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12-1 Chemical Sensors *
* 12-1
Detect presence and concentration of chemicals.
Two types will be discussed:
- Humidity.
Measure humidity in air.
- Oxygen.
Measure amount of oxygen in air.
Model functions will not be given for any transducer described in
this set of lecture notes.
12-1 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-2 Humidity Transducer *
* 12-2
Two common types, capacitance and resistance.
Construction and Operation, Capacitance Type
Transducer is a capacitor.
Capacitor's dielectric is exposed to environment.
The dielectric's permittivity changes with humidity : : :
: : :and so the capacitance changes with humidity.
12-2 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-3 *
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Construction and Operation, Resistance Type
Transducer consists of leads placed on a special material.
Water from the air adsorbs onto the surface.
This causes a change in resistance, which is measured.
Output of both types are highly temperature dependent.
Sensors usually sold with integrated conditioning circuits.
12-3 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-4 Oxygen Sensors *
* 12-4
Construction and Operation
Consists of zirconium oxide, ZrO2 , between platinum electrodes.
One side exposed to reference gas : : :
: : :other side exposed to test gas.
Gases come in contact with both platinum and zirconium oxide.
On both sides O2 adhere to zirconium oxide's surface.
Reference gas has a known concentration of oxygen.
Test side exposed to gas being measured.
Test gas oxygen concentration determined from electrode voltage.
12-4 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-5 Measurement of Potential in Fluids *
* 12-5
In many biological and chemical sensors : : :
: : :a voltage between two liquids is measured.
As with thermocouple measurement : : :
: : :this is more difficult than it might seem.
Difficulty
A voltage will develop between a metal wire : : :
: : :and the liquid in which it is dipped.
This voltage depends upon the liquid : : :
: : :which can be of unknown composition.
However, the voltage between : : :
: : :a reference electrode and the liquid is zero : : :
: : :avoiding the problem.
Reference electrodes will be used to:
- Measure potentials in a solution, for example a neuron firing.
- Measure the presence of dissolved chemicals.
12-5 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-6 Electrodes in Fluid *
* 12-6
Consider the following circuit:
Current flow in metal is by the motion of electrons, as usual.
Current flow in the solution is by the motion of ions.
Point of interest is the interface between the metal and the solution.
A chemical reaction is occurring in which an electron is transfered to or
from the metal, from or to the solution.
The reaction has a certain energy, which is responsible for the voltage
between the electrode and solution.
12-6 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-7 *
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The reaction might:
- Result in accretion of material on the electrode.
- Result in the eroding of material from the electrode.
- Leave the electrodes unchanged.
The first two cases occur in a battery and in electro-plating : : :
: : :the third case occurs in the electrolysis of water.
12-7 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-8 *
* 12-8
Reversible Electrodes
These should not be confused with reference___ electrodes.
An electrode is reversible if the chemical reaction can occur either way,
depending upon the direction of current flow.
The AgjAgCl Reversible Electrode
Reaction: Ag + Cl AgCl + e .
Current flows in solution by motion of chlorine ions, Cl .
12-8 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-9 *
* 12-9
In forward direction current flows from electrode to solution (elec-
trons flow from solution to electrode).
Chlorine ion gives electron to electrode, then combines with a neutral sil-
ver atom to form AgCl, which deposits on surface of electrode.
In reverse direction current flows from solution to electrode (elec-
trons flow from electrode to solution).
AgCl on electrode splits into silver and chlorine, an electron from elec-
trode ionizes the chlorine.
The reaction energy depends upon the concentration of the reac-
tants.
In measurement problems, these may be unknown.
12-9 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli1*
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12-10 Reference Electrode *
* 12-10
In a reference electrode current can flow with a zero potential drop.
Construction:
A reversible electrode is placed in a small container with a solution of
known concentration.
The container and its contents are the reference electrode.
The reference electrode is placed in test solution.
The container is sealed except for a tiny plug.
The plug is usually filled with gel, fibers, or a tightly packed powder.
12-10 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli*
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12-11 *
* 12-11
Operation:
A small amount of ions can flow through the plug.
Flow is small enough so that concentration of solution in reference elec-
trode does not change by a significant amount.
Plug is large enough so that current-carrying ions will not be impeded.
Solution in reference electrode is chosen so that there will be no potential
across the plug.
For example, consider KCl, which is frequently used.
Both potassium, K+ , and chlorine ions, Cl , will flow through plug.
Materials were chosen because their flow rate is such that their poten-
tials cancel.
Therefore, reference electrode potential does not depend upon test solu-
tion.
12-11 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli*
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12-12 Microelectrodes For Biological Use *
* 12-12
Used for measurement of biological potential.
A type of reference electrode.
Construction:
Reference electrode container in shape of tapered tube.
Fat end holds the reversible electrode and lead.
Thin end has the hole. Hole is open, there is no gel or other plug.
Hole diameter about 1 m.
Thin end can be inserted into an individual cell.
12-12 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli*
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12-13 Measurement of Ion Concentration *
* 12-13
Sensor measures concentration of ions dissolved in water.
Construction:
Consists of a container divided into two halves by a membrane.
In one half is a solution of known composition.
In the other half is the test solution.
Membrane will allow ion being measured to cross, but will not allow oth-
ers to cross.
A reference electrode is placed in each half, the voltage across the elec-
trodes are measured.
12-13 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli*
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12-14 *
* 12-14
Operation
Test ions cross membrane, causing a potential to develop across the mem-
brane.
The potential will eventually stop the flow of ions.
The strength of the potential is proportional to ion concentration.
The potential is measured by the reference electrodes.
12-14 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli*
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12-15 Measurement of pH *
* 12-15
The pH of a solution is the negative log of hydrogen ion, H+ , con-
centration.
Construction:
Reference electrode and membrane are combined to form probe.
Probe is used with a second, conventional, reference electrode.
Membrane, which must be permeable only to protons, is glass.
Operation:
Hydrogen ions (protons) cross membrane until potential impedes their
flow.
Potential is measured at reference electrode leads.
Because of reference-electrode properties, the voltage across leads is same
as voltage across membrane.
Because of glass membrane (which has a high resistance) conditioning
circuit must have a high input impedance.
12-15 EE 4770 Lecture Transparency. Formatted 8:14, 26 February 1999 from lsli*
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| David M. Koppelman - koppel@ee.lsu.edu | Modified 26 Feb 1999 8:15 (14:15 UTC) |