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Detection of mixed gases

- Dec 20, 2017 -

When a mixture of gases is treated as a pure substance, the density of the mixed gas used in the reduced molar mass μ and the equivalent gas constant R is equal to the sum of the product of the density of each constituent gas at the total pressure and the temperature of the mixed gas, that is, there are many different methods for detecting the gas concentration, which are widely used in the following ways:

(1) Electrochemical method: The use of electrical parameters such as resistance or conductivity of the material with the change of gas concentration to make gas sensors. The two electrodes are inserted into the electrolyte at the same time, and the electrons are transferred by reaction, which produces the current change and the reaction concentration.

(2) Solid gas method: The heater will be prepared into beads or flaky metal oxides heating, oxygen on their surface agglomeration, change the resistance value, reaction concentration of information changes.

(3) Catalytic combustible gas: The resistance value varies with the temperature on the screw ring, the reaction concentration information, the balance of the bridge, and the form of electrical signals.

(4) Infrared absorption method: Based on Lamberbille law, the concentration-related information is expressed by means of gas characteristic absorption.

Infrared hybrid gas detection has been widely used in many fields, including atmospheric chemical analysis, industrial process Control, agricultural production management, urban environmental quality testing, life science research and so on.

In the detection of hybrid gas based on infrared spectroscopy, the optical multiplexing structure is the core part of the detection system, which is both the medium of infrared light transmission and the main place of gas absorbing infrared energy. According to the difference of light conversion process, the infrared spectrum detection technology can be divided into two kinds: direct detection and indirect detection. In the ndir spectrum technique, a broadband light source is used to obtain the approximate monochromatic light which is matched with the gas absorbing characteristic by selecting the filter element with different central wavelength and bandwidth, and then directly detected by the detector.

The detector usually integrates the filter element, including the action channel and the reference channel, and the signal of the channel output is calculated with the reference signal respectively to eliminate the influence of the light source, the detector instability and the outside interference. The selection and control of the light source, the selection of the filter parameters and the matching degree of the light source and the detector are all worthy of further study in the ndir spectrum technology. Danilova and so on in the experiment has proved that the proper reduction of driving current pulse duty ratio can improve the luminous efficiency of light source. It is advantageous to determine the optimal parameters of gas absorption by selecting the filter element rationally. In order to avoid the Kam noise brought by the infrared light source in low modulation frequency (less than 10Hz), the high modulating frequency LED light source and the detector with low time constant can be used to improve the detection speed of the whole system and reduce the noise.

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