Radiation in the infrared (IR) part of the electromagnetic spectrum is usually absorbed by gas molecules. When the wavelength of the IR corresponds to the natural resonances or frequencies of the molecules, the energy states of atoms moving in the molecules vary significantly. In other words, the atoms’ vibrations increase when stimulated by a suitable wavelength of light. A collective absorption of IR radiation effectively increases the temperature of gas.
In case the molecules’ chemical structure is complex, there will be a series of resonant vibrations. This is usually presented as a range of absorption peaks plotted against wavelength known as absorption spectrum. Figures 1, 2 and 3 show some of the gas absorption spectra.
Figure 1. Water infrared spectrum
Figure 2. Carbon-dioxide infrared spectrum
Figure 3. Methane infrared spectrum
IR reacts with the gas only when a dipole on the molecule is created. Molecular dipoles can occur when the vibration modes are non-symmetrical or when atoms in the molecules are organized in a non-symmetrical fashion. The vibrations comprise bending and stretching of the molecules that deform them to produce dipole or multi-pole moments. This is shown in Figure 4 for carbon dioxide (CO2).
Figure 4. Dipole and multi-pole moments of CO2
Molecules that are symmetrical are not stimulated by IR as their modes of vibration and structures fail to produce any net dipoles. Some examples of symmetrical molecules include the binaries O2, H2 and N2. CO2 is an excellent example of a symmetrical molecule having non-symmetrical and symmetrical modes of vibration (Figure 4) and hence it absorbs IR. Hydrocarbons, in particular, are active in the IR mainly through C-H stretching modes. When the number of C-H bonds is greater, the absorption lines will be stronger, most of which combine into bands.
Absorption spectra of IR gases display a range of fine line structures and broad peaks, and cover a wide range between 2.0 to 20µm. These aspects, rather like separate fingerprints, are characteristics of the gases. The wavelength positions of the absorption spectra help in identifying the gases, whilst their heights offer data from which the concentration of gas can be measured. An apparatus used to plot this type of spectra is called 'dispersive' and serves as a gas spectrometer.
Majority of dispersive IR instruments available in the market are large, costly and fragile. They can only be used in laboratories and are not effective at detecting and monitoring gases in field applications. As a result, only a few dispersive IR gas-sensing solutions are used in dangerous locations.
In many IR gas sensing applications, gas spectrometry is not usually carried out as the nature of the target gases are already known. Although users might accept some amount of cross-sensitivity between different types of gases when their absorption lines overlap, the concentration of the target gas would still need to be measured. A non-dispersive infrared (NDIR) sensing method is suitable to address these needs.
In the NDIR technique, fixed narrow-band filters are utilized with separate IR detectors to detect a few gas absorption lines across a restricted wavelength range. As a result, low-cost sensor parts can be used, and compact but robust sensor packages can be equipped into the instruments. This way, the concentration of gas can be obtained in real time from easy algorithms employed in the microprocessor of the instrument.
Choice of Sensing Wavelengths
The choice of sensing wavelengths is controlled by the existing output range of IR sources and also the need to work within the 'water windows'. In Figure 1, the water absorption spectrum displays strong absorptions less then 3µm, from 5-8µm and beyond 16µm. If an effort is made to detect the gas spectral lines in these regions, it would be subject to strong interference if humidity is present in the target gas. Hence, it is better to operate in the 8-16 micron or 3-5 micron windows where a number of practical gas lines exist. Here, the 3-5 micron window is selected for the following reasons:
- There are useful absorption lines at 3.0 to 3.5µm for hydrocarbon sensing and at 4.2µm for CO2 sensing (Figures 2-3)
- IR lamps with glass envelopes emit to 5µm.
- No gas absorption lines are there at 4.0µm, allowing a reference signal to be obtained at this wavelength
For gas sensing in the region further than 8µm, a costly IR source with an IR transmissive and sealed window is needed and hence this approach is not quite popular.
The Beer-Lambert Law for NDIR Gas Absorption
Figure 5 illustrates a typical set up for detecting gas by NDIR and includes an IR source, IR active detector, an absorption cell with gas admission and reflective surfaces, and IR reference detector with neutral filter for background monitoring.
Figure 5. Schematic diagram of NDIR gas sensor
Given that pyroelectric detectors react to fluctuations in light level, the IR source had to be modulated. At 1 to 10Hz pulse rate, the detectors’ output is sinusoidal with a peak-to-peak AC voltage proportional to the intensity of the IR incident on the detector. In case an IR-absorbent gas enters the cell, the IR intensity on the active detector will reduce in accordance with an exponential relationship known as the Beer-Lambert Law:
I = lo exp(- K L C)............................................................(1)
From this law, it is clear that the gas concentration C can be measured.
The Modified Beer-Lambert Law and Normalized Absorbance
The Beer-Lambert expression is suitable when a practical gas-sensing device is used. However, efforts should be made to reduce variances through an improved build standard. This can be achieved by using a fit-for-purpose sensor and reliable components. In order to obtain reliable performance from sensor-to-sensor, strong optical and mechanical alignment is important.
Effects of Ambient Pressure and Temperature
If ambient temperature and pressure are fluctuating, the concentration of gas can be determined using the ideal gas law: C = k P V / T. Corrections to the algorithms can be made, if the system is set up with separate pressure and temperature sensors. The ideal gas law however will not consider any effect introduced by the sensing system itself.
Although an IR gas sensor can detect temperature by absorbing radiation, it also reacts easily to ambient temperature changes and this can result in false signals. By using apposite components and reliable build standards, the sensor responses to temperature can be estimated which is near to linear over the -40 to +75°C range. This makes it considerably easy to offset for in software after the characteristics of the sensor are completely understood. Also, a separate temperature sensor is needed to effect the compensation. The IRIxxx series 2 and the IR15T series of e2v IR gas sensors are integrated with temperature sensors.
Benefits of NDIR Technique
The NDIR technique offers the following benefits:
- Not affected by hazardous chemical environments
- Unlike catalytic sensors, NDIR gas sensors do not suffer from any poisoning effects
- Hydrogen is not detected, which eliminates cross-sensitivity
- Gases can be easily detected in anaerobic conditions
- CO2 is detected sans interference from other gases
- No sensor burn-out or deterioration upon exposure to high gas concentrations, or if monitoring gases for long period of time
- Stable and long-term operation needs minimum recalibration
- Stable even after extended storage
- Low cost of ownership when compared to catalytic sensors
Non-dispersive infrared (NDIR) is a physical sensing technique that provides a number of advantages over catalytic gas sensing method. The main feature of this technique is that fixed narrow-band filters are utilized with separate IR detectors to detect a few gas absorption lines across a restricted wavelength range. This helps in eliminating the dispersive potion of a gas spectrometer. The simple design of the NDIR gas sensor makes it easy for the sensor package to meet certifiable safety standards.
About SGX Sensortech (IS)
SGX Sensortech is a market leader in innovative sensor and detector devices that offer unrivalled performance, robustness and cost- effectiveness.
SGX have been designing and manufacturing gas sensors for use in industrial applications for over 50 years, offering excellent applications support for an extensive range of gas sensors and the expert capability for custom design or own label.
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- Oil and gas
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This information has been sourced, reviewed and adapted from materials provided by SGX Sensortech (IS) Ltd.
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