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Course module description:
This course gives an overview of Categories of Measurements,
Calculation of Errors in electrical measurements, types of sensors and
their uses in measuring systems Bridge circuits, application of
measurement system in the Biomedical Devices, Active filters
Course module objectives:
The goals of the course are
1-Introduce students to know the fundamentals of electric
measurements.
2- The student should be able to use the measuring devices, calculate
the measuring errors,
3- be able to design and use simple measuring circuits
4- Understanding the main basics of different electric measurement
processes especially during the use of biomedical equipments
Course/ module components
Books:
Medical Instrumentation Application and Design, by Webster
Introduction to biomedical Equipment Technology by J Car
Electronic Instruments and Measurements, I.D. Jones and A.F. Chin
Learning outcomes
Knowledge and understanding
1-Understanding the basic system of units
2- Understanding different processes of measurements
3- Understanding the types of measuring processes and their basics
specially in the medical equipments
4- Understanding the basic electric measuring circuits including filters
and amplifiers
Cognitive skills (thinking and analysis).
ability to investigate the types of sensors in any measuring device
ability to feel with the errors in the measurements
Operate , maintain and use of the measuring instrumentation
Communication skills (personal and academic).
. Ability to work with medical team
- Ability to work within a one team
- Ability deal with the biomedical equipment
Practical and subject specific skills (Transferable Skills).
Medical Devices Troubleshooting and Maintenance
Assessment instruments.
Quizzes,
Term Exam,
Small team project
Practical Exam,
Oral Exam
.
Allocation of Marks
Mark Assessment
10%Quizzes
30%Med Term Exams
20%Practical Exam
40%Final Exam
100Total
Expected workload:
On average students need to spend 2 hours of study and preparation for each 50minute lecture/tutorial.
Attendance policy:
Absence from lectures and/or tutorials shall not exceed 25%. Students who exceed
the 25% limit without a medical or emergency excuse acceptable to and approved
by the Dean shall not be allowed to take the final examination and shall receive a
mark of zero for the course.
Biomedical Measurement Course Contents
Ch1
Introduction
- Definition of Measuring Process
- Measuring Types (Direct –Indirect, Null)
- Measurement system components
- Generalized medical instrumentation system
Ch2
Errors in Measurements
- Error definition
- Accuracy , sensitivity, resolution
- Types and sources of errors
-Statistical analysis of error
-Static characteristics of measuring system
Biomedical Measurement Course Contents
Ch3
Sensing Element (Sensors)
- Types of sensors
- Resistive sensors
- Resistive strain Gauges
- Capacitive sensor- Inductive sensors
- Temp Sensor
- Piezo Electric sensor -Electrodes
Ch4
Direct Current Bridge
- Wheatstone bridge
Biomedical Measurement Course Contents
Ch5
Blood Pressure and other Cardio Vascular Measurements
- Important physiological definitions
- Pressure measurements
- Blood pressure measurements (direct – indirect )
- Blood flow measurement
Ch6
Signal Conditioning
- Op-amp and wave shaping
- Filters
Biomedical Measurement Course Contents
Lab
1- Units , Dimensions and some important definitions
2- Temperature Measurements
3- Blood Pressure measurements
4- Spiro meter
5- Pulse Oximeter
6-ECG (Electro cardio gram) Signal
7-Skin resistance
8-Time reaction
9- Strain gauge
Ch1
Introduction
1- Definition of Measuring Process:
Measurement is a process for comparing an unknown quantity with an accepted
standard (calibrated) quantity.
This process involves connecting a measuring instrument into the system and
observing the response of instrument.
Categories of measurements (Types)
There are three main types of measurements
Direct measurement
Indirect measurement
Null measurement
• Direct measurement
Direct measurements are made by holding the measurand (required quantity to be
measured) up to some calibrated standard and comparing the two. A good
example is the meter stick ruler used to measure and cut a piece of cable to the
correct length
• Indirect measurement
Indirect measurements are made by measuring some thing other than the actual
measurand
The most common example of indirect measurement is the blood pressure
measurements
• Null measurement
Null measurements are made by comparing a calibrated source to an unknown
measurand and then adjust the reference until the difference between them
become zero.
2- Electronic Measurement System
The block diagram of Electronic Measurement System is shown in Fig .
I/P = Input
O/P = Output
I/P
Sensing Element
Signal
Conditioning
Signal
Processing
Data Output
O/P
3- Generalized Medical (Measurement ) Instrumentation system
1- Measurand (I/P)
The physical quantity/ property that system measured is called measurand.
Most medically important measurands can be grouped in the following categories:
Bio-potential (ECG), Pressure, Flow, Dimension (imaging), displacement, Velocity,
Force , Acceleration, Temperature and Chemical Concentration.
2- Sensors (Sensing Element)
A sensor is defined as the device that converts a physical measurand to electric output.
The sensor should be sensitive, Bio-compatible
Many sensors have a primary sensing element such as diaphragm which converts the
pressure to displacement
A variable sensing conversion element such as the strain gauge that converts this
displacement to volt.
Displacement
Strain Gauges
mV
T oC
Thermocouple
mV
3- Signal Conditioning
Usually the sensors output can't be directly coupled to the display device
Amplifier and filter are used to modify the signal
4- Output Display
The measured
value must be
displayed in a form that the
operator can analyze easily. The
best form for display may be
numerical or graphical.
Example of
simple Display
Example of simple Display
Example of simple Display
Example of simple instrument
Thermometer
Scale
Temp
Hg
Measurand is Temp
Sensing Element is Mercury
Signal Conditioning is (H) High of Mercury
Data Presentation is scale
Example of simple instrument
Load Cell using Strain Gauge
Load
I/P
Body of
Load Cell
Strain
Gauge
Sensing Element
V
A/D
Signal Processing
∆Ω
Wheatstone
Bridge
Signal Conditioning
PC
Display
V
Amplifier
and Filter
Ch2
Errors in Measurements
Any measurement system is affected by many factors.
Some of these factors are related to the instrument themselves
and the other factor related to the person who using the instrument.
1- The deviation of the measured value from the true value is called the error in the
instrument.
Error = True (Expected) value – measured value
e  XT  X m
X T True (Expected) Value of Measurement
Xm
Measured Value
e
Error
%e
Percentage Error =
e
100 %
XT
XT  X m
% e
100 %
XT
2- Accuracy
The accuracy of measurement is the degree of closeness of the measured value to
the true value.
Percentage Accuracy =
100 %  e %
3- Sensitivity
Sensitivity is the ratio of output signal to the change in the input signal
Sensitivity
S 
Xo
X i
Xo
Output Signal
X i
Change in the Input
For Example
Thermocouple ,, the input is change in temp ,,
the output is Volt
If temp changes 1 oC
What is the output volt ,,,, say 1mv
Then
S 
1 mvolt
1 oC
4- Resolution
Resolution is the minimum change in the measured value can be sensed by the
instrument
2-Types of Errors
Errors are generally categorized under the following three main types
2-1 Gross Error
This error are generally the fault of the person using the instrument (error in
reading, error in recording, incorrect use of the instrument)
2-2 Systemic Errors
These errors due to the problem with the instrument environment effects and it can
be classified to the following.
 Instrumentation Errors
The source of these errors are the measuring device (internal error due to friction in
the bearing of the meter movement, incorrect spring tension, improper calibration)
The instrumentation errors can be reduced by good maintenance and use of the
instrument
 Environmental Errors
These errors are happened due to the change in the surrounded environment (Temp,
Pressure and Humidity
2-3 Random Errors
These errors is due to unknown causes and occur even when all gross and
environmental errors have been reduced
Systemic Errors
2-4 Limiting Error
This error is due to the wrong choose of the measuring scale.
For example the manufacturer of a certain voltmeter may specify the instrument to
be accurate within ±2% of the full scale. This is the limiting error and means that a
full scale reading will be within the limits of ±2% error. But if the measured value
are less than the full scale, the limiting error will increase.
So it is important to obtain the measurements as close as possible to the full scale of
the used instrument.
Example (1)
A 300 Volt voltmeter is specified to be accurate within ±2% at the full scale.
Calculate the limiting error when the instrument is used to measure
1- 120 Volt and 2- a source of 220 volt
Solution:
Case 1
2%(
2
) ( full scale )
100
Limiting Error =
 2 

300  6 V
100


For 120 volt this error become
6
100 %  5%
120
Case 2
For 220 volt this error become
6
100 %  2.7%
220
Example (2)
A voltmeter and Ammeter are used to determine the power dissipated in a resistor.
Both instruments are guaranteed to be an accurate within ±1% at the full scale. If the
voltammeter reads 80 Volt on its 150 Volt Range,, and the ammeter reads 70mA on
its 100mA Full Scale.
Calculate the limiting error for power calculation
For Voltmeter
1
(150) V
Limiting Error =
100
 1.5 V
Then Limiting Error for 80V reading become
1.5
100 %  1.86%
80
For Ammeter
1
100mA %  1mA
Limiting Error =
100
1
Limiting Error for 70mA become
 1.43 %
70
Then the limiting error for the power = sum of individual limiting
errors = 1.86+1.43= 3.29%
Statistical Analysis of Error in Measurements
1- Arithmetic Mean X
If a measurement is repeated several times, at the same conditions, the reading
may differ because of the founding of different errors causing
If we have several values for one measurement At the same conditions
x1 , x2 ,.........xn
The arithmetic mean= Average =
x1  x2  x3  ........  xn
n
x1  x2  x3  ........  xn
X 
n
2- Deviation (d)
If we have several readings for one measurement At the same conditions
The deviation is the difference between each reading and the average
d1  X 1  X
d2  X 2  X
dn  X n  X
3- Standard Deviation (S) (Root Mean Square Value)
It is defined as the variance of the measured value about the mean value
Example 3
d12  d 22  ............d n2
S
n 1
The expected value of the voltage across a resistor is 50Volt, however
measurement yields a value of 49Volt. Calculate
1-The absolute error 2-The percentage error 3-The percentage accuracy
Solution
1-The absolute error =
2-Percentage Error=
XT  Xm
=50-49=1Volt
Error
1
100% 
100%  2%
XT
50
3-Percentage Accuracy=100%-e%=98%
Example 4
During measurements of the volt across battery the following readings are given
at the same measuring conditions
V1=50.1 V2= 49.7 V3=49.6 and V4=50.2
Find 1-The Arithmetic mean
2-The deviation of each value
3- The sum of deviations
4- Standard deviation
Solution
1- The arithmetic mean (Average)

X 
x1  x2  x3  ........  xn
n
50.1  49.7  49.6  50.2
 49.9 V
4
2-Deviation of each value
d1= 50.1 - 49.9 = 0.2 V
d3= 49.6 - 49.9 = -0.3 V
d2= 49.7 - 49.9 = -0.2 V
d4= 50. - 49.9 = 0.3 V
3-Sum of Deviation= 0.2-0.2+0.3-0.3=0
4-Standard Deviation
Reading become
(0.2)2  (0.2)2  (0.3)2  (0.3)2

3
 0 .3
49.9
S=0.295=0.3
Static Characteristics of Measuring Element
A- Range
B- Span
C- Ideal Straight Line D-Non-linearity
E- Environmental Effects
A-Range
F- Wear and Aging
It is specified by the minimum and maximum values of I/P and O/P
(10-104) Pa
(4-80)mV
Pressure
Transducer
Min input =10Pa
Min output 4mA
Max input 104 Pa
Max output 80mA
B-Span
Is the maximum variation in I/P and in the O/P
Span of input =Imax-Imin
Span of output =Omax-Omin
C- Ideal Straight Line
The relation between the input and the output should be straight line
Output
Output
K = slope
K = slope
a
Input
Output = K input
Input
Output =K Input +a
K is calibration factor (Slope)
D- Non-Linearity
The relation between the input and the output is non linear
Output Actual
Output
Output
Therortical
N
K = slope
Input
N (non-linearity) = O actual – O Theoretical
E-Environmental Effects
Generally the output depends not only on the input but also on the environmental
conditions Temp 20-25 oC,,, Pressure =1Bar and Humidity =80%
If these conditions changed the output also changed
F-Wear and Aging
Characteristics changes with time
Example1
During pressure measurement using a resistive strain gauge the following data are
given
Input (Pressure Pa)
0
10
20
30
40
Output (mV)
0
7
14
21
28
Find the calibration factor and write the relation between the input and output
Example2
For the above example if the reading become as the following
Input (Pressure Pa)
0
10
20
30
40
Output (mV)
5
12
19
26
33
Find the calibration factor and write the relation between the input and output
Types of Signals
Signals can be categorized in several ways, but the most fundamental is according to
time domain behaviour and the other major one according to the frequency domain
If we assume that the signal of the form V=f (t). The time domain classes of signals
include Static and Quasi-Static, Periodic, Repetitive, Transient and Random
Static and Quasi-static Signal
(a) Static signal is unchanged with the time
(b) Quasi-Static is nearly unchanged with time
( C) Periodic ,, repeated its self on a regular basis
(like square wave , sine, cosine wave)
(d) Repetitive is quasi-periodic , the difference
between periodic and repetitive is seen by comparing
the signal f (t) and f (t +T) T is the period of signal
This point may be not identical in in repetitive signal
But it is identical in the periodic signal
(d) Transient is a one time event
Or periodical event in which T1< < < < < T2
Fourier Series
All continuous periodical signals can be represented by a fundamental frequency
sine wave and a collection of harmonics of that fundamental sine wave that are
summarized linearly. These frequencies make up the Fourier series of the wave form
CH3
Sensing Elements
3-1 Sensing element is the first element in the measurement system that converts
the measurand (Physical property) to electric signal
Sensors
Selection of transducer criteria
1- Operating Range
Mechanical
Electric
2-Sensitivity
3-Frequancy response
Active
4- Environmental Compatibility
Passive
5-Accurcy
Resistive
Capacitive
Inductive
Active sensors Required an external AC or DC source to power the device such
as Strain Gauge.
Passive Sensor Provide its own energy or derived its energy from the measured
value such as thermocouple.
3-Displacement Measurements
The biomedical researches are interested in measuring the size and shape and
position of the organs and tissue of the body. Variation in theses parameters are
important to know the normal and abnormal function of the organs. The
displacement sensor can be used to measure the change in the blood vessels
diameter, diameter, volume, shape of cardiac chambers.
3-1 Resistive sensors
I
The simplest form
of potentiometer is
Ein
RL

Eo
RX
RX  X , , ,
RL  L
X
En
L
L
X 
Eo
Ein
Eo 
the slide-wire resistor
shown in fig. The sensor
consists of a length L
of resistive wire attached
Across a voltage source
Ein. A wiper moves along The length of the wire
Eo
The relation between the output voltage Eo and measured distance X is X  L E
in
Wire has low resistance and this required excessive power for the input voltage to
get sensible output voltage
So We use High resistance wire wounded potentiometer
2- Angular potentiometer
It is used to measure the
change in angle Like Knee
Elbow angles
E
I  in
R

Eo
R
I
Ein


Eo

The angle is related to the
input volt, output volt and
total potentiometer angle as
follow
Eo


Ein
 is required angle to be measured
 is total potentiometer angle
High Resistance wire
wounded
2- Angular
potentiometer
A displacement transducer with stroke length of 3 in is applied in
the circuit shown in Fig. The total resistance of the potentiometer is
5KΩ and the applied volt VT= 5 V. when the wiper is 0.9 in from B
what is the value of the output volt Vo
Solution
X
Eo  En
L
0.9
R2 
5000   1500
3
R2
Vo 
VT  1.5V
R1  R 2
3-Resistive Strain Gauge
Before discussing the strain gauge we should know the concept of stress,
strain, elastic (Young’s) modulus and Poisson’s ratio
Force ( F in N )
Stress 
Area ( A in m 2 )
Pa
Strain Longitudinal ( ) 
Young ' s mod ulus ( E ) 
Lateral Strain 
W ( in mm)
W ( in mm)


L ( in mm)
L ( in mm)
Pa
  ( ), , , , , , is Poisson ' s Ratio  0.3
The Relation between length and resistance
When a fine wire is strained the wire resistance changes because of the
changes in the length and wire diameter
L
R

A
R is the total resistance of the wire in Ohm
L is length in meter
A is the wire cross sectional area in m2
 Is the electric resistivity of the metal of wire (const for each material )
The strain gauge is used to measure the small displacement
Load
Gauge Factor 
R / R R / R

L / L

R
(Gauge )R+dR
Vo
Vi
R
R
Strain Gauge in Electronic Measuring System
I/P
Body of
Load Cell
Strain
Gauge
Sensing Element
V
A/D
Wheatstone
Bridge
Signal Conditioning
PC
Signal Processing
∆Ω
Display
V
Amplifier
and Filter
Different Types of strain Gauge Bridges
R
R+dR
R+dR
Vo
Vi
R
Vo
Vi
R
R
R
R+dR
Half Bridge
Quarter Bridge
R-dR
R+dR
Vo
Vi
R-dR
Full Bridge
R+dR
Vo  Function of Vin , R, R
A resistive strain gauge with a gauge factor of 2 is fastened to a steel bar
with a diameter of 0.02m, and Young's modulus of 2x1010 Kg/m2 . This
bar is subjected to load of 33Kg Find the change in resistance of the
strain gauge if its initial resistance is 130 Ω .
Solution
R / R R / R
Gauge Factor 

L / L

Stress 

4
(E) 
33
Stress 
Force ( F )
Area ( A )
 105095 Kg / m 2
(0.02) 2
105095

 2 x1010   5.25 x10 6
R / R
R / 130
Gauge Factor 

L / L 5.25 x10 6
R  0.001
2
Young ' s mod ulus ( E ) 


+Q
2-Capacitive Sensor
Q
The capacitive sensor consists of two parallel
plates separated by insulation When charged, the
plates carry equal charges of opposite sign.
o r A
C 
d
d
C  Capaci tan ce in Farad
 o  Dielectric Cons tan t of the Free Space  8.8 x10 12 Farad / m

 r  K  Re lative Dielectric Cons tan t  I
o
 I  Dielectric Cons tan t of the Insulating material
A is area of the plate in m 2
d is the dis tan ce between the plates in m
Area = A
A-Type 1 Variable separation
The type of sensors used to measure the pressure between foot and shoe (variable
distance between plates
o r A
C
dx
w
L
Type 2-Variable Area
L
C
o r A o r W . X

d
d
C
Type 3-Variable dielectric
or1 A1
d
A1  W . X

A2  (l  x) W
or2 A2
d
Capacitive Bridge
C1
Vi
C3
Vo
C2
C4
Vo  Function of Vin , d , X , 
An electrode diaphragm pressure transducer has plates whose
area is 5x10-3 and whose distance between plate is 1x10-3 m.
Calculate its capacitance if it measures air pressure.
The dielectric constant of air K=εr =1
Solution
o r A
C
d
3
2
12
(1)(5 x10 m )(8.854 x10 F / m)

3
1x10 m
 44.25 x10 12
Farad
Inductive Sensor
Vi
‫ تعتمد الحساسات الحثية على التغير فى الحث الناتج‬
‫عن ملف نتيجة اإلزاحة الحادثة على قلب هذا الملف‬
L  n G............Henry
2
L=inductance,
n=No. of turns,
G=Factor depends on coil geometry,
u=Permeability of the medium
u air = 1.25x10-6 Henry /m
The inductance changes due to external magnetic field. The device works on
the principle that alternations in the self-inductance of a coil may be
produced by changing the geometric form factor or the movement of a
magnetic core within the coil
c
Linear Variable Deferential Transformer (LVDT)
LVDT is widely used to measure displacement, pressure
and force
a
The LVDT is composed of a primary coil (a-b) and two
secondary coils (c-e) and (d-e) connected in series.
b
The coupling between these two coils is changed by the
motion of high permeability alloy slug between them. The
output volt
Vout
d
e
Vcd=Vce-Vde
When the slug is symmetrically placed (middle) the two
secondary voltages are equal and the output=zero
Materials cause the flux lines to move apart called
diamagnetic
V
Materials concentrate the flux
o
‫اإلزاحة‬
called paramagnetic
‫ ميللى متر‬0.1 ‫ ميللى فولت لكل‬2 ‫الحساسية حوالى‬

Piezoelectric Transducers
Piezo electric sensor is used to
measure displacement and record
heart sound.
Piezo electric material generate an
electric potential when
mechanically strained, and
conversely and electric potential can
cause physical deformation for the
material
q=kf
,
q=surface charge
coulomb
f=force, (N)
k=constant
Coulomb/N
++
Crystal
V
‫مكبر‬
Amplifier
The Piezo electric material can be act like parallel plate capacitor
K f
q
V 
V 
C
C

K f
o r
d
A
K=2.3x10-12 C/N for quartz
K=140x10-12 C/N for barium
Piezo electric materials have a high resistance
The equivalent circuit of Piezo electric circuit as
shown
Charge generator
Ia=0
IS
IR
Ic
C
R
Vo
‫الدائرة المكافئة للبلورة المبسطة‬
Temperature Transducer
There are 4 main types of common temperature transducers
1- Thermocouple
2- Thermistor (Thermal Resistor)
3- Radiation Thermometry
4- Solid state PN Junction (diode)
Thermocouple
Voltmeter
A thermocouple consists of two dissimilar
conductors or semiconductors joined together
at one end. Due to the contact of different
materials at junction , a potential will be
generated when junction is heated. This
potential changes linearly with temp
Copper
Iron ------T1 , Hot junction
(Measuring probe)
100
V mv
Type E
80
Type J
60
Type K
Type W
40
Type S
20
10
Temp
0
200
1600
Junction
Material
Temp
Range
Output
Voltage
J
- Iron-(Copper Nickel)
0 to 750
5.268mv
K
(Nickel-Chrome)-(Nickel-
-200 to 1250
4.095mv
-200 to 900
6.317mv
Aluminum)
E
(Nickel-Chrome)-(copperNickel)
T
Copper-(Copper-Nickel)
-200 to 350
4.277mv
S
(Platinum10%-Rhodium)-
0 to 1450
0.645mv
0 to 1450
0.647mv
0 to 1700
0.033mv
Platinum
R
(Platinum13%-Rhodium)Platinum
B
(Platinum30%-Rhodium)Platinum6%- Rhodium)
Thermistor
Thermistor is a semiconductor made of ceramic. The material react to temp changes.
There are two types of Thermistor
1- Positive temp Coefficient (PTC) device where the resistance increase with temp.
increase
2- Negative temp Coefficient (NTC) device where the resistance decrease with temp
increase
The resistivity of the Thermistor used in the biomedical applications ranges from
0.1 to 100 Ohm. The device is small in size
Radiation Thermometry
The basics of radiation thermometer is that there is a known relationship between
surface temp and object radiation power. This principle makes it is possible to
measure the skin temp without physical contact This method is also used to detect the
breast cancer
Electrodes for Biophysical Sensing
1- Bioelectricity is a phenomenon that arises from the fact that the living organisms
Are composed of ions at different quantities
2- Ionic conduction involves the migration of ions positively and negatively
charged molecules through out a region.
3- Electric conduction involves the flow of electrons under the influence of an
electric field
4- In an electrolytic solution, ions are easily available. Potential difference occur
when concentration of ions is differ from point to point.
5- Bio-electrodes are class of sensors that transducer ionic conduction to electronic
conduction so that the signal can be recorded easily
6- Bio-electrodes are used to detect the bioelectric signal such as
-
Electro-cardiograph (ECG)
-
Electro-miograph (EMG)
-
Electro-encephalograph (EEG)
Electrodes Potentials
The skin and tissue of living organisms (human) are electrolytic and can be modeled
as electrolytic solution
Imagine a metallic electrode immersed in an electrolytic solution, then the electrode
will begin to discharge some of metallic ions into the solution. Also at the same
time some of ions in the solution start combining with the metallic electrode
(Electro-plating – Anodizing)
After a short time a potential difference or electric potential (Ve) or half cell potential
has built
Then at the interface between electrode and electrolyte, ions
migrate words one side of the electrode forming two parallel
layers of ions of opposite charge. This layer is called the
electrode double layer. This ionic difference is the source of
half cell potential
This means that if electrode (metal) is placed on the skin
(Electrolyte ),, there will be a a value of volt (Ve) have cell
potential,, depending on the metal of electrode and
electrolyte solution at the region ( Region or position of
electrode in the skin = electrolyte concentration)
If we have 2 electrodes A ≠B ,, then we will have Vea and Veb
Offset potential Vout = Vea-Veb
If A=B and the same electrode ,, Vea = Veb and Vout = 0
If A=B and not the same electrolyte (another place in the body),, Vout = Value
(Due to electrolyte effect only)
The electrode must made of specific material because body fluids are are corrosive to
metal
Silver (Ag) –Silver Chloride (Ag Cl) electrode is the
most common one the electrode consists of body
silver coated with a thin layer of silver chloride . T
he Ag Cl gives (Ag+) and Cl- which prevent the
double layer forming.
‫ إلى الجسم‬Stimulation ‫ أو إلدخال إثارة‬، ‫تستخدم اإللكترودات إما لقراءة إشارة من الجسم‬
: ‫تصنف إلى صنفين‬

.1
‫ الثانى هو اإللكترودات الداخلية‬.2
Body surface electrodes ‫األول هو اإللكترودات السطحية‬
Internal electrodes

‫‪ -1‬إلكترودات السطح ‪Body surface electrodes‬‬
‫‪‬‬
‫كمية التيار التى يتم قياسها باإللكترود تكون صغيرة جدا (ميكروأمبير) ‪ ،‬لذلك البد من‬
‫مالمسة سطح اإللكترود لسطح الجسم تماما ‪.‬‬
‫‪‬‬
‫‪‬‬
‫البد من استخدام جيال تين معين كوسط موصل بين سطح اإللكترود وسطح الجسم‬
‫السلك الموصل بين اإللكترود والمكبر يجب أن يكون قصيرا ومعزوال (يستحسن أن‬
‫يكون كابل محورى)‬
‫‪ ‬بعض اإللكترودات الحديثة يكون المكبر صغير الحجم ويوضع مع اإللكترود مباشرة‬
‫والبعض حتى يضع المحول االنسيابى الرقمى مع اإللكترود لتقليل تأثير هذه المسافة‬
‫وتجنب الضوضاء بقدر اإلمكان وبالذات الضوضاء الناتجة من خطوط القدرة (‪60/50‬‬
‫هرتز) ‪.‬‬
‫‪ ‬معظمها مصنع من شريحة من كلوريد الفضة معزولة بطريقة أو أخرى حسب شكل‬
‫اإللكترود‬
‫توجد إلكترودات السطح فى أكثر من شكل‬
‫جيال تين‬
‫مادة صمغية‬
‫قرص معدنى‬
‫قرص معدنى يستخدم لمرة واحدة‬
‫أشكال أخرى‬
‫اإللكترودات الداخلية‬
‫قمة معدنية حادة‬
‫إبرة تحت الجلد‬
‫‪Hypodermic needle‬‬
‫للقياسات البسيطة‬
‫عازل‬
‫اإللكترود‬
‫‪Acute measurements‬‬
‫قاعدة ماصة‬
‫اإللكترود‬
‫لقياس ضربات قلب الجنين‬
‫عضلة‬
‫للقياسات الدائمة ‪Chronic recording‬‬
‫‪‬‬
‫المايكروإلكترود ‪Microelectrodes‬‬
‫تستخدم للقياسات من داخل الخلية ‪ ،‬يجب أن تكون أبعاده أقل من أبعاد الخلية‬
‫حتى ال يسبب تخريب لها عند اختراقها ‪ ،‬ويجب أن يكون صلب بالرغم من هذا‬
‫القطر الصغير ‪.‬‬
‫‪‬يتراوح قطرها من ‪ 0.05‬حتى ‪ 10‬ميكرومتر‬
‫عازل زجاج‬
‫الرأس‬
‫معدن‬
‫مايكروإلكترود معدنى ‪Metal Microelectrode‬‬
‫الرأس ‪ 1‬ميكرون‬
‫المايكروأنبوبة ‪Micropipette‬‬
‫محلول ملحى ‪KCl‬‬
‫أنبوبة شعرية زجاجية‬
The equivalent circuit for a biopotential electrode.
The circuit model of
surface
electrode
contains
Op-amp (difference) ,,
so that the half cell
potential
of
each
electrode is cancelled
Ch4
Wheatstone Bridge
(Direct Current Bridge)
• Direct current bridge is an instrument that used to measure the resistance or
change in resistance and converts it to output current
• This bridge circuit are also used in control circuit, when one arm of the bridge
contains a resistive element that is sensitive to the physical parameters (Temp,
Pressure , Load)
Wheatstone Bridge
• It consists of 2 parallel resistance branches, each branch contains two series
elements (Resistance)
I
I2
I1
I1
•DC volt is used as a power
source
•Null detector (Galvanometer)
is connected to detect balance
R
+
R2
1
a
I2
R2
R1

b
a
b
I3
R3
R4
I3
R3
R4
I4
Using the bridge to determine unknown resistance
1- Assume R4 is unknown ,,
I1
2- Assume we can change one resistor in the bridge
(say R1) till the balance condition
Va
 Vb
R
I2
R
1
or Vo  0
2
a
b
I3
Vb  E  I 2 R2          (1)
Va  E  I1 R1          (2)
Vb  0  I 4 R4          (3)
Va  0  I 3 R3          (4)
At case of unbalance (reading= Va-Vb)
R3
R4
I4
At balance
Va  Vb
Vb  Va  0
and I 2  I 4 , , , , , I1  I 3
From 1 , 2
E  I 2 R2  E  I1 R1
 I 2 R2  I1 R1          (5)
From 3 , 4
I 4 R4  I 3 R3            (6)
I1
R
I2
R
1
2
I1 R1  I 2 R2
a
I 3 R3  I 4 R4
R1 R2

R3 R4
b
I3
or
R1 R4  R2 R3
R
R
3
I4
4
Example (1)
Determine the value of unknown resistor Rx in Fig. assuming the balance
condition
R1  12 K
I1
R2  15K
+
R3  32 K

R2
R1
R x  ????
Solution
At Balance Vo=0
R2 R3  R1 R x
R2 R3
Rx 
R1
I2
(15) K (32) K

 R x  40 K
12 K
Vo
b
a
R3
Rx
• Sensitivity of the bridge
• When the bridge is in an unbalance, current flows through the
galvanometer, causing deflection of the pointer.
• The sensitivity of the bridge = S
 deg ree


A
A
Thevenin Theory for the bridge
E 0
I1 
R1  R3
E 0
,, I2 
R2  R4
R3
Va  I1 R3  0  E
R1  R3
Vb  I 2 R4  0
Vth  Va  Vb
E
R4
R 2  R4
E
I
E
I2
I1
I1
R1
R2
+
R2
R1

a
I2
b
a
b
I3
R3
R4
I3
0
R3
R4
I4
0
Thevenin Theory for the bridge
R2
R1
R1
R
2
b
a
b
a
R3
R4
R4
R3
b
a
Vth  E
R3
R4
E
R1  R3
R2  R4
Vth  E (
Rth  (
R1R3
R1  R3
R2 R4
R2  R4
R3
R4
E
)
R1  R3
R2  R4
R1 R3
R2 R4

)
R1  R3
R2  R4
Vth
I Galv 
Rth  RLoad
a
Vth
+
Load

b
Example
Calculate the current through the galvanometer shown in Fig
E  6V R1  1K
R2  1.6 K R3  3.5K R4  7.5K
and Rgalv  200
Solution
Vth  E (
R3
R4

)
R1  R3
R4  R2
3.5
7.5

)  0.276 Volt
3.5  1
7.5  1.6
R1 R3
R2 R4
Rth  (

)
R1  R3
R2  R4
Vth  6 (
(1) (3)
(1.6)(7.5)
Rth  (

)  2.097 K
13
1.6  7.5
Vth
I Galv 
Rth  RLoad
0.276
I Galv 
 120A
3
(2.097  0.2)10
I1
+
I2
R2
R1

a
Rg
b
I3
R3
R4
I4
‫تهيئة إشارة القنطرة لتوصيلها على الحاسب‬
R1
+
R2
V1
Vi
R1
+
V2
Isolator
(Buffer)
+
+
Vo
R2
Vo  (V 1  V 2)
R1
R2
Difference
Amp.
Ch5
Blood Pressure and other Cardio Vascular Measurements
Firstly the blood pressure is measured in arteries
There are two kind of arteries pressure
1-Systolic pressure
It is the pressure in arteries at case of heart contraction ≈ 120 mmHg
2- Diastolic pressure
It is the pressure in arteries in the case of heart relaxation ≈ 80 mmHg
F
(Pa)
A
F = Force in Newton
A is the area in m2
1 Pa = N/m2
Pressure =
A small coin has a diameter of 1cm and a mass of 1.59 gram
Find
(a) Gravitational force (Weight)
(b) The pressure caused by the coin
Solution
(a ) Force = Mass. Acceleration
Force  1.5 x10
F
(b) Pr essure 
A
3
m
3
Kg x 10

15
x
10
N
2
sec
15 x10 3

 (1x10 2 ) 2
4
N
 191 2 or ( Pa)
m
Pressure Measurements
•The air on the surface of the earth has a pressure value called
atmosphere (1 atom) = 760 mmHg ( zero pressure is reference)
•If the pressure is measured with respect to vacuum (0 atom) it is
called absolute pressure ( zero pressure is reference)
•If the pressure is measured with respect to atmospheric pressure (1
atom) it is called gauge pressure (1 atom pressure (760mmHg) is
reference )
•Pressure in human circulatory system is measured with respect to
atmospheric pressure (gauge pressure)
•Gauge pressure is usually given in mmHg above or below the
atmospheric pressure
•Zero gauge pressure is 1 atom
Blood pressure measurement
There are many methods that can be used for blood pressure
measurement
1- Direct measurements (Invasive)
2- Indirect measurements (Noninvasive)
1- Indirect Measurements
This method is used for routine clinical measurements of blood
pressure in human, a suitable technique without painful or hazard
is required
The instrument consists of an inflatable rubber bladder called cuff,
rubber squeeze ball pump, assembly valve and manometer.
The manometer might be a mercury column or dial gauge
1.
The cuff is wrapped around the patient upper arm; the stethoscope is placed over
the artery.
2. The cuff is inflated so that the pressure inside the cuff becomes greater than the
expected systolic pressure. This pressure compresses the artery against the bone
and shuts off the flow of the blood in the artery.
3-The pressure in the cuff then slowly released (using the valve) when the pressure of
the cuff equal the systolic blood pressure the blood starts to flow and the operator
can hear a crashing sound in the stethoscope. Then the systolic pressure can be
watched on the dial gauge or in the mercury column.
4-The pressure of the cuff is lowered
more and more and when the cuff
pressure equals the diastolic pressure
, the sound in the stethoscope is
disappeared. Then the diastolic
pressure can be watched
The Ultra-sound blood Pressure Measurements
The Ultra sound determination of blood pressure uses a Doppler
sensor to detect the motion of blood vessel walls.
The Fig. shows the placement of compression cuff over two small
transmitting and receiving ultra sound crystals (8MHz) on the arm.
The reflected signal (shifted in frequency) is detected with the
receiving crystal. The difference in frequencies in the range from 40
t0 50 Hz, depends on the velocity of the wall motion and blood
velocity.
•As the cuff pressure increased above the diastolic pressure (80mmHg) but
below the systolic pressure, the vessel opens and close with each heart beat,
the opening and closing of vessel are detected by ultra sound system.
•As pressured increased as shown in Fig. the time between the opening and
closing decreases until they coincide. The reading at this point in the
manometer or dial gauge is the systolic pressure.
•Conversely, when the pressure in
the cuff is reduced, the time between
opening and closing increases until
the closing signal on pulse coincide
with opening signal of the next one.
The reading in this case is the
diastolic pressure.
•The advantages of the Ultra sound
method
•can be used with infants
•can be used in high noise
environment
Automatic Blood Pressure Measurements
1- The user adjusts the pressure of the system (by touching digital bottoms) this
expected pressure above the expected systolic pressure
2- The pump triggers and gives a pressure to the cuff
3- The cuff pressure is measured using strain gauge system (calibrated)
4- The adjusted pressure from step 1 and the measured pressure by strain gauge and
applied by the pump (step 2) are compared through a comparator
5- When the values of 2 pressures are equal, the comparator works in 2 ways
(a) Give signal to
stop the pump
(b) Give signal to
solenoid valve to start
to release the cuff
pressure gradually
6- The pressure is always measured by the strain gauges and recorded all the
time using a memory system
7- The microphone (which placed under the cuff) detect the first sound when
the cuff pressure equal the systolic pressure. This will trigger the memory and
then store s the systolic value
8- The output of microphone is connected with a comparator with a minimum
level of sound can be recorded
9- When the pressure reaches to the diastolic pressure the microphone output
sound reaches to the value of minimum level of sound, then the comparator
works and gives a signal to the memory to record the value of diastolic
pressure ,, also it gives a signal to repeat the process again
(a) Close the solenoid
(b) Trigger the pump to be ready
to work again
Invasive Blood Pressure Measurements
1-Extra Vascular measurement of blood pressure
An electronic pressure transducer can be connected to the patient
through a thin piece of tubing called a catheter is filled with a salineheparin solution and inserted in the patient.
The pressure transducer diaphragm is coupled to the patient’s blood
stream; the diaphragm senses the pressure of the blood which
transmuted through the fluid in the catheter
The diaphragm is attached to strain gauge that converts the
diaphragm displacement to electric current
2- The intravascular method
At this case the sensor is placed inside the blood stream in the body
The optical pressure sensor is good example for invasive pressure sensor
The sensor consists of 2 bundles of optical fibers which inserted into a thin catheter in
the vessel. The first bundle is used to transmit light from source to the end of the
catheter (the diaphragm). The other bundle is used to transmit the reflected light to
the photo detector. At the end of the catheter, a very thin metal membrane
(diaphragm) is attached. The inner surface of the membrane is polished to reflect the
light. Due to the pressure of the blood, the metal membrane deflects with respect to
the pressure value.
The reflected light changes with the
value of membrane deflection. This
reflected light converted to current
by the photo detector
A calibration is needed to know the
relation between reflected light
(current) and blood pressure.
Blood Flow Measurement
The velocity of blood flow can be measured using many methods like (a)
Electromagnetic flow meter (b) Ultra-sound flow meter
(a) Electromagnetic flow meter
The popularity of the magnetic flow meter results from the following factors
1- It measures volume flow rate independent of the velocity
2- It produces accuracy up to ± 5%
3- It can measures velocity in vessels from 1mm to 20mm diameter
Theory
We know from basic electrical theory
that a voltage is created when moving
a conductor cuts a magnetic flux. If
that conductor is a blood carrying
vessel of diameter EE’, the voltage
generated will be
QB
E (in Volt ) 
50  a
E is the resultant potential
Q is the volume flow rate m3/sec
B is the magnetic flux in gauss (G)
A is the vessel radius in (m)
Example
Find the potential generated if blood flowing in a vessel with a radius
of 0.9cm cuts magnetic flux of 250G. Assuming a volume flow rate
175 cm3/sec.
Solution
QB
(175 x10 6 m 3 / sec)( 250G)
E 

50  a
(50)( )(0.9 x10 2 m)
 309 x10 6 volt
If the blood flows in a magnetic field, an emf will be generated and
picked by 2 electrodes. The electrodes must be small
The Ultra-sound Flow-meter
Ultra Sound waves are acoustical waves (like regular sound
waves, 30Hz to 20 KHz) in the range above human hearing (more
than 20 KHz). Like all acoustical waves, ultra sound waves are
subjected to Doppler shift; this effect is a slight alteration of
frequency (ΔF) when reflected from moving object.
A transmitter piezoelectric crystal sends a sound wave with
known frequency. These waves are reflected with different
frequency and received by the receiver crystal. This difference in
frequency corresponds to the velocity of the flow (blood).
The ultra sound flow meter can be used to measure blood and
gases flow rates in patient circuits because it’s known area.
Measurement of Gas Flow
The volume flow and volume flow rate are used to measure/estimate rate of
changes of lung volume. The instrument used to measure the volume flow
rate is called flow meter
1- The flow meter is a device that contains a calibrated tube to indicate gas
flow in Litters/min and a valve to control the flow. The flow meter must be
in upright position to obtain an accurate reading.
The relation between shape and weight of floating ball pressure and flow
rate are as follows
Other different types of flow meters
1- Rotating Vane flow meters
This type of sensor has a small turbine in the flow path. The rotation of the turbine
can be related to the flow meter of gas by using a calibration technique. Interruption
of a light beam by the turbine has also been sensed and converted to voltage
potential to flow and / or its integral to be recorded or displayed continuously
2- Ultra sound flow meters
3- Thermal convection flow meters
4- Difference pressure flow meters
First Order High Pass Filter
second Order High Pass Filter