Infrared gas online analyzer

Infrared is an invisible light with a wavelength range of 0.78-1000 micrometers. It is named infrared because it is outside the red light boundary. Infrared radiation can be divided into three parts, namely near-infrared radiation, with a wavelength between 0.75 and 1.50 μ m; Mid infrared, with a wavelength between 1.50-6.0 μ m; Far infrared radiation with a wavelength between 6.0 and 1000 μ m.

Wavelength - The distance between two adjacent light waves in the same phase point in the direction of light propagation is called wavelength.

Wave number - Wave number is a parameter that describes infrared radiation, referring to the number of infrared waves contained per centimeter length.

Frequency - the number of cycles of light wave vibration per unit time.

Photon energy - the energy emitted, propagated, or received by light waves in the form of radiation, represented by E and measured in J.

Characteristic absorption wavelength - In the near-infrared and mid infrared bands, infrared radiation energy is small and cannot cause transitions in the electronic energy levels of molecules. It can only be absorbed by sample molecules, causing transitions in the vibrational energy levels of molecules. Therefore, infrared absorption spectroscopy is also known as molecular vibrational spectroscopy. When the energy of infrared radiation at a certain wavelength is exactly equal to the difference in energy between the vibrational energy levels of a certain molecule, it will be absorbed by that molecule and produce a corresponding vibrational energy level transition. This wavelength is called the characteristic absorption wavelength of that molecule.

　　2.1Infrared gas analyzerThe basic principle of

Its working principle is based on the selective absorption of infrared radiation by certain gases. The commonly used infrared wavelength for infrared analyzers is 2-12 µ m. Simply put, the gas to be tested is continuously passed through a container of a certain length and volume, and a beam of infrared light is incident from one of the two transparent end faces of the container. Then, the radiation intensity of the infrared light is measured on the other end face, and the concentration of the gas to be tested can be determined based on the proportional relationship between the absorption of infrared light and the concentration of the absorbing substance.

Lambert Beer law - its physical meaning is that when a beam of parallel monochromatic light passes vertically through a uniformly non scattering absorbing substance, its absorbance is proportional to the concentration of the absorbing substance and the thickness of the absorbing layer. This is the measurement basis of the infrared gas analyzer.

2.2 Characteristics of Infrared Gas Analyzer

1. Capable of measuring multiple gases

In addition to single atom inert gases and diatomic molecular gases with symmetrical structures and no polarity, inorganic substances such as CO, CO2, NO, NO2, NH3, as well as alkanes, alkenes, and other hydrocarbons and organic compounds such as CH4 and C2H4 can be measured using infrared analyzers;

2. Wide measurement range

The upper limit of gas analysis can reach 100%, and the lower limit can reach concentrations of several ppm. After fine processing, trace analysis can also be performed;

3. It has high monitoring sensitivity and can distinguish even small changes in gas concentration;

4. High measurement accuracy

Generally, it is within+/-2% FS, and many products achieve+/-1% FS. Compared with other analytical methods, it has higher accuracy and better stability;

5. Quick response time

The response time is generally within 10 seconds

6. Has good selectivity

Infrared analyzer has a high selectivity coefficient, making it particularly suitable for measuring a certain component to be analyzed in multi-component mixed gases. Moreover, when the concentration of one or several components in the mixed gas changes, it does not affect the measurement of the component to be analyzed.

2.3 Basic Structure and Main Components of Infrared Analyzer

Infrared gas analyzer generally consists of two parts: gas path and circuit. The connecting component between the gas path and circuit is also the core part, which is the transmitter. The transmitter is the "heart" part of the infrared analyzer, which converts changes in the concentration of the measured component into changes in certain electrical parameters and outputs them as voltage or current through the corresponding circuit. The transmitter consists of two parts: an optical system and a detector. The main components include an infrared radiation source, an air chamber and filter element, and a detector

Measuring principle

One is the measurement room, and the other is the reference room. The two rooms open and close the light path simultaneously or alternately at a certain period through a light cutting plate. After introducing the measured gas into the measurement chamber, light with the wavelength of the measured gas is absorbed, thereby reducing the amount of light passing through the measurement chamber and entering the infrared receiving chamber. The higher the gas concentration, the less light flux enters the infrared receiving chamber; The luminous flux passing through the reference chamber is constant, and the luminous flux entering the infrared receiving chamber is also constant. Therefore, the higher the concentration of the measured gas and sensitivity, the greater the difference in luminous flux between the measurement chamber and the reference chamber. This difference in luminous flux is projected onto the infrared receiving chamber with a certain amplitude of periodic vibration. The receiving chamber is divided into two halves by a few microns thick metal film, and the chamber is sealed with a high concentration of the measured component gas. Within the absorption wavelength range, all the incoming infrared rays can be absorbed, thereby converting the pulsating luminous flux into a periodic change in temperature. Then, the temperature change can be converted into a pressure change according to the gas equation, and a capacitive sensor can be used to detect it. After amplification processing, the measured gas concentration is indicated.

2.4 Main components of transmitter

light source

According to the structure of the light source, it can be classified into two types: single light source and dual light source. According to the classification of luminescent materials, there are mainly the following types: alloy luminescent sources, ceramic luminescent sources, and laser luminescent sources

Cut light slice

The function of the cutting plate is to turn the infrared light from the radiation source into intermittent light, that is, to modulate the infrared light. The purpose of modulation is to convert the signal generated by the detector into an AC signal, which facilitates amplifier amplification and improves the response time characteristics of the detector.

air chamber

The gas chamber in an infrared analyzer includes a measuring gas chamber, a reference gas chamber, and a filtering gas chamber. Their structures are basically the same, all cylindrical, and both ends are sealed with chips. The gas chamber requires high inner wall smoothness, no absorption of infrared radiation, no adsorption of gases, and stable chemical properties. The material of the chamber is brass plated with gold, glass plated with gold, or aluminum alloy, and the inner wall surface requires polishing. The chemical properties of gold are extremely stable, and the inner wall of the gas chamber never oxidizes, so it can maintain a high reflection coefficient. The commonly used window materials for gas chambers include: lithium fluoride transmission limit of 6.5 μ m, calcium fluoride transmission limit of 13 μ m, sapphire transmission limit of 5.5 μ m, fused quartz transmission limit of 4.5 μ m, and sodium chloride transmission limit of 25 μ m. The reference chamber and the filtering chamber are sealed and non removable. The measuring chamber may be contaminated, so rubber sealing should be used. Attention should be paid to maintenance and regular replacement. Dust, dirt, and fuzzing on the chip can cause sensitivity reduction, measurement error, and zero drift to increase. Therefore, it is necessary to keep the chip clean, which can be wiped with mirror paper or silk cloth. Be careful not to touch the surface of the chip with your hands.

optical filter

A filter is an optical filtering element. It works based on various optical phenomena such as absorption, interference, selective reflection, polarization, etc. The use of a filter can change the radiation energy and spectral composition of the measurement chamber, eliminate or reduce the influence of scattering and interference components absorbing radiation, and allow infrared radiation with characteristic absorption wavelengths to pass through. Interference filter is a bandpass filter made based on the interference phenomenon that occurs when light passes through a thin film. Interference filter can obtain a narrow passband, and its transmission wavelength can be adjusted by the refractive index, thickness, and layer of the coating material.

detector

Thin film capacitance detector, semiconductor detector, microflow detector.

The working principle and characteristics of thin film capacitor detection

Thin film capacitor detector, also known as thin film microphone, is a capacitor composed of a metal thin film moving electrode and a fixed electrode. When the gas pressure in the receiving chamber changes due to the influence of infrared radiation energy, it pushes the moving plate of the capacitor relative to the fixed plate, converting the change in concentration of the measured component into a change in capacitance

Features: Low impact of temperature changes, good selectivity, and high sensitivity. The disadvantages are that the thin film is susceptible to mechanical vibration, the modulation frequency cannot be increased, the amplifier production is difficult, and the volume is relatively large.

The working principle and characteristics of semiconductor detectors

A semiconductor detector is made using the principle of semiconductor photoelectric effect. When infrared light is irradiated onto a semiconductor, it absorbs photon energy, causing a change in the electronic state and generating free electrons or holes, resulting in a change in conductivity, that is, a change in resistance value. Therefore, it is also known as a photoelectric conductivity detector or a photoresistor.

Features: Simple structure, easy manufacturing, small size, long lifespan, and rapid response. Higher modulation frequencies can be used to make amplifier production easier. It can be used in conjunction with narrowband interference filters to create a versatile and fast response infrared detector. When changing the measurement components, only the wavelength of the interference filter and the instrument scale need to be changed. Its disadvantage is that indium antimonide is greatly affected by temperature changes.

Principle and characteristics of microflow detector

Microflow detector is a new type of detection device for measuring small gas flow rates. Its sensing element consists of two micro hot wire resistors and two additional auxiliary resistors forming a Wheatstone bridge. The hot wire resistor is electrified and heated to a certain temperature. When gas flows through, it takes away some of the heat to cool the hot wire, causing the resistance to change and be converted into a voltage signal through an electric bridge.

Features: Low price, reduced size of optical system, improved reliability and vibration resistance.

2.5 Structure types of infrared gas analyzer

It can be divided into two types based on whether infrared light is converted into monochromatic light: dispersive type (dispersion type) and non dispersive type (non dispersion type).

The advantages of spectral type: good selectivity and high sensitivity; The disadvantage is that the energy after splitting is small, and any slight displacement of any component in the splitting system can affect the wavelength of the split light.

The advantages of non dispersive type include high sensitivity, high signal-to-noise ratio, and good stability. The disadvantage is that overlapping absorption peaks between the components of the sample to be tested can interfere with the measurement.

According to the optical system, it can be divided into two types: dual path and single path

Two parallel infrared beams are emitted from two identical light sources or a precisely allocated light source, and enter the detector after passing through the analysis chamber and reference chamber with the same geometric light path.

A single beam of infrared light emitted from a light source passes through only one geometric optical path. However, for the detector, it still accepts two infrared beams of different wavelengths, which only arrive at the detector at different times. It uses the rotation of the tuning disk to modulate the light emitted by the light source into infrared beams of different wavelengths, which are alternately sent to the detector through the analysis chamber, achieving dual optical paths in time.

Divided by the types of detectors used, there are currently mainly thin film capacitive detectors, semiconductor detectors, and microfluidics detectors.

2.6 Main content and requirements for calibration of infrared gas analyzer

Phase balance adjustment adjusts the axis position of the cutting plate to be at the symmetrical point of the two infrared light beams. It is required to simultaneously block or leak two light sources with the cutting film, which is called synchronization, so that the light area of the two light paths acting on the windows on both sides of the detector room is equal.

Adjustment of optical path balance: Adjust the eccentric shading film on the reference optical path to change the luminous flux of the reference optical path, so that the optical energy of the measurement and reference optical paths is equal.

Zero point and range calibration are performed by using zero point gas and range gas respectively, and the instrument's zero point and range are repeatedly calibrated.

2.7 Common faults and solutions

There are many types of infrared gas analyzers, and their faults and solutions are also different. The following table lists some common faults and their solutions for reference:

Optical path imbalance interference:

When a preheated infrared gas analyzer is filled with nitrogen gas, the output is very large due to the phase imbalance of the cutting plate and the optical path imbalance. Therefore, simply adjust the phase adjustment knob to make the output small, and then adjust the optical path balance knob to make the output minimum. Then calibrate the zero point and range of the instrument repeatedly with the zero point gas and range gas.

Moisture interference:

If there is moisture in the zero point gas, the calibration of the infrared gas analyzer will cause negative errors. In the near-infrared region, water has a continuous characteristic absorption spectrum. If the zero point gas used for calibration contains moisture, it will cause a negative bias of the instrument's zero position. After calibration, the instrument's displayed value will inevitably be lower than the actual value, resulting in negative errors.

Interference caused by temperature changes:

The detection process of infrared gas analyzer needs to be carried out at a constant temperature. Changes in environmental temperature will directly affect the stability of infrared light sources, the intensity of infrared radiation, the density of gas samples flowing continuously in the measurement chamber, and the normal operation of detectors. If the temperature significantly exceeds the normal state, the output impedance of the detector will decrease, causing the instrument to malfunction and even damaging the detector. Infrared analyzers generally have temperature control devices and over temperature protection circuits inside. However, even so, some instruments, especially trace analysis instruments, can still observe the impact of environmental temperature changes on detection, especially during high ambient temperatures in summer. In this case, changing the ambient temperature and setting up air conditioning is a solution.

Interference from atmospheric pressure fluctuations:

Atmospheric pressure can vary even within the same region and on the same day. If the weather suddenly changes, the magnitude of the change is significant. The change in atmospheric pressure has a direct impact on the flow rate of gas sample venting. The gas sample directly released after measuring the gas chamber will cause an additional error in the density of the gas sample in the chamber due to changes in atmospheric pressure.

　　

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