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).
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Figure 1. potential barrier
(in the absence of gases)
Figure 2. Model of
inter-grain potential barrier
(in the presence of gases)
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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
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