Infrared Gas Sensors exploit the property that many gases absorb radiation in the 2-14 micron, infrared region of the spectrum. These spectral absorbance show features which may be regarded as 'fingerprints' to identify the gases and enable their concentrations to be deduced. The sensor bodies contain an infrared source and infrared detectors inside a compact and combined gas cavity/ optical cell. The detectors have infrared band pass filters placed in front, which tune them to the specific gases to be sensed. When the specific gas enters the cavity it is registered as a change in detector signal. The magnitude of this change is related to the concentration of that gas via a simple exponential formula.By utilizing different infrared filters a range of gases can be sensed and discriminated with these devices.
In cases where spectral lines overlap, then an individual sensor may show cross sensitivities to a gas range. Infrared gas sensors are very robust devices not affected by contact with a harsh chemical environment. Any changes in ambient conditions such as temperature are compensated for in software. Their dimensions and power requirements are compatible with and complementary to pellistor gas sensors. After over thirty years of successful manufacture of pellistor-based flammable gas sensors, the range of Non-Dispersive Infra-red (NDIR) gas sensors represents the first of many diversifications into other areas of gas sensor technology.
In a molecule, absorption or emission of energy can occur in transitions between different energy levels. These transitions can be associated with changes in the vibrational energy and changes in the rotational energy of the molecule. Such internal energies are quantized, so that the molecule can exist only in certain discrete vibrational and rotational energy levels. The energy related to transitions between vibrational energy levels is equivalent to radiation in the near infra-red region of the electromagnetic spectrum. Each vibrational level is associated with a set of rotational levels, which results in several closely spaced energy levels existing within a frequency band in the infra-red spectrum of the molecule. The fundamental frequencies at which the bands exist are functions of the particular bond and the mode of vibration, e.g. stretching or bending. When a molecule is exposed to infra-red radiation with an energy equivalent to a vibrational transition, the radiation is absorbed and the molecule undergoes the transition. This absorption is used as the means to determine the amount of target gas molecules present.
The NDIR technique uses a broad-spectrum source, such as a filament lamp, to expose the gas to a wide range of infra-red frequencies. An associated detector is fitted with an optical filter such that it can only monitor the intensity of a certain narrow frequency band. This frequency band is selected to match a frequency band within the absorption spectrum of the target gas and the detector output is therefore affected by the concentration of the target gas. The frequency of radiation, for our purposes, is more often expressed in terms of its wavelength, as the two terms are directly related
In cases where spectral lines overlap, then an individual sensor may show cross sensitivities to a gas range. Infrared gas sensors are very robust devices not affected by contact with a harsh chemical environment. Any changes in ambient conditions such as temperature are compensated for in software. Their dimensions and power requirements are compatible with and complementary to pellistor gas sensors. After over thirty years of successful manufacture of pellistor-based flammable gas sensors, the range of Non-Dispersive Infra-red (NDIR) gas sensors represents the first of many diversifications into other areas of gas sensor technology.
In a molecule, absorption or emission of energy can occur in transitions between different energy levels. These transitions can be associated with changes in the vibrational energy and changes in the rotational energy of the molecule. Such internal energies are quantized, so that the molecule can exist only in certain discrete vibrational and rotational energy levels. The energy related to transitions between vibrational energy levels is equivalent to radiation in the near infra-red region of the electromagnetic spectrum. Each vibrational level is associated with a set of rotational levels, which results in several closely spaced energy levels existing within a frequency band in the infra-red spectrum of the molecule. The fundamental frequencies at which the bands exist are functions of the particular bond and the mode of vibration, e.g. stretching or bending. When a molecule is exposed to infra-red radiation with an energy equivalent to a vibrational transition, the radiation is absorbed and the molecule undergoes the transition. This absorption is used as the means to determine the amount of target gas molecules present.
The NDIR technique uses a broad-spectrum source, such as a filament lamp, to expose the gas to a wide range of infra-red frequencies. An associated detector is fitted with an optical filter such that it can only monitor the intensity of a certain narrow frequency band. This frequency band is selected to match a frequency band within the absorption spectrum of the target gas and the detector output is therefore affected by the concentration of the target gas. The frequency of radiation, for our purposes, is more often expressed in terms of its wavelength, as the two terms are directly related
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