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 are1-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 componentsBooks:Medical Instrumentation Application and Design, by WebsterIntroduction to biomedical Equipment Technology by J CarElectronic Instruments and Measurements, I.D. Jones and A.F. Chin

Learning outcomesKnowledge and understanding1-Understanding the basic system of units2- Understanding different processes of measurements 3- Understanding the types of measuring processes and their basics specially in the medical equipments4- 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 deviceability 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 MarksMark Assessment10%Quizzes30%Med Term Exams20%Practical Exam40%Final Exam100Total

Expected workload:On average students need to spend 2 hours of study and preparation for each 50-minute 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

Signal Conditioning

Sensing Element Signal Processing

Data OutputI/P 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.

ThermocoupleT oC mVStrain GaugesDisplacement

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 instrument

Thermometer

Measurand is Temp

Sensing Element is Mercury

Signal Conditioning is (H) High of Mercury

Data Presentation is scale

Scale

Temp Hg

Example of simple Display

Example of simple instrument

Load Cell using Strain Gauge

Load

StrainGauge

Body of Load Cell

Wheatstone Bridge

Amplifier and Filter

I/P

Sensing Element

∆Ω V

Signal Conditioning

PCA/D DisplayV

Signal Processing

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

mT XXe

TX

mXTrue (Expected) Value of Measurement

Measured Value

e Error

2- Accuracy

The accuracy of measurement is the degree of closeness of the measured value to the true value.

e% Percentage Error = %100TX

e

%100%T

mT

X

XXe

%%100 ePercentage Accuracy =

3- Sensitivity

Sensitivity is the ratio of output signal to the change in the input signal

i

o

X

XS

oX

iX

Sensitivity

Output Signal

Change in the Input

For Example

Thermocouple ,, the input is change in temp ,,

the output is Volt

If temp changes

What is the output volt ,,,, say 1mv

Then

4- Resolution

Resolution is the minimum change in the measured value can be sensed by the instrument

C

mvoltS

o1

1

Co1

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 instrumentthe fault of the person using the instrument (error in (error in reading, error in recording, incorrect use of the instrument)reading, error in recording, incorrect use of the instrument)

2-2 Systemic Errors2-2 Systemic Errors

These errors due to the problem with the instrument environment effects and it can errors due to the problem with the instrument environment effects and it can be classified to the following.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 Errors2-3 Random Errors

These errors is due to unknown causes and occur even when all gross and These errors is due to unknown causes and occur even when all gross and environmental errors have been reducedenvironmental errors have been reduced

Systemic ErrorsSystemic 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)Example (1)

A 300 Volt voltmeter is specified to be accurate within A 300 Volt voltmeter is specified to be accurate within ±2% at the full scale. at the full scale. Calculate the limiting error when the instrument is used to measureCalculate the limiting error when the instrument is used to measure

1- 120 Volt and 2- a source of 220 volt1- 120 Volt and 2- a source of 220 volt

Solution:Solution:

)()100

2(%2 scalefull

V6300100

2

Limiting Error =

For 120 volt this error become %5%100120

6

Case 1

Case 2

For 220 volt this error become %7.2%100220

6

Example (2)Example (2)

A voltmeter and Ammeter are used to determine the power dissipated in a resistor. A voltmeter and Ammeter are used to determine the power dissipated in a resistor.

Both instruments are guaranteed to be an accurate within Both instruments are guaranteed to be an accurate within ±1% at the full scale. If the at the full scale. If the voltammeter reads 80 Volt on its 150 Volt Range,, and the ammeter reads 70mA on voltammeter reads 80 Volt on its 150 Volt Range,, and the ammeter reads 70mA on its 100mA Full Scale. its 100mA Full Scale.

Calculate the limiting error for power calculationCalculate the limiting error for power calculation

Limiting Error =

Then Limiting Error for 80V reading become

%86.1%10080

5.1

For Voltmeter

Limiting Error for 70mA become

mAmA 1%100100

1

VV 5.1)150(100

1

For Ammeter

Limiting Error =

%43.170

1

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 MeanIf 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

nxxx ,........., 21

The arithmetic mean= Average =n

xxxx n ........321

n

xxxxX n

........321

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

X

XXd

XXd

XXd

nn

22

11

3- Standard Deviation (S) (Root Mean Square Value)It is defined as the variance of the measured value about the mean value

Example 3

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 = =50-49=1Volt

1

............ 222

21

n

dddS n

mT XX

2-Percentage Error= %2%10050

1%100

TX

Error

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)n

xxxxX n

........321

V9.494

2.506.497.491.50

2-Deviation of each value

d1= 50.1 - 49.9 = 0.2 V d2= 49.7 - 49.9 = -0.2 V d3= 49.6 - 49.9 = -0.3 V d4= 50. - 49.9 = 0.3 V

3-Sum of Deviation= 0.2-0.2+0.3-0.3=0

4-Standard Deviation3

)3.0()3.0()2.0()2.0( 2222 S=0.295=0.3

Reading become 3.09.49

Static Characteristics of Measuring Element

A- Range B- Span C- Ideal Straight Line D-Non-linearity

E- Environmental Effects F- Wear and Aging

A-Range

It is specified by the minimum and maximum values of I/P and O/P

(4-80)mVPressure

Transducer

(10-104) Pa

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 = K input Output =K Input +a

K is calibration factor (Slope)

K = slope

Output

Input

a

Output

Input

K = slope

D- Non-Linearity

The relation between the input and the output is non linear

N (non-linearity) = O actual – O Theoretical

Output

Input

K = slope

Output Therortical

Output Actual

N

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

PassiveActive

ElectricMechanical

Sensors

InductiveResistive Capacitive

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.

Selection of transducer criteria

1- Operating Range

2-Sensitivity

3-Frequancy response

4- Environmental Compatibility

5-Accurcy

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

The simplest form

of potentiometer is

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

The relation between the output voltage Eo and measured distance X is in

o

E

ELX

X

o

L

in

R

E

R

EI

oin

no

LX

EE

LX

EL

XE

LRXR

,,,

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

High Resistance wire wounded

2- Angular potentiometer

It is used to measure the change in angle Like Knee Elbow angles

The angle is related to the input volt, output volt and total potentiometer angle as follow

2- Angular potentiometer

in

o

E

E

angleterpotentiometotalis

measuredbetoanglerequiredis

oin EE

I R

E

R

EI oin

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

VVRR

RV

R

To 5.1

150050003

9.0

21

2

2

no EL

XE

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

PaminAArea

NinFForceStress

)(

)(2

)(

)()(

mminL

mminLalLongitudinStrain

PaEulussYoung

)(mod'

3.0',,,,,),()(

)(

RatiosPoissonis

mminW

mminWStrainLateral

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

A

LR

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

RR

LL

RRFactorGauge

/

/

/

R(Gauge )R+dR

ViVo

RR

StrainGauge

Body of Load Cell

Wheatstone Bridge

Amplifier and Filter

I/P

Sensing Element

∆Ω V

Signal Conditioning

PCA/D DisplayV

Signal Processing

Strain Gauge in Electronic Measuring System

Different Types of strain Gauge Bridges RR+dR

Vi Vo

RR

Vi Vo

R+dR

R+dR R-dR

R-dR

Vi Vo

R

R

R+dR

R+dR

Full Bridge

Quarter Bridge Half Bridge

RRVofFunctionVo in ,,

A resistive strain gauge with a gauge factor of 22 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

RR

LL

RRFactorGauge

/

/

/

)(

)(

AArea

FForceStress

)(mod' EulussYoung

2

2/105095

)02.0(4

33mKgStress

610 1025.5102105095

)( xxE

001.0

21025.5

130/

/

/6

Rx

R

LL

RRFactorGauge

2-Capacitive Sensor

The capacitive sensor consists of two parallel plates separated by insulation When charged, the plates carry equal charges of opposite sign.

+Q Q

d Area = A

d

AC ro

minplatesthebetweencedistheisd

minplatetheofareaisA

materialInsulatingtheoftConsDielectric

tConsDielectriclativeK

mFaradxSpaceFreetheoftConsDielectric

FaradinceCapaciC

I

o

Ir

o

tan

tan

tanRe

/108.8tan

tan

2

12

A-Type 1 Variable separation

The type of sensors used to measure the pressure between foot and shoe (variable distance between plates

xd

AC ro

Type 2-Variable Area

L

w

L

d

XW

d

AC roro .

Type 3-Variable dielectric

WxlA

XWAd

A

d

AC roro

)(

.

2

1

21 21

C1

C2

C3

C4

VoVi

Capacitive Bridge

,,, XdVofFunctionVo in

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

Faradx

mx

mFxmx

d

AC ro

12

3

1223

1025.44

101

)/10854.8)(105)(1(

Inductive Sensor

تعتمد الحساسات الحثية علىالتغير فى الحث الناتج عن ملف نتيجة اإلزاحة الحادثة على قلب هذا الملف

HenryGnL ............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

Vi

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

Linear Variable Deferential Transformer (LVDT)

e

a

b

c

d

Vout

Vo اإلزا

حة حوالى لكل 2الحساسية فولت ميللىمتر 0.1 ميللى

LVDT is widely used to measure displacement, pressure and force

The LVDT is composed of a primary coil (a-b) and two secondary coils (c-e) and (d-e) connected in series.

The coupling between these two coils is changed by the motion of high permeability alloy slug between them. The output volt

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

Materials concentrate the flux

called paramagnetic

Piezoelectric Transducers

q=kf ,

q=surface charge coulomb

f=force, (N)

k=constant Coulomb/N

مكبرAmplifier

Crystal

++V

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

The Piezo electric material can be act like parallel plate capacitor

A

dfK

C

fKV

C

qV

ro

K=2.3x10-12 C/N for quartzK=140x10-12 C/N for barium

Piezo electric materials have a high resistance

The equivalent circuit of Piezo electric circuit as shown

Charge generator

IR

RCVo

الدائرة المكافئة للبلورة المبسطة

Ia=0IS

Ic

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)

Voltmeter

Iron------- Copper

T1 ,Hot junction(Measuring probe)

Thermocouple

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

100

80

60

40

20

10

0 Temp

200 1600

V mvType E

Type J Type KType W

Type S

Junction Material Temp Range

Output Voltage

J - Iron-(Copper Nickel) 0 to 750 5.268mv

K (Nickel-Chrome)-(Nickel-Aluminum)

-200 to 1250 4.095mv

E (Nickel-Chrome)-(copper-Nickel)

-200 to 900 6.317mv

T Copper-(Copper-Nickel) -200 to 350 4.277mv

S (Platinum10%-Rhodium)-Platinum

0 to 1450 0.645mv

R (Platinum13%-Rhodium)-Platinum

0 to 1450 0.647mv

B (Platinum30%-Rhodium)-Platinum6%- Rhodium)

0 to 1700 0.033mv

ThermistorThermistor 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.

تستخدم اإللكترودات إما لقراءة إشارة من الجسم ، أو إلدخال إلى الجسم Stimulationإثارة

: تصنف إلى صنفين

Body surface األول هو اإللكترودات السطحية 1.electrodes

Internalالثانى هو اإللكترودات الداخلية 2.electrodes

Body surface- إلكترودات السطح 1electrodes

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

هرتز( .60/50خطوط القدرة )

معظمها مصنع من شريحة من كلوريد الفضة معزولة بطريقة أو أخرى حسب شكل اإللكترود

كمية التيار التى يتم قياسها باإللكترود تكون صغيرة جدا)ميكروأمبير( ، لذلك البد من مالمسة سطح اإللكترود لسطح

الجسم تماما .

توجد إلكترودات السطح فى أكثر من شكل

قرص قرص معدنى يستخدم معدنى

لمرة واحدة

مادة صمغية

جيال تين

أشكال أخرى

اإللكترودات الداخلية

إبرة تحت الجلد

Hypodermic needleاإللكترو

د Acuteللقياسات البسيطة measurements

عازل

قمة معدنية حادة

عضلة

Chronic للقياسات الدائمةrecording

قاعدة ماصةاإللكتر

لقياس ضربات قلب ودالجنين

Microelectrodesالمايكروإلكترود 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 .

ميكرومتر10 حتى 0.05يتراوح قطرها من

معدن

عازل زجاج الرأ

س

1 الرأسميكرون

محلول ملحى KCl

المايكروأنبوبة Micropipette أنبوبة شعرية

زجاجية

Metalمايكروإلكترود معدنى Microelectrode

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

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)

+

R1 R2

R3 R4

a b

I1 I2

I4

I3

I

1

3

R

4

a b

R 2

RR

3

I2

I1

I

•DC volt is used as a power source

•Null detector (Galvanometer) is connected to detect balance

Ch4

Using the bridge to determine unknown resistance

1- Assume R4 is unknown ,,

2- Assume we can change one resistor in the bridge (say R1) till the balance condition 2

4

R

3R

1 2

I

1R

R4

a b

I I

I

3

0 oba VorVV

)4(0

)3(0

)2(

)1(

33

44

11

22

RIV

RIV

RIEV

RIEV

a

b

a

b

At case of unbalance (reading= Va-Vb)

2

4

R

3R

1 2

I

1R

R4

a b

I I

I

3

)6(

4,3)5(

2,1,,,,,

0

3344

1122

1122

3142

RIRI

FromRIRI

RIERIE

FromIIIIandVV

VVbalanceAt

ba

ab

32414

2

3

1

4433

2211

RRRRorR

R

R

R

RIRI

RIRI

Example (1)Determine the value of unknown resistor Rx in Fig. assuming the balance

condition

????

32

15

12

3

2

1

xR

KR

KR

KR

SolutionAt Balance Vo=0

Vo

+

R1

R2

R3 R

x

a b

I1

I2

KRK

KK

R

RRR

RRRR

xx

x

4012

)32()15(

1

32

132

• 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

A

ree

A deg

Thevenin Theory for the bridge

I

1

3

R

4

a b

R 2

RR

3

I2I1

IE E

0 0

+

R1 R2

R3 R4

a b

I1 I2

I4

I3

422

311

0,,

0

RR

EI

RR

EI

bath

b

a

VVV

RR

RERIV

RR

RERIV

42

442

31

331

0

0

Thevenin Theory for the bridge

R1

R3

a b

R2

R4

R1

2

R

R

a

4

b

R3ba

31

31

RR

RR

42

42

RR

RR

Loadth

thGalv

th

th

th

RR

VI

RR

RR

RR

RRR

RR

RE

RR

REV

RR

RE

RR

REV

)(

)(

42

42

31

31

42

4

31

3

42

4

31

3

Vth +

a

Load

b

ExampleCalculate the current through the galvanometer shown in Fig

+

R1 R2

R3 R4

a b

I1 I2

I4

I3

Rg

200

5.75.36.116 4321

galvRand

KRKRKRKRVE

VoltV

RR

R

RR

REV

th

th

276.0)6.15.7

5.7

15.3

5.3(6

)(24

4

31

3

Solution

KR

RR

RR

RR

RRR

th

th

097.2)5.76.1

)5.7)(6.1(

31

)3()1((

)(42

42

31

31

AI

RR

VI

Galv

Loadth

thGalv

12010)2.0097.2(

276.03

تهيئة إشارة القنطرة لتوصيلها على الحاسب

+

Vi Vo

R1

R1

R2

R2

Difference Amp.

Isolator (Buffer)

+

+

+ V1

V21

2)21(R

RVVVo

Ch5

Blood Pressure and other Cardio Vascular Measurements

Firstly the blood pressure is measured in arteriesThere are two kind of arteries pressure 1-Systolic pressure It is the pressure in arteries at case of heart contraction ≈ 120 mmHg2- Diastolic pressure It is the pressure in arteries in the case of heart relaxation ≈ 80 mmHg

Pressure =

F = Force in Newton A is the area in m2

1 Pa = N/m2

)(PaA

F

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

Nxm

xKgxForce 32

3 1015sec

10105.1

)(191

4

)101(

1015Pr)(

222

3

Paorm

N

x

x

A

Fessureb

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 measurement1- 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 Measurements1- The user adjusts the pressure of the system (by touching digital bottoms) this

expected pressure above the expected systolic pressure2- 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 comparator5- 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 recorded9- 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 Measurements1-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 saline-

heparin 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 methodAt 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 MeasurementThe 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 velocity2- 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

a

BQVoltinE

50)(

E is the resultant potentialQ is the volume flow rate m3/secB 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

voltxmx

Gmx

a

BQE 6

2

36

10309)109.0)()(50(

)250sec)(/10175(

50

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 meter1- 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 metersThis 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 continuously2- Ultra sound flow meters3- Thermal convection flow meters4- Difference pressure flow meters

First Order High Pass Filter

second Order High Pass Filter