14 Şubat 2014 Cuma

Arsalan Bayatmakoo (030100920), Gas sensor(TGS) in English



                                                                   
 



                                                               Gas Sensor (TGS)
      
         We are facing with a different type of pollutants and gases in our circumstance , which
Causes environmental degradation, greenhouse effect, ozone layer destruction and poisoning .
Sensor the device which be used for distinguish these gases and chemicals since 1970 . In gas
Sensors industry  the  signature technology is the metal oxide semi conductor type gas sensor. 
Taguchi gas sensor (TGS)  is the most common one .


Operation Principle : 
              
       Metal oxide (generally SnO2)  is a sensing material in TGS gas sensors . When a metal oxidecrystal  ( like SnO2) is heated at high temperature in air, oxygen is adsorbed on the crystalsurface with a negative charge. “Then  electrons in the surface are transferred to the adsorbedoxygen, resulting in leaving positive charges in a space charge layer”. Thus, surface potential is formed to serve as a potential barrier against electron flow .(Figure 1).
                                                                                                                  
 
Figure 1.  potential barrier
    (in the absence of gases)





                     Figure 2. Model of inter-grain potential barrier
                                     (in the presence of gases)












  
                   
  Figure 3.  (reaction between CO and SnO2)




        Current flow through the grain boundary of  SnO2 micro crystal. The absorbed
oxygen form Potential barrier at that grain boundaries which prevents carriers from
moving freely.The electrical resistance of the sensor is attributed to this potential barrier. In the presenceof a deoxidizing gas, the surface density of the negatively charged oxygen decreases,so  barrier height   is reduced (Figures 2 and 3).
 

                 
           Rs = A[C] ^ -a

 Rs = electrical resistance of the sensor
A = constant
[C] = gas concentration
a = slope of Rs curve


Signal processing :


         When VRi get bigger than Vref (we set Vref amount before ) then the comparator 
will activate  the LED .However most of the time using microprocessor is desirable.


                                               

References :

http://laitec.ir/wp-content/uploads/2013/10/78.pdf
http://www.sos.sk/a_info/resource/c/figaro/digest_cata.pdf


1 yorum:

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