Infrared analyzer type
Infrared analyzer type, there are many types of infrared gas analyzers currently used, and there are many classification methods. 1. From whether or not to convert infrared light into monochromatic light, it can be divided into two types: non-dispersive type (non-dispersive type) and split type (dispersive type).
2. From the optical system, it can be divided into two types: double light path and single light path.
3. Divide from the type of detector used.
4. From the number of detection components, there are two components, one component and multiple components.
This article mainly focuses on whether to convert infrared light into monochromatic light, which can be divided into two types: non-dispersive type (non-dispersive type) and split type (dispersive type).
The continuous spectrum emitted by the non-dispersive (NDIR) light source is all projected onto the sample to be tested. The component to be tested absorbs the infrared light of its characteristic absorption band. Since the component to be tested often has more than one absorption band, the detection of NDIR is performed. In terms of the way, it has an integral nature. Therefore, the sensitivity of the non-spectral type instrument is much higher than that of the spectroscopic type, and it has a high signal-to-noise ratio and good stability. The main disadvantage is that when there are overlapping absorption peaks between the components of the sample to be tested, it will cause interference to the measurement.
The fixed spectroscopic (CDIR) spectrometer uses a jacketed spectroscopic system to match the radiation spectrum of the measuring chamber to the characteristic absorption spectrum of the component to be tested. The advantage is that the selectivity is good and the sensitivity is high; the disadvantage is that the beam energy after splitting is small, and the slight displacement of any component of the spectroscopic system affects the wavelength of the splitting light. Therefore, spectroscopic instruments have been used in laboratories with good conditions and have not been used for online analysis for a long time. In recent years, with the widespread use of narrowband interference filters, spectroscopic instruments have begun to enter online analysis. However, the splitting of such a narrow-band interference filter is different from that of a grating system. It cannot form a continuous spectrum and can only strobe a narrow band near one or several specific wavelengths. Therefore, it is called a fixed beam type. (CDIR) instruments, unlike continuous spectroscopic instruments.
How infrared analyzer works
The working principle of the infrared analyzer is divided by the type of detector used. The detectors used in gas analyzers mainly include film capacitor detectors, micro flow detectors, photoconductive detectors, and pyroelectric detectors. According to the difference in structure and working principle, we can divide it into two categories, the first two are pneumatic detectors, and the latter two are solid detectors. The pneumatic detector operates on the pneumatic pressure difference, and the film vibration in the film capacitance detector is driven by this pressure difference, and the flow fluctuation in the micro flow detector is also caused by the pressure difference. This pressure difference is derived from the energy difference of the infrared radiation, which is formed by the measuring beam path and the reference beam path, so the pneumatic detector is generally used in conjunction with the dual beam path system.
Non-dispersive infrared (NDIR) is derived from the gas sealed in the pneumatic detector and the gas to be tested is the same (usually a mixture of gas and argon), so after the continuous radiation of the source spectrum reaches the detector, it only treats the gas. The spectrum of the characteristic absorption wavelength is sensitive, and good selectivity is obtained without splitting.
There are no movable parts in the photoconductive detector and the pyroelectric detector, and the detecting elements are solid devices, which we call a solid state detector according to this feature. The solid state detector is directly responsive to the infrared radiation energy and has no selectivity to the infrared radiation spectrum. The selectivity of the gas characteristic absorption spectrum to be measured is achieved by means of a narrow band interference filter. The optical system with which it is used is generally a single optical path structure, and the modulation of the relevant filter wheel forms a dual optical path in time. This infrared analyzer is a fixed spectrometer.
It can be seen from the above that the two types of detectors work differently, and the optical path system used has different structures. The two are different from the perspective of whether or not the light is required to be split. Therefore, it is possible to classify the infrared gas analyzer into two types: a non-dispersive infrared analyzer using a pneumatic detector and a fixed spectroscopic infrared analyzer using a solid detector.
Compared with the two types of instruments, the sensitivity and detection limit of the former are significantly better than the latter, while the latter is simple in structure, easy to adjust, small in size, and low in price. The former is a winding product of an infrared gas analyzer. It is also the current mainstream product. In the aspects of the whole working principle, optical system structure, signal modulation method and detection method, these two types of devices have significant differences.
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