2.1 What are the physical fundamentals of IR gas analysis?
The molecular vibrations of many chemical compounds can be excited by energies that lie in the infrared wavelength range. Therefore, these substances absorb infrared radiation. The absorption spectrum depends mainly on the structure of the molecule and thus the degrees of freedom for the movement of the molecular components, their mass, their compositions, their spacing and their binding forces. Therefore, each substance has a characteristic absorption spectrum. For example, in carbon dioxide molecules can be excited bending and stretching vibrations. Additionally rotational movements around different molecular axes are possible, which superpose the vibrational spectrum and generate their fine structures.
2.2 How does a gas analyzer with an infrared spectrometer (IR spectrometer) work?
An IR spectrometer measures the absorption spectrums of gases. The comparison with the spectrums that are stored in a database allows a qualitative and quantitative reference for the substance.
2.3 How does non-dispersive infrared gas analysis (NDIR gas analysis) work?
For this form of gas analysis, the spectral sensitivity of a broadband thermal detector is limited by an optical bandpass filter. This is done for the range in which the absorption bands are found for the gas to be determined. With the thermal detector, the transmission of the measured gas mixture is determined in a defined arrangement. If the gas being searched for is not present, most of the infrared radiation will reach the detector and the signal will be at its maximum. If the concentration of the gas increases, absorption will also increase according to Lambert-Beer's law, and the signal will reduce accordingly.
2.4 What advantage does the non-dispersive infrared gas analysis (NDIR gas analysis) have?
Compared to infrared spectrometers, the NDIR gas analysis is significantly cheaper. However, their use is only possible if the gases to be measured are known, and their number is low.
2.5 Which gases are not suitable for analysis with non-dispersive infrared gas analysis (NDIR gas analysis)?
Noble gases consist only of individual atoms. In order to vibrate, however, at least one bond is required. Noble gases can therefore not be detected with this method. In diatomic elemental gases such as oxygen (O2) or nitrogen (N2), only a few vibrational modes can be excited by infrared radiation, so that even here the method fails.
2.6 What are the main components of an NDIR gas analyzer?
An NDIR gas analyzer consists of an electrically or mechanically modulated infrared source, a gas cell and usually a pyroelectric detector. An electronic device calculates the gas concentration based on signal voltage. InfraTec offers a large number of standard IR narrow bandpass filters (NBP), which are optimally matched to the absorption properties of the gases to be measured. The filters are mounted in the cap of the detector, which is welded to the detector base body. With multiple gases to be measured, the use of multi-channel detectors is recommended.
2.7 Are there advantages of having an absorption-free reference channel for non-dispersive infrared gas analysis (NDIR gas analysis)?
Yes, for NDIR gas analyzers, it is advisable to use a reference channel and normalize the signal of the gas duct via the quotient process on this reference. The optical, mechanical and electronic drift of the overall system is reduced considerably, and the interval between calibrations can be significantly extended. The spectral position of the optical reference should be located as close as possible to the spectral lines of the gases to be measured. A reference channel can be shared for multiple gases, if all absorption bands lie within a spectral window from 3 to 5 µm or from 8 to 12 µm. Without an optical reference channel, a reference can still be done by periodically introducing a reference gas into the channel, such as nitrogen.
2.8 How does an infrared flame sensor work?
The pyroelectric detector of the flame sensor detects the typical spectral radiance of burning organic materials such as wood, natural gas, oil or plastic. In order to prevent a false alarm due to sunlight or other intense light sources, such as light from arc welding, two independent criteria of a flame are analyzed: First a typical flame is characterized by a flicker frequency of 1 to 5 Hz. Secondly, a hydrocarbon flame contains the combustion gases carbon monoxide (CO) and carbon dioxide (CO2). Their emission bands lie in the infrared spectral range of 4.0 to 4.8 µm. In order to obtain a high signal, one uses wide bandpass filters for the detector window, which include both the radiation emission of CO and of CO2. Optionally, a further channel can be used to recognize a further combustion by product, water.
2.9 Can pyroelectric standard detectors by InfraTec detect a flame distance up to 100 m reliably even without additional optics?
Yes, especially InfraTec’s pyroelectric detectors in current mode with a chip size of (2 x 2) mm2 or (3 x 3) mm2 have a high signal-to-noise ratio and a very small popcorn noise. This allows for reliable detection.
2.10 Can pyroelectric detectors be used without integrated reinforcing elements in gas analyzers and flame sensors?
The charges generated in the pyroelectric crystal are very low, requiring a preamplifier with extremely high input impedances of up to some 10 GΩ. Even at normal humidity of about 60 % at an ambient temperature of 22 °C, such preamplifier circuits no longer work without interference. Therefore, it must be inside the sealed detector housing. Due to the necessary reinforcement, detectors without integrated junction field effect transistors (JFET) or operational amplifier (OpAmp) are hardly suitable for gas analysis or flame sensors.