1. The document discusses chemical sensors, providing definitions and explaining their basic working principles. Chemical sensors contain a receptor that interacts with analyte molecules and a transducer that converts this interaction into an electrical signal. 2. Chemical sensors can be classified by their operating principle and type of substance detected. Common types include optical, electrochemical, electrical, mass sensitive, magnetic, and thermometric sensors. Thermometric sensors like the catalytic sensor measure the heat of a chemical reaction. 3. The document provides examples of specific chemical sensors, such as the optical chemical pH sensor, coulometric oxygen sensor, and gas sensors for detecting carbon dioxide, carbon monoxide, and hydrogen.

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2. INTRODUCTION
 DEFINITION
 BRIEF EXPLANATION OF CHEMICAL SENSOR
 GENERAL WORKING PRINCIPLE
 CLASSIFICATION OF CHEMICAL SENSOR
 DISCUSSION OF THE TYPES OF CHEMICAL SENSOR
 SUMMARY/CONCLUSION
 QUESTIONS FROM THE PRESENTATION
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3. INTRODUCTION
Definition of the Chemical Sensor
• A chemical sensor is a device that transforms chemical
information (composition, presence of a particular
element or ion, concentration, chemical activity, partial
pressure.) into an analytically useful signal. The
chemical information, mentioned above, may originate
from a chemical reaction of the analyte or from a
physical property of the system investigated. They can
have applications in different areas such as medicine,
home safety, environmental pollution and many others.
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4. WORKING PRINCIPLE OF THE CHEMICAL SENSOR
• Chemical sensors usually contain two basic components
connected in series: a chemical (molecular) recognition system
(receptor) and a physicochemical transducer. In the majority of
chemical sensors, the receptor interacts with analyte molecules.
As a result, its physical properties are changed in such a way
that the appending transducer can gain an electrical signal.
Receptor: The function of the receptor is fulfilled in many cases
by a thin layer which is able to interact with the analyte
molecules, catalyze a reaction selectively, or participate in a
chemical equilibrium together with the analyte. The receptor
layer can respond selectively to particular substances or to a
group of substances.
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5. WORKING PRINCIPLE OF THE CHEMICAL SENSOR
The term molecular recognition is used to describe this behavior.
Among the interaction processes, the most important for
chemical sensors are adsorption, ion exchange and liquid-liquid
extraction. Primarily these phenomena act at the interface
between analyte and receptor surface.
Transducer: Nowadays, signals are processed almost exclusively
by means of electrical instrumentation. Accordingly, every
sensor should include a transducing function, i.e. the actual
concentration value, a non-electric quantity must be
transformed into an electric quantity, voltage, current or
resistance. Some of them develop their sensor function only in
combination with an additional receptor layer. In other types,
receptor operation is an inherent function of the transducer.
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6. DIAGRAM OF CHEMICAL SENSOR
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7. CLASSIFICATION OF CHEMICAL SENSOR
Chemical sensors may be classified according to:
 The operating principle of the transducer.
 The type of substance either, chemicl,biochemical or physical(
in terms of optical) it is sensing from its vicinity.
And they types include the folowing:
1. Optical devices transform changes of optical phenomena,
which are the result of an interaction of the analyte with the
receptor part.
2. Electrochemical devices transform the effect of the
electrochemical interaction analyte - electrode into a useful
signal.
3. . Electrical devices based on measurements, where no
electrochemical processes take place, but the signal arises from
the change of electrical properties caused by the interaction of
the analyte. CHEMICAL SENSORS 7

8. CLASSES OF THE CHEMICAL SENSOR
4. Mass sensitive devices transform the mass change at a specially
modified surface into a change of a property of the support
material.
5. Magnetic devices based on the change of paramagnetic
properties of a gas being analysed. These are represented by
certain types of oxygen monitors.
6. Thermometric devices based on the measurement of the heat
effects of a specific chemical reaction or adsorption which
involve the analyte. for example in the so called catalytic
sensors the heat of a combustion reaction or an enzymatic
reaction is measured by use of a thermistor.
7. Other physical properties as for example X-, p- or r- radiation
may form the basis for a chemical sensor in case they are used
for determination of chemical composition.
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9. THERMOMETRIC SENSOR(CATALYTIC SENSOR)
• A thermometric sensor can be set up easily by coating
the surface of a thermometer with a catalytic layer. If
the catalysed reaction has a considerable heat effect,
then the reaction heat is preferably released locally at
the active surface.Thermistors are micro thermometers
useful as a basis for thermometric chemical sensors.
They mainly consist of a semiconductor body with a
temperature-dependent conductivity. As an example, a
hydrogen sensor is created by coating a thermistor
with a thin layer of black platinum Hydrogen traces in
the air burn in the catalytic area. The reaction heat
causes the temperature to rise
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10. • The resulting temperature difference compared to
ambient temperature can be measured in terms of
resistance change. This difference depends on the
hydrogen content in the air. Other combustible gases
like hydrogen sulphide or carbon monoxide can be
analysed by means of the same arrangement, but with
different catalysts. It is essential to find a catalyst as
selective as possible.Thermistors are available in
numerous sizes and shapes. Common forms are balls
with diameters as small as 0.1mm or thin films on a
substrate.
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11. CATALYTIC SENSOR(PELLISTOR)
• A catalyst is a chemical or substance that increases the rate of a
reaction without being itself consumed.Heat is liberated as a result of
a catalysed reaction.The temperature related to the chemical reaction
is measured, using a calorimeter.Catalytic sensors are widely used to
detect to detect low concentrations of flammable gases.
• Pellistors are used to detect the presence of flammable gases.Any
combustible gases present will oxidise on the catalyst bead, raising
the temperature of the coil.The change in resistance is detected by
comparing with an uncatalysed reference sensor.
CHEMICAL SENSORShttp://www.citytech.com/technology/pellistors.asp
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12. Pellistor operating Modes.
CHEMICAL SENSORS
• The platinum coil is embedded in a ceramic pellet coated with a porous
catalytic metal (palladium or platinum). This coil acts as both the heater
and temperature sensor (like in the Mass Flow Controller). When the
combustible gas reacts at the catalytic surface, the heat evolved
increases the temperature inside the thermal shield.
• This is raises the temperature of the platinum coil and thus its resistance
• www.ipc.uni-tuebingen.de/weimar/pictures/chem_sensor.gif
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13. • Pellistors have two operating modes:
• Isothermal, where an electronic circuit controls the current in the coil
required to maintain constant temperature.
• Non-isothermal, where the sensor is connected as part of a
wheatstone bridge whose output voltage is a measure of the gas
concentration.Catalytic sensors are widely used in the industries for
detectation of flammable gases.
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14. OPTICAL CHEMICAL SENSORS.
• Optical sensors represent a group of chemical sensors in
which electromagnetic (EM) radiation is used to generate the
analytical signal in a transduction element. The interaction of
this radiation with the sample is evaluated from the change of
a particular optical parameter and is related to the
concentration of the analyte.
• Typically, an optical chemical sensor consists of a chemical
recognition phase (sensing element or receptor) coupled with
a transduction element. The receptor identifies a parameter,
e.g., the concentration of a given compound, pH, etc., and
provides an optical signal proportional to the magnitude of
this parameter. The function of the receptor is fulfilled in
many cases by a thin layer that is able to interact with the
analyte molecules, catalyse a reaction selectively, or
participate in a chemical equilibrium together with the
analyte. The transducer translates the optical signal produced
by the receptor into a measurable signal that is suitable for
processing by amplification, filtering, recording, display, etc.
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15. • The figure below shows a schematic of the basic components of an optical
chemical sensor, namely, the sample (analyte), the transduction platform, and
signal processing element (electronics) leading to the optical signal
measurement which is related to the analyte concentration.
CHEMICAL SENSORS
Principal stages in the operation of a Chemical pH Sensor.
Optical sensors can be based on various optical principles (absorbance, reflectance,
luminescence, fluorescence), covering different regions of the spectra (UV, Visible)
and allowing the measurement not only of the intensity of light, but also of other
related properties, such as lifetime, refractive index, scattering, diffraction and
polarization.
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16. • As an example, a luminescent sensor can be constructed by associating a sensing
element, which emits light when in contact with a specific analyte, with a
photodiode, which converts the energy of the incident light into a measurable signal.
• OPTICAL DETECTION PRINCIPLES
For sensor applications only part of spectroscopic wavelength range is useful. From the
practical point of view the following ways (Figure below) in which radiation can
interact with an analytical sample are the most useful.
• • absorption
• • emission (fluorescence or phosphorescence)
• • reflexion and refraction
CHEMICAL SENSORS
Fig. General arrangement of
spectroscopic measurements: A
– light reflection, B – light
refraction, C – light absorption,
D – light emission.
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17. • Basically, Optical sensors employ optical transduction techniques and are
based on reagents that change their optical properties on interaction with the
analyte of interest. The most commonly measured optical properties are
absorption, fluorescence intensity, and decay time, but in addition,
reflectance, refractive index, light scattering, and light polarization have also
been used as analytical parameters.
A very good example of the Optical Chemical Sensor is the Optical
Chemical PH Sensor.
OPTICAL CHEMICAL PH SENSOR
• The Optical chemical pH sensors was developed in the last 2 years (August
2011−August 2013). Optical Chemical pH sensors could be reagent-based,
namely, absorption- and fluorescence-based, as most optical pH sensors uses
of colorimetric or fluorescent indicator dyes.
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18. • In more specific terms, pH is of concern in life sciences, food and beverage
processing, soil examination, and marine and pharmaceutical research to
name a few. Thus the development of an optical pH sensor which can be
applied in real world applications is not trivial.
WORKING PRINCIPLE OF THE OPTICAL CHEMICAL pH SENSOR.
• Free hydrogen ions do not exist in aqueous solution and should be described
as hydronium ions H3O+. The hydrogen ion is very small and possesses very
high charge density. These characteristics promote a reaction with a water
molecule resulting in very strong association. Very often, the term hydrogen
ion or proton is used, but it has to be kept in mind that this is only an
accepted convention. Søren Sørensen,(a Danish chemist) defines pH as the
negative logarithm of hydrogen ions concentration.
• Today, the pH of a solution is defined in terms of a hydrogen ion H+ activity
(sometimes called protons, or more correctly hydronium ions, H3O+):
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19. • pH = −log aH+
where aH+ is the activity of the hydrogen ions.
The negative sign assures that pH of most solutions is
always positive.
Optical pH sensors exploit pH indicator dyes which are
typically weak organic acids or bases with distinct
optical properties associated with their protonated
(acidic) and deprotonate (basic) forms. The absorption
(color) or fluorescence properties of these dyes are
modified with a change of concentration of the hydrogen
ions (pH). A schematic representation of absorption- and
luminescence-based sensing is shown in the Figure
below.
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20. CHEMICAL SENSORS
• Schematic representation of the principle of (A) absorptionbased and (B) fluorescence-based pH
sensing mechanisms.
Basically, the working Principle of the Optical Chemical pH Sensors is passing Light
through the Analyte, then depending the the pH Concentration of the Analyte, we
will have different Degrees of Absorption and Fluorescence of Light which are given
as Signals. These Signals are then Processed and Amplified and digitally displayed
on a Screen. A Chemical Parameter (pH Concentration) has been Converted to an
Electrical Signal.
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21. Coulometric Oxygen Sensors
• A Coulometric oxygen sensor is system that
employs two chambers with a specimen
mounted as a sealed semi-barrier between them.
• One chamber contains oxygen while the other is
slowly purged with a stream of a carrier gas
such as nitrogen.
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22. Mechanism of the Coulometric Oxygen sensor
• As the oxygen gas permeates through the
specimen into the carrier gas, it is transported to
the coulometric detector where it creates an
electric current with a magnitude that is
proportional to the number of oxygen atoms
flowing into the detector
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23. • The solid state coulometric oxygen analyzer measures oxygen
concentration by “counting” the number of electrons flowing
through its circuit. Coulometric oxygen sensor is not consumed
when it is exposed to oxygen, it does not require a constant
purge to protect the sensor when the analyser is not being used
and its lifetime is not dependant on how much oxygen it is
exposed to. Coulometric oxygen sensors are suitable when
oxygen concentrations are low in the gas to be measured.
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24. Summary of testing Method
CHEMICAL SENSORS
• The specimen is mounted as a sealed semi-
barrier between two chambers at ambient
atmospheric pressure. One chamber is slowly
purged by a stream of nitrogen and the other
chamber contains oxygen. As oxygen gas
permeates through the film into the nitrogen
carrier gas, it is transported to the coulometric
detector where it produces an electrical current,
the magnitude of which is proportional to the
amount of oxygen flowing into the detector per
unit time. Among that, oxygen gas is the testing
gas and nitrogen gas is the carrier gas. Oxygen
gas concentration of upper chamber is higher
than that of lower chamber, due to which certain
concentration difference is formed between two
sides of specimen. During the permeability
process, oxygen gas transmits from upper
chamber through specimen into lower chamber.
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25. Instrument Calibration
• The oxygen sensor used in this test method is a
coulometric device that yields a linear output as
predicted by Faraday's Law. In principle, four
electrons are produced by the sensor for each
molecule of oxygen that passes into it. Experience
has shown under some circumstances the sensor
may become depleted or damaged to the extent
that efficiency and response are impaired. For that
reason, this test method incorporates means for a
periodic sensor evaluation. standard film is needed
in instrument calibration. Since the data of
standard film can directly influence the
determination of calibration coefficient Q, special
attention should be paid to film preparation and
calibration repeatability.
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26. GAS SENSORS
• Definition
 Gas sensor is a subclass of chemical sensors.
• -Gas sensor it a subclass chemical sensor that
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.
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27. 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
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28. Disadvantages:
 Low gas sensitivity due to the limited surface-
to-volume ratio
 Bulky or very heavy.
 Consume lots of power in order to increase the
sensitivity ( 500 degree)
 Require “risky” high voltage to operate.
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29. Types of gas sensors
• Carbone dioxide gas sensor
• Carbone monoxide gas sensor
• Hydrogen gas sensor
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30. 1-Carbone dioxide (CO2) gas sensor
• it is based on infrared light absorption (by
CO2) principle.
 The infrared detector detects the infrared light
which is not absorbed by CO2 between source
and detector. And then measures the heat
produced by the non absorbed light. A voltage
is produced due to the increasing of
temperature in the infrared sensor.
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31. 2- carbone 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. 100ppm).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.
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32. Types of carbone monoxide gas sensors
 Semiconductor sensor
 Electrochemical sensor
 Digital sensor
 Biomimetic sensor (chem-optical or gel cell
sensor
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33. 3- hydrogen gas sensor
• Palladium is commontly used to detect
hydrogen because palladium selectively
absorbs hydrogen gas and forms the chemical
palladium hydride.
• Types of hydrogen gas sensor:
 Optical fiber hydrogen sensors
 Nanoparticle-based hydrogen microsensors
 Diode based sensor
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34. GAS SENSING TECHNOLOGIES
 Metal Oxide Based Gas Sensors
 Capacitance Based Gas Sensors
 Acoustic Wave Based Gas Sensors
 Calorimetric Gas Sensors
 Optical gas sensors
 Electrochemical gas sensors
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35. NEW TECHNOLOGIES IN GAS SENSOR
• Advanced technology today by using nanomaterials offers
possibility to improve gas detection. we can have the new
carbon nanotube with has one of the best surface-of-volume
ratio which is very important for hight sensitivity fast
response and low temperature.
• The recent progress in developing MEMS (Micro-Electro-
Mechanical Systems) leads to a new 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 micro H2 gas sensor that has several unique
advantages in terms of speed, sensitivity, stability and
amenability to large scale manufacture.
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36. The new flexible carbon nano tube
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37. Some Companies involved in gas sensor fabrication and prices
• RAE system: offers different types of sensors
and prices depends on the performance of the
sensor. ex QRAEII which can senses hydrogen
sulfide oxygen and carbon( 176 pound),carbon
monoxide and oxygen(132 pound) ,oxygen(88
pound per unit)..
• Barharach
• crowncon
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38. SUMMARY/CONCLUSION
DEFINITION
BRIEF EXPLANATION OF CHEMICAL
SENSORS
GENERAL WORKING PRINCIPLES
CLASSIFICATION OF CHEMICAL
SENSORS
SUMMARY/CONCLUSION
QUESTIONS FROM THE
PRESENTATION
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39. QUESTIONS FROM THE PRESENTATION
1. DEFINE CHEMICAL SENSORS
2. OUTLINE ANY 5 OF THE TYPES OF
CHEMICAL SENSORS WE HAVE
3. EXPLAIN ANY ONE OF TYPES OF THE
CHEMICAL SENSOR.
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40. •THANK YOU
FOR GOOD
LISTENING
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