Topic-3 : Basic transducer principles & Electrodes

COMPILED BY: PROF G B RATHOD
EC department-BVM College,
Email: ghansyam.rathod@bvmengineering.ac.in
BASIC TRANSDUCER PRINCIPLES &
ELECTRODES
Prof.G.B.Rathod, EC Dept.BVM-EC453

THE TRANSDUCERS AND TRANDSDUCTION
PRINCIPALES
 A variable is any quantity whose value changes
with time. A variable associated with
physiological process of the body is called a
physiological variable.
 A transducer is required to convert each variable
into an electrical signal which can be amplified
or otherwise processed and converted into some
form of display.
 The device that performs the conversion of one
form of variable into another is called a
transducer.
 Here we discuss the transducer which is having
input some other quantity and output will be
electrical quantity.

THE TRANSDUCERS AND TRANDSDUCTION
PRINCIPALES
 Two different principles are involved in
the process of converting nonelectrical
variables into electrical signals.
 One of these is energy conversion:
Transducers based on this principle is
called active transducers.
 Second of these is control of an
excitation voltage or modulation of a
carrier signal. Transducers based on
this principle is called a passive
transducers.

ACTIVE TRANSDUCERS

ACTIVE TRANSDUCERS
 Some conversion principles are describe here
 Magnetic Induction: if an electrical conductor
is moved in a magnetic field in such a way that
the magnetic flux through the conductor is
changed, a voltage is induced which is
proportional to the rate of change of magnetic
flux.
 Basically two basic configuration are used
using this concept: one is linear motion and the
other is rotary motion.
 The applications are heart sound microphones,
pulse transducers and electromagnetic blood
flow meters.

ACTIVE TRANSDUCERS
 The natural materials in which this piezoelectric
effect can be observed are primarily slices from
crystals of quartz(SiO2) or Rochelle salt(sodium
potassium tartrate).
 The piezoelectric process is reversible. If an
electric field is applied to a slab of material that has
piezoelectric properties, it changes its dimensions.
 We will see equivalent circuit of the piezoelectric
transducer connected to an amplifier.
 The piezoelectric principal is also used in
ultrasonic instruments.

ACTIVE TRANSDUCERS
Equivalent circuit

PASSIVE TRANSDUCERS
 Passive transducers utilize the principle of
controlling a dc excitation voltage or an Ac
carrier signal.
 There are only three passive circuit elements
that can be utilized as a passive transducers.
Resistors, capacitors, and Inductors.
 Passive Transducers Using Resistive Elements:
Special linear potentiometer can be used to
convert displacement into a resistance change.

PASSIVE TRANSDUCERS

PASSIVE TRANSDUCERS
 Passive transducer using Capacitive elements.
 The capacitance of the capacitor can be
changed by varying the physical dimension of
the plate structure or by varying the
dielectric contestant of the medium between
the plates. E.g., the capacitance
plethysmography.
 Passive Transducers Using Active Circuit
Elements. Transistor , Photoelectric
transducers

TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS
 Pressure Transducers: Pressure transducers are closely
related to force transducers. Force summing members
used in pressure transducers are shown in figure.
 We can use the strain gage also for designing of such
pressure transducers.
 Diaphragm type pressure transducers can be designed for a
wide range of operating pressures, depending on the
diameter and stiffness of the diaphragm, bourdon tube
transducers are usually used for high pressure ranges.
 In differential pressure transducers the two pressure are
applied to opposite side of the diaphragm.

APPLICATIONS

APPLICATIONS
 Flow Transducers: For fluids and gases flow rate
measurements, the methods are described in upcoming
topics.
 Transducers with Digital Output: ADC can be used to
convert analog signal to digital output for analog
transducers.
 For digital output, specially design encoding disks are
to be used in the process of the conversion from the
transducers circuit. Usually photo diodes or photo
transistors related circuit are used for the digital out
put data.

Electrode Theory
 Electrodes: Devices that convert ionic potentials
into electronic potentials are called electrodes.
 The interface of metallic ions in solution with
their associated metals results in an electrical
potential that is called the electrode potential.
 At the equilibrium, the double layer charge
produce with opposite sign.
 The hydrogen is taken as a reference electrode in
international agreement. The other potentials are
taken by taking hydrogen as a reference electrode.
The electrodes potentials for variety of other
electrodes are listed in table.

Electrode Theory

Electrode Theory
 When the ionic movement occurs and the new potential developed at the membrane, the
value of that potential can be found out by Nernst Equation.
 Where R=gas constant
 T = absolute temperature, degrees kelvin
 n=valence of the ion
 F=Faraday constant
 C1,C2 = two concentrations of the ion on the two sides of the membrane
1 1
2 2
ln
C f
RT
E
nF C f
 

Electrode Theory
 f1,f2=respective activity coefficients of the ion on the two sides of
the membrane
 This above f1 and f2 are depend on such factors as the charges of
all ions in the solution and the distance between ions.
 The product of C1f1 of concentration and its associated activity
coefficient is called the activity of the ion responsible for the
electrode potential.
 The metal-electrolyte interface developed and the potential
generated.

Biopotential electrodes
 Basically three types.
 Microelectrodes: Electrodes used to measure bioelectric
potentials near or within a single cell.
 Skin surface electrodes: Electrodes used to measure ECG,
EEG, and EMG potentials from the surface of the skin.
 Needle electrodes: Electrodes used to penetrate the skin to
record EEG potentials from a local region of the brain or EMG
potentials from a specific group of muscles.
 The equivalent circuit of the electrode in upcoming figure.

 Two electrodes are require to do measurements.
 If the same type of electrodes are used, the potential difference is
usually small and depends on the actual difference of ionic
potential between the two points of the body.
 If the electrodes are different, the dc voltage generated which is
nothing but a electrode offset voltage. Which can cause an error in
the measurement.
 Some dc also produce in the same type of electrodes we use.
 To reduce that error by choice of materials, or by special
treatment, such as coating the electrodes by some……contd

 ….contd….electrolytic method to improve stability.
 E.g : silver silver chloride electrode is very stable prepared by
electrolytically coating a piece of pure silver with silver chloride.
 We can see the equivalent diagram of the use of two electrodes for the
biopotential measurements.
 In that the impedance is varies according to the polarization which is a
result of direct current passing through the metal electrolyte interface.
 Size and type of electrodes also affects the impedance . Higher the size
lower impedance. E.g surface electrodes….have 2 to 10 kohm, where as
small needle electrodes have much larger value.

 Microelectrodes: Electrodes with tips sufficiently small to
penetrate a single cell in order to obtain readings from within the
cell.
 Basically two types: 1. Metal , 2. Micropipet.
 Metal type are formed by electrolytically etching the tip of a fine
tungsten or stainless steel wire to the desired size. Then wire is
coated with the an insulating material.
 Micropipet as shown in upcoming diagram.
 The problem with such electrodes is that high impedance and for
that amplifier with very high impedance required.

Fig: Physical Look of Microelectrodes

Fig:Array of microelectrodes connected with brain

Fig: Probe of the Microelectrodes.. Dimensions compared with the mosquito

Prof.G.B.Rathod, EC Dept.BVM-EC453 Click here to see one the application of such electrodes

 Body Surface Electrodes:
 The earliest bioelectric potential measurements used immersion
electrodes, which were buckets of saline solution into which the
subject placed his hands and feet, one bucket for each extremity.
Shown in upcoming image.
 After that improvements done and plate electrodes introduced in
1917. These electrodes are separated from the skin by cotton or
felt pads socked in saline solution.After that jelly introduced.

 Another most popular old type electrodes used today also is a
suction cup electrode shown in figure.

 One difficulty in using plate electrodes is that possibility of electrode
slippage or movement.
 This also occurs with the suction cup electrode after a sufficient length
of time. Number of attempts were made to overcome this problem.
 All the preceding electrodes suffer from a common problem. They are
sensitive to movement, some to a greater degree than others.
 The adhesive tape and “nutmeg grater” electrodes reduce this movement
artifact by limiting electrode movement and reducing the interface
impedance, but neither is satisfactory insensitive to movement.

 A new type of electrode, the floating electrode, was introduced in
varying forms by several manufacturers. This principle of this
electrode is to practically eliminate movement artifact by avoiding
any direct contact of the metal with the skin.
 The only conductive path between metal and skin is the electrolyte
paste or jelly.
 Floating electrodes are generally attached to the skin by means of
two sided adhesive rings.
 ECG measurement for long time can make some problem.

 Various types of disposable electrodes have been introduced in
recent years to eliminate the requirement of cleaning and care
after each use.
 Special types of have been developed for other applications. For
example, a special ear-clip electrode was developed for use as a
reference electrode for EEG measurements.
 Scalp surface electrodes for EEG are usually small disks about 7
mm in diameter or small solder pellets that are placed on the
cleaned scalp, using an electrolyte paste.

 Needle Electrodes: To reduce interface impedance and,
consequently, movement artifacts, some electroencephalographers
use smalls subdermal needles to penetrate the scalp for EEG
measurements.
 In animal research longer needles are actually inserted into the
brain to obtain localized measurement of potentials from a specific
part of the brain.
 Sometimes a special instrument, called stereotaxic instrument, is
used to hold the animal’s head and guide the placement of
electrodes.

 Needle electrodes for EMG consist merely of fine insulated wires,
placed so that their tips are in contact with the nerve muscle. Or
other tissue from which the measurement is made.
 Wire electrodes of copper or platinum are often used for EMG
pickup from specific muscles.
 A single wire inside the needle serves as a unipolar electrode, If a
two wire placed inside the needle, the measurement is called
bipolar and provide a very localized measurement between the
two wire tips.

BIOCHEMICAL TRANSDUCERS
 Reference Electrode
 The pH electrode
 Blood Gas Electrodes
 Specific Ion ELectrodes

 Reference Electrode: Normally Hydrogen is used as a reference
electrode.
 These electrodes make use of the principle that an inert metal,
such as platinum, readily absorbs hydrogen gas.
 Unfortunately, the hydrogen electrode is not sufficiently stable to
serve as a good reference electrode.
 Measurement of electrochemical concentration simply requires a
change of potential proportional to a change in concentration.
 Two types: silver-silver chloride and the calomel electrode.

 The silver-silver chloride electrode used as a reference in
electrochemical measurements utilizes the same type of interface
described before.
 In chemical transducer silver chloride side of the interface is connected
to the solution by an electrolyte bridge which is filling solution KCl.
 The reference electrode with 0.01 mole solution, potential is 0.343 V
and for 1.0 mole solution, potential is 0.236V
 The another is calomel electrode which is also called mercurous
chloride same as a Silver-silver chloride.
 0.01 mole, potential will be 0.388V and 3.5 moles, 0.247V

Fig: Reference
Electrode
Basic configuration

 The pH Electrode
 To know chemical balance in the body, pH of the blood and other fluids
are very important.
 Equation of pH is
 pH is a measure of the acid base balance of a fluid.
 A natural solution has a pH of 7. Lower pH numbers indicate acidity,
whereas higher pH values define a basic solution. Most human body
fluids are slightly basic. The pH of normal arterial blood ranges between
7.38 and 7.42. The pH of venous blood is 7.35, because of the extra
CO2.
10 10
1
log [ ] log
[ ]
pH H
H


  

 In the measurement of pH and in any electrochemical
measurement, each of the two electrode required to obtain
the measurement is called half cell and its sometimes called
the half cell potential.
 The glass electrodes quite adequate for pH measurements in
physiological range(around pH 7).
 Special hydroscopic glass that readily absorbs water provides
the best pH response.

 Blood Gas Electrodes:
 One of the important physiological chemical measurements
is pressure of oxygen and carbon dioxide in the blood.
 The effectiveness of both the respiratory and cardiovascular
systems is reflected in these important parameters.
 The diagram of Po2 electrode with platinum cathode will be
in upcoming slide which shows a principle of operation.

Fig: diagram of Po2 electrode
With platinum cathode showing
Principle of operation

Fig: combination of Pco2
And Po2 electrode

Fig: Diagram showing
Construction of flow-
Through liquid membrane
Specific ion electrode.

Fig: specific ion electrodes
With pH glass electrode
(1) Sodium ion
(2) Cationic electrode
(3) pH glass
(4) ammonia

Outcomes
 From this unit, we come to know about various types of
transducers used for the physiological potential
measurements. The real time use and the benefit with some
major and minor artifacts also discussed.
 Biochemical transducers and related to pH measurement is
also focused and shows it own stability related advantages in
various measurements.

Reference
 Book:“Biomedical instrumentation and measurements “ ,by
L. Cromwell, F .Weibell, and E. Pfeiffer. PHI publication 2nd
Edition

Thank You
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