GAS SENSORS
Prepared by:
Jiturvi Chokshi
ENPM-808B
Gas sensors
Introduction:
Sensor:
 A sensor is a technological device that
detects / senses a signal, physical condition
and chemical compounds.
 It is also defined as any device that converts
a signal from one form to another.
Sensors are mostly electrical or electronic.
Gas sensors
Introduction:
Sensor:
Applications
Automobiles
Machines
Robotics
Industry
Medicine
Aerospace etc.
Gas sensors
Introduction:
Sensor:
Examples of sensors
Thermometer
Thermocouple
Phototransistor
Photo resistor
Microphone
Seismometer
Hydrophone etc.
Gas sensors
Introduction:
Gas Sensor:
Gas sensor is a subclass of chemical
sensors.
Gas sensor measures the concentration of
gas in its vicinity. Gas sensor interacts with a
gas to measure its concentration. Each gas
has a unique breakdown voltage i.e. the
electric field at which it is ionized. Sensor
identifies gases by measuring these voltages.
The concentration of the gas can be
determined by measuring the current
discharge in the device.
Gas sensors
Applications of Gas Sensor:
Process control industries
Environmental monitoring
Boiler control
Fire detection
Alcohol breath tests
Detection of harmful gases in mines
Home safety
Grading of agro-products like coffee and
spices
Gas sensors
Operating parameters:
Operating temperature
Operating humidity
Disadvantages:
Bulky
Consume lots of power
Require “risky” high voltage to operate.
Gas sensors
Gas sensing technologies:
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Metal Oxide Based Gas Sensors
Capacitance Based Gas Sensors
Acoustic Wave Based Gas Sensors
Calorimetric Gas Sensors
Optical gas sensors
Electrochemical gas sensors
Gas sensors
Gas sensing technologies:
1. Metal Oxide Based Gas Sensors

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Metal oxide sensors are also known as
chemiresistors.
The detection principle of resistive sensors
is based on change of the resistance of a
thin film upon adsorption of the gas
molecules
on
the
surface
of
a
semiconductor.
The gas-solid interactions affect the
resistance of the film because of the
density of electronic species in the film.
Gas sensors
Gas sensing technologies:
2. Capacitance Based Gas Sensors
 They measure the change in dielectric
constant of films between the electrodes
as a function of the gas concentration.
 The capacitive sensor relies on interdigitated electrode structures, which
correspond to the two plates of a standard
capacitor, to monitor changes of the
dielectric coefficient of the film.
Gas sensors
Gas sensing technologies:
2. Capacitance Based Gas Sensors (Contd.)

The simple theory behind it is if the
dielectric constant of the film is lower than
that of the analyte, the capacitance will
increase and vice versa.
Gas sensors
Gas sensing technologies:
3. Acoustic Wave Based Gas Sensors

Sound based gas sensors are known as
acoustic wave based gas sensors.

To launch the acoustic waves, this type of
sensor use piezoelectric material either in
the thin film form or in bulk form which has
one or more transducers on its surface.
Gas sensors
Gas sensing technologies:
3. Acoustic Wave Based Gas Sensors (Contd.)

Then type of acoustic wave generated and
device resonant frequency has been
determined.

Depending on that, it is possible to measure
properties, processes, or chemical species in
the gas phase, liquid phase, vacuum or thin
solid films.
Gas sensors
Gas sensing technologies:
4. Calorimetric Gas Sensors
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The principle of calorimetric gas sensors
based on change in temperature at
catalytic surfaces.
It consists of a surface of a film of a
catalytically active metal (e.g. Platinum,
Palladium or Rhodium) .
It burns combustible gases. Heat is
generated due to the combustion.
This heat is balanced by a reduction in the
electrical heating power. Thus the power
consumption indicates the concentration of
gas.
Gas sensors
Gas sensing technologies:
5. Optical gas sensors
Following methods are used :

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Ellipsometry (Technique for the
investigation of the dielectric properties)
Spectroscopy (luminescence,
phosphorescence, fluorescence, Raman)
Interferometry (white light Interferometry,
modal Interferometry in optical waveguide
structures)
Gas sensors
Gas sensing technologies:
5. Optical gas sensors (Contd.)
In these sensors a desired quantity is
determined by:


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Refractive index (Speed of the light)
Absorbance and
Fluorescence properties (of the analyte
molecules or a chemo-optical transducing
element.)
Gas sensors
Gas sensing technologies:
5. Optical gas sensors (Contd.)
Gas sensors
Gas sensing technologies:
6. Electrochemical gas sensors
It consists of:
 Chemical reactants (electrolytes or gels)
 Two terminals (an anode and a cathode)

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Anode is responsible for oxidization
process and cathode is responsible for
reduction process.
As a result, current is created.
Positive ions flow to the cathode and the
negative ions flow to the anode.
Gas sensors
Gas sensing technologies:
6. Electrochemical gas sensors (Contd.)

We can find reducible gases (such as
oxygen, nitrogen oxides and chlorine) at
the cathode and oxidizable gases (carbon
monoxide, nitrogen dioxide, and hydrogen
sulfide) at the anode.

The output is directly proportional to the
concentration or partial pressure of the
gaseous species.
Gas sensors
Discussion on some gas sensors:

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Carbon monoxide (CO) gas sensor
Carbon dioxide (CO2) gas sensor
Hydrogen gas sensor
Gas sensors
1. Carbon monoxide gas sensor
 It can either be battery-operated or AC
powered.
 Mostly the sensor will not sound an alarm
at lower concentrations (e.g. 100 ppm).
The alarm will sound within a few minutes
at 400 ppm. So the function is specific to
concentration-time. Figure shows simple
carbon monoxide sensor.
Gas sensors
1. Carbon monoxide gas sensor (Contd.)
Carbon monoxide sensor can be of
different types such as:

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Semiconductor sensor
Electrochemical sensor
Digital sensor
Biomimetic sensor (chem-optical or gel cell
sensor)
Gas sensors
2. Carbon dioxide (CO2) gas sensor
 CO2 absorbs infrared light therefore CO2
sensor consists of a tube containing an
infrared source at one end and an infrared
detector at the other end.
 The infrared detector detects the infrared
light which is not absorbed by CO2
between source and detector.
Gas sensors
2. Carbon dioxide (CO2) gas sensor (Contd.)
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Infrared radiation which is not being
absorbed by CO2 produces heat so the
temperature will increase.
The infrared detector measures the
temperature.
A voltage is produced due to the
temperature increase in the infrared
sensor.
We can read amplified voltage into the
data logger.
Gas sensors
3. Hydrogen gas sensor

Mostly palladium is used to detect hydrogen
because palladium selectively absorbs
hydrogen gas and forms the chemical
palladium hydride.
Types of hydrogen gas sensor:

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Optical fiber hydrogen sensors
Nanoparticle-based hydrogen microsensors
Diode based sensor
Gas sensors
3. Hydrogen gas sensor (Contd.)
Development:
 There is a recent progress in developing
MEMS
(Micro-Electro-Mechanical
Systems) based H2 gas sensors. These
sensors couple novel thin films as the
active layer with a MEMS structure known
as a Micro-Hotplate. This coupling results
in a H2 gas sensor that has several unique
advantages in terms of speed, sensitivity,
stability and amenability to large scale
manufacture. Preliminary results are
extremely encouraging and suggest that
this technology has substantial potential for
meeting the sensing requirements of a
hydrogen based energy economy.
Gas sensors
LITERATURE CITED:
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http://scholar.google.com/scholar?hl=en&lr=&q=ga
s+sensor
http://en.wikipedia.org/wiki/Gas_sensor
http://en.wikipedia.org/wiki/Hydrogen_microsensor
http://vernier.com/probes/co2-bta.html
http://sensorstransducers.globalspec.com/LearnMore/Sensors_T
ransducers_Detectors/Gas_Sensing/Gas_Sensors
http://etd.ncsi.iisc.ernet.in/handle/2005/281
Proceeding of the 2000 Hydrogen Program Review,
NREL/CP-570-28890
http://www.gasdetection.com/TECH/sensorprincipl
e2.htm
http://physicsworld.com/cws/article/news/17869