One of the most established methods is non-dispersive infrared measurement, or NDIR. NDIR gas sensors use a physical property of many gases: they absorb infrared light at characteristic wavelengths.
This absorption can be used to determine whether a particular gas is present and at what concentration. The method is used in industrial gas analysis, process measurement technology, emissions monitoring, and gas detection, among other applications.
But how exactly does an NDIR gas sensor work? Which components are required? And why is the technology particularly well suited to durable and stable measurement solutions?
What does NDIR mean?
NDIR stands for non-dispersive infrared measurement.
“Non-dispersive” means that the light is not completely separated into its individual wavelengths by a conventional spectrometer. Instead, the measurement focuses specifically on the part of the infrared spectrum in which the target gas absorbs light.
To do this, the sensor uses an optical filter or a suitably designed detector. It allows only a defined wavelength range to pass. The sensor therefore concentrates on the spectral range relevant to the respective target gas.
The measuring principle in one sentence: An infrared beam passes through a measurement chamber. The target gas absorbs some of the light - and the concentration is calculated from the resulting attenuation.
Why do gases absorb infrared light?
Gas molecules are constantly in motion. Their atoms can vibrate relative to one another, and their bonds can stretch or bend. Certain molecular vibrations can be excited by infrared radiation.
A molecule does not absorb every wavelength. Absorption occurs only in specific ranges related to the structure of the molecule. These ranges form a kind of characteristic spectral fingerprint.
Carbon dioxide, for example, absorbs infrared light in different ranges than methane or nitrous oxide. An NDIR gas sensor is therefore designed to capture the absorption range of the required target gas as precisely as possible.
This gas-specific absorption is the basis for the selectivity of the measurement.
However, NDIR is not suitable for every gas. Under typical measurement conditions, molecules such as oxygen, nitrogen, and hydrogen do not have sufficiently suitable infrared absorption bands for conventional NDIR measurement. Other methods are required for these gases, such as paramagnetic, electrochemical, or thermal measurement principles.
What components make up an NDIR gas sensor?
At its core, an NDIR gas sensor consists of several coordinated components. Only their interaction enables stable and reproducible measurement.
1. Infrared light source
The light source generates the infrared radiation that is directed through the measurement chamber. It must provide sufficiently stable radiant power over a long period.
If the intensity of the light source changes as a result of ageing or temperature fluctuations, the measurement result may be affected. High-quality sensors account for such changes through reference measurements, signal processing, and suitable compensation methods.
2. Measurement chamber or optical gas path
Inside the measurement chamber, the infrared light comes into contact with the gas being analysed. The length of this optical path affects the sensitivity of the sensor.
A longer gas path generally increases the measurable absorption. This can be advantageous at very low concentrations. At the same time, the measurement chamber must be designed so that the gas can enter and leave reliably without creating unnecessary dead volumes.
Depending on the application, the gas reaches the sensor by natural diffusion or is actively supplied by a pump and sampling system.
3. Optical filter
The optical filter limits the wavelength range considered for the measurement. It is matched to the absorption band of the target gas.
Selecting a suitable filter is crucial because the absorption ranges of different gases can partially overlap. Background gases or water vapour may affect the measurement signal if the spectral design does not match the actual application.
4. Infrared detector
The detector measures how much infrared radiation remains after passing through the measurement chamber.
Without the target gas, a certain light intensity reaches the detector. When the absorbing gas is present in the measurement chamber, the measured intensity decreases. This difference produces the actual sensor signal.
Many systems also use a reference channel. It observes a wavelength range in which the target gas absorbs as little as possible or not at all. This makes it easier to identify and compensate for changes in the light source, contamination, or other optical effects.
5. Electronics and signal processing
The raw detector signal alone is not yet a reliable concentration value. The electronics amplify, filter, and process the signal.
The following factors, among others, may be taken into account:
- sensor temperature
- ambient pressure
- zero-point behaviour
- target-gas characteristic curve
- reference signal
- stored calibration data
The result is then transmitted to the higher-level system via an analogue or digital interface.
How does an NDIR measurement work?
The measurement is performed continuously or in short, regularly recurring cycles.
First, the infrared source sends light through the measurement chamber. The gas mixture inside the chamber absorbs some of the radiation. The detector then measures the remaining light intensity in the relevant wavelength range.
The electronics compare the measurement signal with stored reference and calibration data. This is used to calculate the current concentration of the target gas.
In practice, this calculation is more complex than a simple linear comparison. Absorption does not develop uniformly across every concentration range. The sensor characteristic must therefore be adapted to the intended measurement range.
A sensor for low concentrations in the ppm range requires a different optical and mathematical design than a sensor intended to measure a gas in the volume-percent range. The correct measurement range should therefore be defined when the measurement task is specified.
What are the advantages of NDIR technology?
NDIR gas sensors are used in many industrial applications because the optical measurement method offers several technical advantages.
High selectivity
By selecting a suitable wavelength range, an NDIR sensor can be tuned specifically to a particular gas.
However, selectivity does not depend on the target gas alone. The rest of the gas mixture must also be considered. Other infrared-active gases may cause cross-sensitivities.
Good long-term stability
In an NDIR measurement, the target gas is not consumed by a chemical reaction. The underlying measurement principle therefore does not continuously consume electrolyte or reagents.
Long-term stability nevertheless depends on the entire sensor design. The light source, detector, optics, electronics, and calibration must be coordinated.
Long service life
Because the optical method operates without a consumptive measurement reaction, NDIR sensors can be suitable for long-term industrial applications.
The actual operating conditions are decisive. Dust, aerosols, condensation, aggressive gas components, or strong temperature fluctuations can also place stress on an optical sensor.
Wide measurement ranges
Depending on their design, NDIR sensors can be used both for low concentrations and for measurements in the volume-percent range.
The measurement chamber, optical path length, filter, and calibration must match the required concentration range. The widest possible measurement range is not automatically the best solution. A clearly defined, application-specific range is often more accurate and makes more technical sense.
Continuous measurement
NDIR is suitable for the continuous monitoring of gas concentrations. It can detect not only individual threshold violations, but also changes over time and process trends.
This is particularly relevant for process control, quality assurance, and emissions monitoring.