reference wavelength automatically offset the aging effects of the light source or gas cell, the dual-wavelength sensor guarantees stable readings over an extended period of operation.

3. Components of NDIR Gas Sensor

3.1 Infrared Light Emitter

Generally speaking, light emitters fall into two categories: thermal and optical. The thermal type emits light by heating the object through the current supplied to the heating element, while the optical type utilizes the recombination of electrons and holes in a semiconductor to directly convert current into light. Each type is described along with the unique characteristics of the product.

Light emitter of NDIR type CO2 sensor

Figure 2. The light emitter of NDIR type CO2 sensor

3.1.1 LED (Optical type)

For gas sensors of the NDIR type, Light light-emitting diodes (LEDs) are the preferred light emitters. LEDs respond much faster than thermal light emitters and can be controlled to limit the duration of light emission, reducing the power consumption of gas sensors. In practice, infrared LEDs and AKM infrared sensors are utilized to develop commercially available gas sensors that operate on batteries for up to four years.

3LED (Infrared Light emitter of NDIR type CO2 sensor)

Figure 3LED (Infrared Light emitter of NDIR type CO2 sensor)

3.1.2Lamp (Thermal type)

The lamp is a common old-fashioned infrared light emitter that can also be used for gas sensors. Depending on the type of gas, infrared rays are naturally absorbed by it. Therefore, a single lamp can accommodate a range of gas types because lamps simultaneously emit light at multiple wavelengths from the visible to the infrared region.

As a result, a lamp can serve as a versatile light source. It takes time for a thermal infrared light emitter to heat up and emit light, which means that applying a voltage also takes longer, resulting in more power consumption. Additionally, when using a lamp as an infrared light emitter for a gas sensor, caution must be taken in the surrounding area of the gas sensor due to the filament’s sensitivity to vibrations.

lamp

Figure 4 lamp

3.1.3 MEMS heater (Thermal type)

The concept of light emission is similar to that of lamps. Among thermal-type light emitters, a MEMS heater is designed with a small heat capacity to enhance heat generation, resulting in a response speed faster than that of a lamp but slower than that of an LED. Moreover, since MEMS heat does not reach the same high temperatures as a lamp, the emission wavelength band is narrower than that of a lamp but wider than that of an LED. When the wavelength of the MEMS heater aligns with the wavelength of a gas absorbent, it can be utilized as a light emitter for a gas sensor.

A MEMS heater’s microstructure makes it susceptible to vibrations similar to those of a lamp. Additionally, due to the exposed hot section, a MEMS heater is not suitable for detecting combustible gases in the same manner as an infrared light emitter.

MEMS heaterFigure5, MEMS heater

3.2 Infrared Sensor

Additionally, there are two general categories of infrared sensors: thermal and optical. The thermal type detects polarization or voltage due to temperature differentials when an object becomes warm, while the optical type uses a semiconductor’s photovoltaic power to directly convert light into current. Each type is described along with the specific characteristics of the product.

Infrared sensor of NDIR type CO2 sensor

Figure 6. Infrared sensor of NDIR type CO2 sensor

3.2.1 Photodiode (Optical type)

Photodiodes function by absorbing light, specifically infrared rays, in the depletion layer formed when two P-type and one N-type semiconductor are combined. This concept is utilized to produce photovoltaic power. Typically, it is considered difficult to differentiate between the current generated by photovoltaic power and the noise current generated by heat without cooling the photodiode that is responsive to mid-infrared light.

The photodiode’s response speed (typ. 2 μs) is significantly faster than that of a thermal-type detector because it is a photodetector. As a result, integrating this photodiode with an infrared LED light emitter can reduce power consumption and enhance its application in an NDIR-type gas sensor.

NDIR type CO2 sensor)

Figure 7. Photodiode (Infrared sensor of NDIR type CO2 sensor)

3.2.2 Thermopile (Thermal type)

An infrared thermopile is a sensor that utilizes the Seebeck phenomenon to convert an object’s temperature difference into a voltage. A thermopile generates a temperature differential. The warm junction denotes the warmer side of the intersection, while the cold junction refers to the colder side. A thermopile is designed to facilitate efficient heat collection through an infrared absorbing layer on the warm junction and heat dissipation on the cold junction to enhance the temperature differential. Connecting a thermopile in series increases the electromotive force.

A thermopile can be used as an infrared sensor in NDIR by employing an optical filter that is suitable for both the thermopile and the target gas due to its broad sensitivity to infrared light. By changing the optical filter, it becomes possible to detect various gases. As a thermal infrared sensor, a thermopile has a slow response speed because it requires time to achieve a temperature differential (>200 msec).

Consequently, the driving duration of the infrared light emitter lengthens, leading to an unavoidable increase in power consumption. Furthermore, due to the vacuum or sealing gas inside the package, this thermopile-type sensor is not suitable for environments where vibrations could potentially damage the package, such as in cars.

Thermopile (Infrared sensor of NDIR type CO2 sensor)

Figure 8. Thermopile (Infrared sensor of NDIR type CO2 sensor)

3.2.3 Pyroelectric Sensor (Thermal type)

Pyroelectric sensors, also known as thermal infrared sensors, require time to heat the ferroelectrics, which slows down the response speed (> 200 msec). Similar to thermopiles, they are sensitive to an infrared region across a broad area, making them suitable for use as NDIR infrared sensors when equipped with an appropriate optical filter for the gas. Moreover, due to the vacuum inside the package, pyroelectric sensors are unsuitable for environments where vibrations could potentially harm the package, such as in automotive applications.

Pyroelectric Sensor (Infrared sensor of NDIR type CO2 sensor)

Figure 9. Pyroelectric Sensor (Infrared sensor of NDIR type CO2 sensor)

4. What is the working principle of an NDIR CO2 sensor?

NDIR technology is a commonly used method for measuring gas concentrations and can detect a variety of gas compounds. In a sensor’s gas measurement chamber, a specific gas (such as CO2, CO, CH4, and other hydroxy CH gases) absorbs infrared light at corresponding wavelengths, as shown in Figure 1. Typically, the higher the concentration of the gas, the more energy it absorbs at the corresponding infrared wavelength. Therefore, we can determine the concentration of a gas by measuring the amount of infrared light absorbed at these wavelengths. Because the light emitted from the source and passing through the gas measurement chamber is composite and non-dispersive, this measurement method is referred to as non-dispersive infrared technology. This article explains the working principle of NDIR sensors using CO2 detection as an example.

IR absorption for different gases [1]

Figure 10. IR absorption for different gases

As shown in Figure 10, different gases have distinct absorption wavelengths. If we want to measure a specific gas, it is usually necessary to place an optical filter in front of the detector (as shown in Figure 2) to block all other light except for the infrared light that the target gas can absorb. For CO2, the most prominent absorption peak occurs around a wavelength of 4.26 μm, which does not significantly overlap with the absorption peaks of other gases. Therefore, a narrowband optical filter with a center wavelength of 4.26 μm is often used as the filter for CO2 sensors.

NDIR sketch of optical structure

Figure11.NDIR sketch of optical structure

Behind the optical filter, a thermopile detector is typically used to measure the intensity of infrared light corresponding to the absorption wavelength of carbon dioxide. The absorption of infrared light follows the Beer-Lambert Law:

I=Io​e−kCL

where:

  • I represent the intensity of the transmitted light (measured in W/m²),
  • I0 represents the intensity of the incident light (measured in W/m²),
  • k is the absorption coefficient of the gas at a specific wavelength,
  • L is the effective optical path length between the infrared light source and the detector (measured in cm),
  • C represents the gas concentration (measured in mol/dm³).

This equation indicates that the intensity of the infrared light decreases exponentially with the concentration of the target gas, the absorption coefficient, and the effective optical path length. Thus, the greater these factors are, the more significant the attenuation of the infrared intensity.

 Working principle of thermocouple

Figure 12. Working principle of thermocouple

A thermopile is composed of a series of thermocouples connected in series. Each thermocouple consists of two conductors made of different materials. According to the Seebeck Effect, if there is a temperature difference between the two junctions of the conductors, a voltage difference is generated between the hot and cold junctions (as shown in Figure 12). In NDIR applications, the infrared light passing through the optical filter is applied to a set of junctions connected in series. These junctions are heated, generating a small thermoelectric voltage relative to another set of reference junctions. This thermoelectric voltage is directly proportional to the intensity of the absorbed infrared light. A typical thermopile detector is shown in Figure 13.

Sectional view of MEMS thermopile sensor

Figure 13. Sectional view of MEMS thermopile sensor

The output from a thermopile is a weak voltage signal that needs to be amplified and filtered by a signal conditioning circuit. Additionally, since the thermopile generates an induced signal by absorbing thermal radiation, fluctuations in the ambient temperature can affect its signal. Therefore, a thermistor is typically included in the detector for temperature reference and compensation.

In applications requiring high precision, the conditioned analog front-end signal and the ambient temperature signal are digitized using an Analog-to-Digital Converter (ADC). Software (compensation algorithms) then processes the sensor signals through Digital Signal Processing (DSP), including filtering and compensation. Finally, the concentration readings are output to the client via digital communication. A block diagram illustrating this principle is shown in Figure 14.

System design of NDIR sensor

Figure 14. System design of NDIR sensor

5. Applications for NDIR CO2 Sensors

  • Indoor Air Quality Monitoring: NDIR CO2 sensors are commonly used in homes, offices, schools, and other indoor environments to monitor air quality. Elevated CO2 levels can indicate poor ventilation and air quality, which can affect comfort and health.
  • HVAC Systems: These sensors are integrated into Heating, Ventilation, and Air Conditioning systems to optimize airflow and ventilation. By monitoring CO2 levels, HVAC systems can adjust ventilation rates to maintain optimal air quality and energy efficiency.
  • Industrial Safety: In industries where CO2 is used or produced, such as breweries, greenhouses, and chemical plants, NDIR CO2 sensors are used to monitor gas concentrations and ensure safe working conditions.
  • Environmental Monitoring: They are used in environmental studies and monitoring stations to track CO2 levels in the atmosphere, which is important for studying climate change and air pollution.
  • Greenhouse Control: In agriculture, NDIR CO2 sensors are used to monitor and control CO2 levels in greenhouses, which can enhance plant growth and productivity.
  • Automotive: They are used in automotive applications to monitor cabin air quality and control ventilation systems.
  • Medical Applications: NDIR CO2 sensors are used in medical devices, such as capnography equipment, to monitor the CO2 levels in patients’ breath during anesthesia or critical care.
  • Breath Analysis: In some medical diagnostics and fitness applications, CO2 sensors are used to analyze the composition of exhaled breath.

6. Features for NDIR CO2 Sensors

  • Refrigeration and Food Storage
  • Safety in Confined Spaces
  • High Accuracy and Precision
  • Selective Gas Detection
  • Long-term Stability
  • Low Maintenance
  • Wide Measurement Range
  • Fast Response Time
  • Temperature Compensation
  • Digital and Analog Outputs
  • Robust and Durable
  • Low Power Consumption
  • Self-diagnosis and Calibration
  • Compact Size

7. FAQ NDIR CO2 Sensors

What is the wavelength of CO2 in NDIR?

The sensing channel detection wavelength overlaps with the absorption band of the gas of interest, while the reference channel wavelength is not absorbed by the gas. For CO2 sensing, wavelengths around 4.26 μm and 3.9 μm are commonly used for the sensing and reference channels, respectively.

What are the disadvantages of NDIR?

Other Limitations with NDIR Sensors
They function poorly in extreme environments or where there is a rapid change in conditions. The design of NDIR sensors also allows humidity, fog, and ambient infrared light into the open chamber, all of which can cause interference.2

What is the most accurate CO2 sensor?

NDIR (non-dispersive infrared) sensors: These sensors use infrared light to measure CO2 levels. They are the most accurate and reliable type of CO2 sensor. Electrochemical sensors: These sensors use a chemical reaction to measure CO2 levels. They are less accurate than NDIR sensors, but they are less expensive.

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