Research of Fluorescence Wireless Detection System for SO2

Abstract. The environmental pollution increasingly caused people's concern， the concentration of sulfur dioxide and other gases in the atmospheric pollution has important effects on the human body.The paper theoretically studies the linear relationship between the concentration of sulfur dioxide in a certain concentration range and the intensity of the fluorescence based on the reality of monitoring SO2 gas from flue，and on this basis，designs the scheme of measurement about the concentration of SO2， researches a new method of detecting the concentration of gas that uses the wireless transmission of the light signals to online telemeter SO2 gas with high sensitivity.

Key words： Wireless gas sensor； Sulfur dioxide； Ultraviolet fluorescence method； Electricity circuit design

Atmospheric pollution is one of the problems concerned by every country in the world. It always brings too many damages to both the development of economy and natural environment.Especially，it is harmful to the human being’s health. SO2 is one of the most important components in atmospheric pollution.In China， it generates from the sulfurous fuel used by industrial and winter heating such as coal and oil.If we want to control and eliminate the pollution to the atmosphere caused by sulfur dioxide， the key is the precise detection of the concentration of sulfur dioxide.Among all methods for detecting sulfur dioxide， ultraviolet fluorescence method is popular in many countries， including our country， and also is promoted by Word Health Organization.

The fluorescence gas sensor has the following advantages：

（1） safe and reliable

（2） resistance to electromagnetic interference， high accuracy of measurement

（3） the large dynamic range， fast response， high sensitivity

At the same time fluorescence gas sensor can adapt to the harsh testing environment， be paid attention widely by both at home and abroad in recent years.

This article researches and designs a new wireless gas concentration measurement system to achieve online telemetry of SO2 gas with high sensitivity based on gas ultraviolet fluorescence detection technology.We design the scheme of measurement about the concentration of SO2 using the fluorescence sensor based on the linear relationship between the concentration of sulfur dioxide in a certain concentration range and the intensity of the fluorescence，which is the reality of monitoring SO2 gas from flue.The design idea of the overall system is：to set a light source， the requiredexciting light is acquired by the light source through a filter.The exciting light and the sample gas react in the fluorescence room and the fluorescence signal is produced， which is detected and converted to the electrical pulse signal through the photomultiplier tubes， the signal is transmitted by wireless technology， then measured and compared through the subsequent amplification filter circuit， alarm woll happen if it is beyond the normal range. This system has the following features：

（1）The system can realize real-time on-line detection of the concentration of so2 gas， and has a high sensitivity；

（2）The system has the high anti-jamming capability， can work safely and reliably in the harsh environments such as strong electromagnetic interference；

（3）The system uses wireless transmission， can realize remote sensing function；

（4）The system responses to change quickly；

（5）The designed of circuit，light path and gas path are modular， the work is stable， easy to repair and maintain.

1. Choice of the detection device of the light source

In the system， in order to guarantee the accuracy and reliability，the signal detection is the key.The light with certain wavelength emitted from the source to the fluorescent chamber， the photons interact with the sample gas， the optical signal with the information is transmitted to the light detector and converted into the electrical signal， this process that the information，which is needed be detected，is isolated by the signal processing unit is the premise of ensuring the system’s accuracy. The error of any link in the process will directly affect the final results of testing， so when the light path is designed，every link must be given sufficient attention，all kinds of devices must be strictly chosen.

The light source has an important influence to the system，is an important part of the sensor system，the sensor system and its important，directly affects the design of the light path and the acception and handle to the light signal.In the process of research， we give full consideration to the requirement of the system design， comprehensive consideration to the all kinds of information，such as cost， the width of the spectrum， the intensity and the quenching effect of the fluorescence’s efficiency.Through the test and comparison，the continuously work alternating current discharge Zn lamp is chosen， its spectral distribution can meet the demands of the system design，which is between 200-700 nm， and the light intensity is the largest at 213.8 nm.

Because the exciting light and the fluorescence in the system are both in the ultraviolet range，so the lens which the light through can't have strong absorption to the uv light， we use the lens made from quartz glass to gather more light.The function of the lens in front of the fluorescence room is to collect the exciting light to the fluorescent room as much as possible，the reaction can be sufficient；The function of the lens at the back is to gather more fluorescence to the photomultiplier tubes，convert the optical signals into the electrical signals as much as possible.

The opticalfilter and monochromator are used for filtering， the principles for selection are：

（1）The spectrum of the exciting light. We use Zn light，its spectral is distributed between 200nm to 700nm， and the light intensity is the largest at 213.8 nm；

（2）The fluorspar spectral lines of the used gas. According to the literature，the range of the fluorspar spectral of sulfur dioxide molecules is between 240nm to 420nm， and the light intensity is the largest at 213.8 nm.

At the same time，due to the presence of other gases in sample gas and the temperature and humidity of the actual environment，the fluorspar spectral distribution of SO2 will be affected by the environment， but it is still the strongestin in its fluorspar spectral range.Using filtering to reduce or even eliminate the influence of the stray light， can raise the strength of the ultra light and the fluorescent signal of SO2， improve the accuracy of measurement。

The choppesr used for the optical modulation periodically pass through and block the beam by five uniform interval light hole turntable drived by the motor with stable speed.In order to more efficiently collect the exciting light and the fluorescent light， a diaphragm of particular aperture is set in front of the reaction chamber.

The light reaction chamber is the core part， considering the cost and the accuracy of the system， its surface is made of common glass materials， all sealed with blackblackened everywhere except the vent.The quartz glass strips will be cohered on the face of the reaction chamber as the windows for incidence and emergence， in order to reduce the extinction of light， ensure to abtain the exciting light and the fluorescence as much as possible.

The photomultiplier tube has simple structure， its role is to detect the optical signal， and proceed photoelectric conversion，realize the internal amplification of the photoelectron signal through a series of double poles. In this process， the photoelectron signal is amplificated， a lot of noise is not attracted， the anti-jamming performance is high， the response is fast.

Because of the influence of the environmental factors，the fluorescent signal will be weak， so we need to choose the photomultiplier tubes with high sensitivity， at the same time the dark count rate of the photomultiplier should be low and the corresponding spectral band also should meet the requirements.

2. Air circuit system

Considering the characteristics of the sulfur dioxide gas in the circuit Air circuit design， all the vent lines use the polytetrafluoroethylene tubes which is antiseptic，acid-resisting and no absorption moisture， all the points use the standard metal joints， in order to prevent reaction with sulfur dioxide gas.The vents of the reaction chamber are designed at both ends， in order that the exciting light and the sample gas can fully react.

3. Circuit system

The detected signals will be sampled and amplified by the measuring circuit， which mainly includes the signal analysis circuit and the alarm circuit. The main function of the measuring circuit is to extract the required information from the weak pulse signal， and amplify it to drive the display， recorder， and alarm circuit. When the content of sulfur dioxide in flue gas is more than the standard of emission， the instrument can automatically give an alarm signal.

The measuring circuit mainly consists of amplifying， filtering， A/D sampling， I/O parts， etc.The luorescent sensors detect the change of the photomultiplier converts the electrical signals by amplifying circuit for amplifying and processing. As a result that the detection system usually works in the bad environment， the random interference into the power is inevitable， therefore， there should be automatic compensation link in the circuit design.

The large circuit of the system adopts the analog switch and general integrated op-amp， which has the advantages such as low drift， high input impedance， high common mode rejection ratio， high precision gain， simple debugging etc.In order to eliminate the effects of various kinds of high frequency and low frequency noise，the large ac signal should be filtered.The system uses the peak-peak sampling principle， the center frequency of the filter is the sinusoidal carrier frequency， and the band range should be narrow， the circuit design uses the infinite gain multiplex feedback band-pass filter.

The transmission of information uses the ZigBee wireless transmission technology，which is a kind of new technology between the wireless markup technology and the bluetooth technology，as a new wireless network technology of short distance， low power consumption， low data rate， low cost， low complexity， it has the advantages of power saving， simple， and low cost. The scheme of transmission ensures the low price， simple installation， convenient location， wireless transmission，achieves the automatic monitoring.

4. Compensation measures

（1）Temperature compensation

in the system the Photomultiplier tubes and the reaction chamber are closely connected together， in order to increase the degree of fluorescence， the reaction chamber must has a certain temperature， but the temperature of the reaction chamber directly affects the photomultiplier tubes， which will affect the accuracy of detection.We can preheat the sampling gas to solve this problem， in front of the reaction chamber obout 50 cm， the method not only ensures the temperature of the gas，but also avoids the affect to the photomultiplier tube，that ensure the accuracy of the system.

（2）Light compensation

The intensity of the exciting light probably fluctuates because of being affected by the factors such as time and voltage，it is the impact on the accuracy of the measurement.Changing the input voltage of Zn lamp can control the intensity of the exciting light， ensure that the volatility of the exciting light is within a reasonable range.

References

[1] Feng Ling，Yang Jingling， Cai Shuzhong.Development and application status of desulfurization technology for flue gas. Environmental Engineering.1997， 15 （2） ： 19-24

[2] Wang Shixiang， wang qin. Sulfur dioxide pollution and the control countermeasures in our country. Journal of Xin Yang Agricultural College.2008， 12 （3） ： 20-21

[3] David G Streets， Sarath K.G uttikunda， Gregory R.C armichael. ATMOSPHERIC ENVI ROMENT.34 （2005） ： 4425-4439

[4] Peng Dingyi，Lin Shaoning.The air pollution and how to control. Beijing， China Environmental Science Ppress. 1991：298-300

[5] Zhang Chengyun. Study of key techniques of uv fluorescence SO2 analyzer.[South China Normal University Graduate Dissertation].2005