The working of diffrent transducers and its priciples are discussed. The various types of sensors, transducers for the biopotential detections are also discussed with necessary diagrams.

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

2. 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.

3. 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.

4. ACTIVE TRANSDUCERS

5. 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.

6. ACTIVE TRANSDUCERS

7. ACTIVE TRANSDUCERS

8. 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.

9. ACTIVE TRANSDUCERS
Equivalent circuit

10. ACTIVE TRANSDUCERS

11. ACTIVE TRANSDUCERS
 The Thermoelectric effect: If two wires of dissimilar
metals (e.g., iron and copper) are connected so that
they form a closed conductive loop, a voltage can be
observed at any point of interruption of the loop which
is proportional to the difference in temperature
between the two junction between the metals.
 The sensitivity of a thermocouple is small and amounts
to only 40 microvolts per degree Celsius. (microV/oC)
for copper-constantan.

12. ACTIVE TRANSDUCERS

13. ACTIVE TRANSDUCERS
 The use of thermoelectric effect to convert from
thermal to electrical energy is called the seeback effect.
In reverse direction it is called the Peltier effect.(its
used to cool parts of instruments. E.g. a microscopic
stage)
 The photoelectric Effect: The selenium cell, has been
used to measure the intensity of light in photographic
exposure meters or the light absorption of chemical
solution.
 The silicon photoelectric cell(solar cell), has a much
higher efficiency than the selenium cell.

14. ACTIVE TRANSDUCERS

15. ACTIVE TRANSDUCERS

16. 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.

17. PASSIVE TRANSDUCERS

18. PASSIVE TRANSDUCERS

19. PASSIVE TRANSDUCERS
 Some passive resistive based transducers are : Photo resistive
cells, photo diode, photo emissive cell(either vacuum or gas
filled)
 Most popular is strain gage.
 The principle of strain gauge is as follow:
 We know
 Gage factor
 The Gage factor of metal is 2 and for silicon is 120.
L
R
A


/
/
R R
G
L L




20. PASSIVE TRANSDUCERS

21. PASSIVE TRANSDUCERS
 Mercury strain gage: Resistive material consists of a column
of mercury enclosed in a piece of silicon rubber tubing. Used
in physiological variable measurement( diameter of blood
vessels).
 First time introduced byWhitney that’s why this gage some
times called aWhitney gages.
 When replacing metallic train gage: there are two types.
 (1) Unbonded strain gage , (2) Bonded strain gage(foil gage,
semiconductor strain gages)

22. PASSIVE TRANSDUCERS

23. PASSIVE TRANSDUCERS

24. PASSIVE TRANSDUCERS
 Passive transducer using inductive elements.
 In inductive transducer the core is a permanent magnet
which when moved induces a voltage in the coil. In this
passive transducer the core is made of a soft magnetic
material which changes the inductance of the coil when it is
moved inside.
 Another type of passive inductive based is variable reluctance
transducer where core remain stationary but the air gape is
varied and ultimately permeability is varied.
 E.g., LVDT

25. PASSIVE TRANSDUCERS

26. PASSIVE TRANSDUCERS

27. 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

28. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS

29. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS
 Force Transducers: A design element frequently used for the
conversion of physical variable is the force summing member.
 By using that summing of force vector are summing and been
zero for the spring that’s why the name given is force
summing member.
 Using this element we can design the force based transducers
for various applications.
 All force transducer should be isometric.( no change in
dimension)
 All displacement transducer should be isotonic.( No
resistance change in displacement)

30. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS

31. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS

32. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS

33. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS

34. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS
 Transducers for Displacement, velocity andAcceleration
dD
V
dt

2
2
dV d D
A
dt dt
 
V Adt 
2
( )D Vdt A dt  

35. 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.

36. TRANSDUCERS FOR BIOMEDICAL
APPLICATIONS

37. TRANSDUCERS FOR BIOMEDICAL
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.

38. 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.

39. Electrode Theory

40. 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 fRT
E
nF C f
 

41. 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.

42. 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.

43. Biopotential electrodes

44. Biopotential electrodes
 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

45. Biopotential electrodes
 ….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.

46. Biopotential electrodes

47. Biopotential electrodes
 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.

48. Biopotential electrodes

49. Biopotential 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.

50. Biopotential electrodes

51. Biopotential electrodes

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

53. Biopotential electrodes
 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.

54. Biopotential electrodes
 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.

55. Biopotential electrodes

56. Biopotential electrodes

57. Biopotential electrodes
 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.

58. Biopotential electrodes

59. Biopotential electrodes

60. Biopotential electrodes

61. Biopotential electrodes
 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.

62. Biopotential electrodes

63. Biopotential 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.

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

65. BIOCHEMICAL TRANSDUCERS
 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.

66. BIOCHEMICAL TRANSDUCERS
 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
0.343V 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

67. BIOCHEMICAL TRANSDUCERS
Fig: Reference
Electrode
Basic configuration

68. BIOCHEMICAL TRANSDUCERS
 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


  

69. BIOCHEMICAL TRANSDUCERS
 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.

70. BIOCHEMICAL TRANSDUCERS

71. BIOCHEMICAL TRANSDUCERS
 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.

72. BIOCHEMICAL TRANSDUCERS
Fig: diagram of Po2 electrode
With platinum cathode showing
Principle of operation

73. BIOCHEMICAL TRANSDUCERS
Fig: combination of Pco2
And Po2 electrode

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

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

76. 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.

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

78. Questions….????
Thank you