Transcript Electrochemical Sensing Elements

Electrochemical Sensing
Elements
Muhajir Ab Rahim
School of Mechatronics
Northern Malaysia College University of Engineering
Flashback…
4 types of element in measurement system
1.
2.
3.
4.
Sensing element
Signal Conditioning element
Signal Processing element
Data Presentation element
Sensing element
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1st element in measurement system , which
in contact with, and draw energy from, the
process or system being measured
Classified by physical principle involved (refer
Table 8.1 in text book)
Electrochemical Sensing Element
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What kind of input we want to measure?
How it works?
Advantages and disadvantages?
Type of input measured by
electrochemical sensing element
1.
2.
Ionic concentration
Gas composition
Application
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Medical- to measure O2, glucose, CO2
Water Treatment- to measure pH, Cl2, O2
Industrial safety- to detect toxic gases
Military- to detect explosive agent
1. Ionic Concentration Measurement
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Use Ion Selective Electrodes (ISEs) as a
sensor
Reaction between the charged species
(selective single ion) in the immersed
solution and those on sensor surface will
establish equilibrium potential difference.
The measurement of e.m.f. of ISEs is based
on Nernst equation
Ion- Selective Electrodes (ISEs)
High input
impedance meter or
buffer amplifier
RISE
Ion- selective
electrode
Test
Solution
Reference
electrode
ZIN
RTH
RREF
Meter or
amplifier
2. Gas Composition Measurement
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The detection method varies depending on
the type of sensor used
Zirconia is a solid state material which gives
electrochemical response to oxygen
Semiconductor based gas sensor is the most
popular gas sensor applied in industry
TGS-813 Gas Sensor
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The working principle is based on the properties of
the elements Iridium (Ir) and Palladium (Pd)
The sensor element is essentially an assembly of a
heater coil buried in SnO2. The heater coil is made of
Ir and Pd.
When the current flows in the heater coil at a given
temperature, the oxygen takes away electrons from
the donor at the surface of the semiconductor
element, creating a potential barrier.
TGS-813 Gas Sensor (cont…)
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Because the potential barrier makes the movement
of the free electrons more difficult, the electrical
resistance of the element increases.
However, the sensitivity of the sensor can be
affected by the ambient temperature and humidity.
Normally, requires 2-3 minutes of warm-up time, and
the resistance of element rises by 20% after the
warm-up time.
Chemically Sensitive Field Effect
Transistors (CHEMFET)
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The basic principle underlying the MOSFET is the
control of a current flowing between two
semiconductor electrodes. Drain and source are
placed on the same element, with a third electrode,
the gate, between them. The metallic gate electrode
is insulated against drain and source by means of
silicon dioxide and can only influence the drainsource current electrostatically. The extremely high
input resistance of the gate electrode means that no
large input power is required to control this current.
Chemically Sensitive Field Effect
Transistors (CHEMFET) cont….
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The MOSFET’s gate consists of a metallic coating
and is used as an electrode to control the drainsource current through the external potential Vgs. In
the case of the ISFET the metallic gate is replaced
by a special oxide-coated gate, that is sensitive to
certain ion concentration. When immersed in a
liquid, the electrical circuit Vgs is closed with the
reference electrode and the certain ion concentration
in solution can influence the drain-source current
Useful Interactive Link
http://www.sensedu.com/