UNESCO-NIGERIA
UNESCO
TECHNICAL &
VOCATIONAL EDUCATION
REVITALISATION PROJECT-PHASE
PROJECT
II
NATIONAL DIPLOMA IN
ELECTRICAL ENGINEERI
ENGINEERING
NG TECHNOLOGY
ELECTRICAL/
RICAL/ ELECTRONIC
INSTRUMENTATION (II)
COURSE CODE: EEC 236
YEAR III SEMESTER III
THEORY
Version 1: December 2008
TABLE OF CONTENTS
Department
Electrical Engineering Technology
Subject
Electrical Electronics Instrumentation 11
Year
2
Semester
3
Course Code
EEC 236
Credit Hours
2
Theoretical
1
Practical
2
WEEK 1: Cathode Ray Oscilloscope
1.1.
Introduction………………………………………………………1
1.2.
Cathode Ray OscilloscopeOperation…………………………….2
WEEK 2: Cathode Ray Tube
2.1. Introduction………………………………………………………5
2.1.1 The electron gun…………………………………………….5
2.1.2. The deflecting system………………………………………5
2.1.3. The fluorescent screen………………………………………6
2.2. A simple circuit diagram of (CRO)………………………………7
WEEK 3: Measurement Of Voltages And Currents
3.1. Introduction…………………………………………………………9
3.2. Measure of phase and frequency……………………………….....10
3.2.1. Frequency measurements……………………………………11
2
WEEK 4: Power Factor Meter
4.1. Introduction………………………………………………………13.
4.2. Electrodynamic power factor meters………………………….13
4.2.1. Principle of operation of electrodynamic power factor meter.15
WEEK 5: Measurement of Power And Power factor
5.1. Introduction………………………………………………………………17.
5.2. Power factor………………………………………………………………17
5.3. Calculating power factor…………………………………………………19
WEEK 6:Wattmeters:
6.1. Introduction……………………………………………………23
6.2. Dynamometer wattmeter………………………………………23
6.2.1. construction………………………………………………..23
6.2.2. Operation…………………………………………………..24
6.2.3. Advantages and disadvantages…………………………….25
6.2.4. Errors……………………………………………………27-28
6.3. Deflecting torque………………………………………………25
WEEK 7: Power measurement in three phase cct
7.1.
Introduction……………………………………………………………………32
7.2. Three wattmeter method………………………………………32.
7.3. Two wattmeter method…………………………………….....33.
7.3.1. Two wattmeter method balanced load……………………36
WEEK 8: One wattmeter method.
8.1. Introduction……………………………………………………….39
8.2. Reading of wattmeter……………………………………………40
8.2.1. Balanced load lagging power factor………………………….40
8.2.2. Balanced load leading power factor. …………………………41
WEEK 9: Instruments selection and specifications.
9.1. Introduction………………………………………………………43
3
9.2. Factors of an instrument selection……………………………42-45
WEEK 10: Importance of instruments in industries.
10.1. Introduction…………………………………………………47
10.2. Importance of measurement in industry……………………47
10.3. Pressure measurement………………………………………48
10.3.1. Importance of pressure measurement……………………49
WEEK 11: Temperature measurement.
11.1. Introduction………………………………………………….50
11.2. Types of thermometers………………………………………50
11.3. Classification of thermometers…………………………......51
11.3.1. Practical thermometers……………………………………51
11.3.2. Common thermometers……………………………………51
11.4. Importance of temperature measurement……………………51
11.5.Level measurement……………………………………………52
11.5.1. How to obtain level measurement………………………52
11.5.2. Importance of level measurement………………………..53
WEEK 12:Flowrate measurement.
12.1. Introduction……………………………………………………55
12.1.1. Importance of flowrate measurement………………………55
12.2. Measurement of density………………………………………56
12.3. Viscosity measurement………………………………………56
12.3.1. Importance of viscosity…………………………………….57
WEEK 13: Humidity measurement.
13.1. Introduction…………………………………………………59
WEEK 14: Electrical electronics instruments.
14.1. Introduction…………………………………………………61
14.2. Types of electrical instruments………………………………61
14.2.1. Indicating instruments……………………………………61
14.2.2. Recording instruments……………………………………61
4
14.2.3. Controlling instruments…………………………………62
14.3. Principles of operation of electrical instruments……………62
WEEK 15: Essentials of indicating instruments.
15.1. Introduction……………………………………………………63
15.1.1. Deflecting torque…………………………………………63
15.1.2. Controlling torque…………………………………………64
15.1.3. Damping torque……………………………………………64
5
This Page is Intentionally Left Blank
6
WEEK 1
INTRODUCTION:
The cathode ray oscilloscope (CRO) is a common laboratory instrument that provides
accurate time and amplitude measurements of voltage signals over a wide range of
frequencies. Its reliability, stability, and ease of operation make it suitable
suitable as general
purpose laboratory instrument. The block diagram of cathode ray oscilloscope is shown
in fig 1.1.
1.2 CRO Operation:
A simplified block diagram of a typical oscilloscope is shown in fig 1.0. in general the
instrument is operated in the following manner.
(i)Vertical amplifier:
This amplifies the input signal to observed. The vertical amplifier is the principle
factor determining the sensitivity and bandwidth of an oscilloscope. Greater sensitivity
expressed in V/cm of the vertical deflecting
deflecting is obtained at the expense of the bandwidth.
(ii)Horizontal amplifier:
The sweep generator output or any signal applied to the horizontal input terminal will
be amplified by the horizontal amplifier and applied to the horizontal deflection plates.
Fig 1.2. Horizontal and vertical amplifier.
(iii)Time base sweep generator:
The linear deflection or sweep of the beam of an oscilloscope horizontally is
accomplished by use of a sweep generator that is incorporated in the oscilloscope circuitry.
The voltage output of such a generator is that of a saw tooth wave as shown in Fig.2.
Application of one cycle of this voltage difference, which increases linearly with time, to the
horizontal plate causes the beam to be deflected linearly with time across the tube face. When
the voltage suddenly falls to zero, as at points (a),(b),(c), etc…, the end of each sweep- the
beam flies back to its initial position. The horizontal deflection of the beam is repeated
periodically, the frequency of this periodicity is adjustable by external controls.
During sweep time (a), the beam is deflected to the right by increasing the amplitude
of the ramp voltage and the fact that the position voltage attracts the negative electrons.
During the retrace fly-back time, the beam returns quickly to left side of the screen, thus
blanking out the beam by means of blanking circuit.
8
WEEK 2
(iv)Triggered sweep:
A trigger circuit is incorporated into the oscilloscope . The trigger circuit may receive
an input from an external source when the trigger selector switch is set to EXT and from a
low amplitude a.c voltage at the line frequency when the switch is set to LINE or from the
vertical amplifier when the switch is set to INT.
When set for internal triggering (INT), the trigger circuit receives it input from the
vertical amplifier. When the vertical input signal is being amplified by the vertical input
signal is being amplified by the vertical amplifier until it reaches a certain level, Then the
trigger circuit provide a pulse to the sweep generator output is synchronized with the signal
that trigger it.
(v)Synchronization:
whatever type of sweep is used, it must be synchronized with the signal being
measured. Synchronization has to be done to obtain a stationary pattern. This requires the
time base to operate at a submultiples frequency of the signal under measurement (signal
applied to Y-plate). See fig. below. If synchronization is not done the pattern is not
stationary, but appears to drift across the screen in a random fashion.
(vi)Blanking circuit:
The control grid is generally “gated off” which blank out the beam during the retrace
(flyback) time to prevents undesirable retrace pattern from appearing on the screen.
9
(vii) Calibration:
The graticules has to be calibrated in order to give the desired scale for measurements
carried out. The graticule is a grid of lines that serves as a scale when making time and
amplitude measurements. See fig below.
10
MEASUREMENT
CURRENTS
OF
VOLTAGES
AND Week 3
1.1 Introduction:
The expression for electrostatic deflection shows that the deflection is proportional to the
deflection-plate voltage. Thus the cathode ray tube will measure voltage. It is usual to
calibrate the tube under the given operating conditions by observing the deflection produced
by a known voltage. Direct voltages may be obtained from the static deflection of the spot,
alternating voltages from the length of the line produced when the voltage is applied to Yplates when no voltage is applied to X-plates. The length of this line corresponds to the peak
to peak voltage. When dealing with sinusoidal voltages, the rms value is given by dividing
the peak to peak voltage by 2
Laboratory oscillographs frequently incorporate voltage measurement a facility by
including constant gain amplifies and calibrate shift controls. The Y-shift control is adjusted
so that positive peak of the test voltage coincides with some datum line on the screen; the
shift control is then operated until the negative peak coincides with the datum. The movement
of control is arranged to read directly the peak to peak voltage. The value of current can be
obtained by measuring the voltage drop across a known resistance connected in the circuit.
To determine the size of the voltage signal appearing at the output of terminals of the
signal generator, an AC (Alternating current) voltmeter is connected in parallel across the
11
terminals (fig 2.1).The AC voltmeter is designed to read the dc “effective value” of the
voltage. This effective value is known as the” root mean square value” (RMS) value of the
voltage.
The peak or maximum voltage seen on the scope(fig 2.1b) is Vm volts and is
represented by the distance from the symmetry line CD to the maximum deflection . The
relationship between the magnitude of the peak voltage displayed on the scope and the
effective or RMS voltage (VRMS) read on the AC voltmeter is
VRMS=0.707Vm (for sine or cosine wave).
Thus
Vm = VRMS/0.707
3.2 Measurement of phase and frequency (Lissajous pattern)
It is interesting to consider the characteristics of patterns that appear on the screen of a
CRT when sinusoidal voltages are simultaneously applied to horizontal and vertical plates.
These patterns are called ‘Lissajous patterns’.
When two sinusoidal voltages of equal frequency which are in phase with each other
are applied to the horizontal and vertical deflection plates, the pattern appearing on the screen
is a straight line.
Two sinusoidal waveforms of the same frequency produce a lissajous pattern, which
may be a straight line, a circle or an eclipse depending upon the phase and magnitude of the
voltages. A circle can be formed only when the magnitudes of the two signals are equal and
the phase difference between them is either 900 or 2700. However, if the two voltages are
not equal or out of phase an ellipse is formed.
When sine wave signals of different frequencies are input to the horizontal and
vertical amplifier as stationary pattern is formed on the CRT when the ratio of the two
frequencies is an integral fraction such as ½, 2/3, 4/3, 1/5,etc. These stationary patterns are
known as lissajous figures and can be used for comparison measurement of frequencies.
3.2.1 Frequency Measurements:
When the horizontal sweep voltage is applied, voltage measurement can still be
taken from the vertical deflection. Moreover, the signal is displayed as a function of time. If
the time base(i.e. sweep) is calibrated, such measurements of pulse duration or signal period
can be made. Frequencies can then be determined as reciprocal of the periods.
12
Set the oscillator to 1000HZ. Display the signal on the CRO and measure the signal of
the oscillations. Use the horizontal distance between two points. Set the horizontal again so
that only one complete waveform is displayed. Then reset the horizontal until 5 waves are
seen. Keep the time base control in a calibrated position. Measure the distance (and hence
time) for 5 complete cycles and calculate the frequency from this measurement.
13
POWER FACTOR METERS
Week 4
4.1Introduction
On measuring the current, voltage and power in a.c circuit ,its power factor can be
calculated from the relationship COS=p/IV. The method of determining the power factor of
an electric circuit , is however, of low accuracy and is rarely used in practice. It is obviously
desirable to an instantaneous indication of the power factor of an a.c circuit, especially where
this is varying continuously, without having resource to mathematical calculation of the
readings of several instruments. Power factor meters indicate directly , by a single reading ,
the power factor of the circuit to which they are connected. The accuracy obtained with the
use of p.f meters is sufficient for most purpose other than high precision testing.
Power factor meters like wattmeters have a current circuit and a pressure circuit. The
current circuit the current (or definite fraction of this current) in the circuit whose power
factor is to be measured. The pressure circuit is connected across the circuit whose power is
to be measured and is usually split up into two parallel paths, one inductive and the other
non-inductive. The deflection of the instruments depend upon the phase difference between
the main current and current in two paths of the pressure circuit, i.e upon the phase angle or
power factor at the circuit. The deflection is indicated by a pointer.
The moving system of power factor meters is perfectly balanced at equilibrium by
two opposing forces and therefore there is no need for a controlling devices. Hence when a
power factor meter is disconnected from a circuit the pointer remain at its position which is
occupied at the instant of disconnection.
14
4.2 ELECTRODYNAMIC POWER FACTOR
METERS
The most commonly used power factor meter is the electrodynamic (Dynamometer)
type. The construction of dynamometer power factor meter is shown in figure 4.1.
I1
Fixed coil
Fixed coil
Movable coil
I2
Multiplier
Load
R
Fig 1.3 Dynamometer wattmeter
The diagram in fig.5.0 consist of a fixed coil which acts as the current coil. The coil
is split up into two parts and carries the current of the circuit under test. Therefore the
magnetic field produced by this coil is proportional to the main current. Two identical
pressure coil A and B pointed on a spindle constitute the moving system. Pressure in coil A
non-inductive resistance R connected in series with it, and coil B has a highly inductive
choke coil L connected in series with it. The two coils are connected across the voltage of the
circuit. The value of R and L are so adjusted that the two coils carry the same values of
current at normal frequency, i.e R=WL. The circuit through coil A is in phase with the circuit
voltage while that through coil B lags the voltage by nearly 90o
15
4.2.1
PRINCIPLES
OF
OPERATION
OF
ELECTRODYNAMIC POWER FACTOR METER.
When the load power factor is unity (COS =1); current is in phase with the voltage.
Then I1 is in phase with the voltage. Then I1 is in phase with I where as I2 lags behind by
90o.Consequently, a torque will act on coil A which will set its plane perpendicular to the
common magnet axis of fixed coils (FA + FB) i.e. corresponding to the pointer position of
unity p.f. However, there will be no torque acting on coil B. When load power factor is zero,
current (I) lags behind voltage (v) by 90o and I2 will be in phase with I where as I1 will be 90o
out phase. As a result, there will be no torque on coil A but that acting on coil B will bring its
plane perpendicular to the common magnetic axis F1 and F2. For intermediate values of
power factor, the deflection of the pointer correspond to the load power factor angle or to
COS , if the instrument has been calibrated to read the power factor directly.
For reliable readings the instrument has to be calibrated at the frequency of the supply
on which is to be used. At any other frequency, the reactance of L will be change so that the
magnitude and the phase of current through coil B will be incorrect and that will lead to
serious errors in the instrument readings.
QUESTIONS
1. Draw a simplified diagram for a power factor meter.
2. Briefly explain the principles of operation of it.
16
MEASUREMENT OF POWER AND POWER Week 5
FACTOR
5.1 Introduction:
Power in a d.c circuit is calculated by multiplying current and voltage (P =I X V) but
the power in an a.c circuit depends on the phase relationship between current and voltage is
taken into consideration by using a power factor in the calculation of a.c power.
5.2 POWER FACTOR:
In a.c circuit analysis the product of r.m.s value of current and voltage, VI is referred
to as the volt-amperes. As has been seen, this product does not in general represent the power
absorbed by a load. The “power factor” is defined as that factor by which the volt-amperes
must be multiplied to give the time power absorbed. Thus power absorbed (watts) =voltamperes x power factor………………………………………(1)
P=IV COS watts…………………………………………………………...(2)
Comparing equation (1) and (2) it is seen that the power factor when the current and
voltage waves are sinusoidal is cos , i.e the power factor (P.F) is the cosine of angle of the
phase difference between the current and voltage. It must be emphasized that this result
obtains only when waves shapes are sinusoidal (see fig 5.o)
V
P.F = cos
I
90
2700
Angle of phase difference
between current
& voltage. The p.f is the consine
of the angle.
For the case of sinusoidal waves the power factor of a circuit is said to be leading or
lagging according to whether the current in the circuit leads or lag the applied voltage.
From equation ( 2) above , the power(P=IVCOS) in watts is often referred to as the
17
true power. We can also determine true power in a circuit by measuring it with a
wattmeter. A wattmeter is constructed so that it takes into account any phase
difference between current voltage.
Sometimes it is as important to apparent power in a circuit as it is to know the true
power. The “apparent power” is the power that appears to be present when the voltage and
current in a circuit are measured separately. The apparent power, then is the product the of
the voltage and current regardless of the phase angle . Apparent power is calculated by the
formular P=IV
watts. When we look closely at the formular for apparent power and true power
P=IVCOS and Papp =IV.
We notice that, the only difference between the two is that the true power includes
COS term. Combining these two formulars yields COS = P/Papp
This relationship makes it relatively easy to determine COS and thus the phase
relationship between current and voltage. All we need to know is the crrent, the voltage and
the true power.
Example 1
An electric motor draws 18A of current from a 240V source. A wattmeter connected to
the current indicates 3024W. what is the power factor of the circuit.
Solution:
power factor = p/Aapp
Papp =IV =18 X 240 =4320VA
Power factor = 3024/4320 = 0.7. Therefore the power factor is 0.7 or 70%
5.3 CALCULATING POWER FACTOR
There are two main method of finding the power factor of a circuit.
(i)By
using
the impedance triangle
(use
the
trigonometric
ratio
COS
=adjacent/hypotenuse where COS =p.f, adjacent =resistance and hypotenuse =
impedance. Therefore power factor =R/Z.
(ii)By using power triangle which is made of three factors
(a)The true power in watts (the adjacent side).
(b)The apparent power in volt-ampere(the hypotenuse)
(The reactive component, measured in volt-ampere reactive (Var)
18
R
Z
XL
Impedance triangle
Apparent power
(P)
(Papp)
XL
Reactive power triangle (IAR)
Example 2:
A coil having a resistance of 7ohms and an inductance of 31.8MHis connected to
230V, 50HZ supply. Calculate (i)the circuit current (ii)phase angle (iii)power factor
(iv)power consumed
Solution:
XL = 2FL =2πx50x31.8x10-3 =10ohms Z2= R2+XL2 =72+102 =12.2ohms
(i)I = V/Z =230/12.2 =18.85A
(II) =Tan-1XL/R = tan-110/7 = 550 lag
(iii)power factor (pf) = cos = cos55 = 0.573 lag
19
(iv)P=IV COS =230 X 18.85 X 0.573 =2484.24W
Example 3:
A 230v , 50HZ a.c supply to a coil of 0.06H inductance and 2.5Ω resistance
connected in series with a 6.8 µf capacitor. Calculate (i) impedance (ii) current (iii) phase
angle between current and voltage (iv) power factor (v) power consumed.
Solution:
XL =2FL =2π X 50 X 0.06 =18.84Ω
XC =1/2FC = 106 /2π X 50 X 6.8 =468Ω
X = XL - XC =18.18 -468 =-449.16Ω
(i)√R2 + X2 = √2.52 + -449..162 = 449.2Ω
(II)I =V/Z =230/449.2 =0.512A
(III) =Tan-1X/R = tan-1 -449.16/2.5 =-89.70
The negative sign with shows that current is leading the voltage.
(iv)power factor = cos = R/Z = 2.5/449.2 = 0.0056 lead
(v)P= IV COS = 230 X 0.152 X 0.0056 = 0.66W.
I reactive
IT = 0.572A
= 89.7
20
WATT METERS
Week 6
6.1 Introduction:
A wattmeter as its name implies , measures electric power given to or developed by
an electric apparatus or circuit. A wattmeter is hardly ever required in a dc ciacuit because
power (p =IV) can be easily determined from voltmeter and ammeter readings. However, in
ac circuit, such a computation is generally speaking impossible. It is because in an ac circuit ,
power (p = IV COS) depends not only on voltage and current but also on the phase shift
between them. Therefore, a wattmeter is necessary for ac power measurement . The
“wattmeter” is an indicating type instruments, generally used for power measurement of the
electrical circuit. There are two principal type of wattmeters VIZ:
(i)Dynamomemeter wattmeter – for both dc and ac
power.
(ii)Induction wattmeter – for ac power only.
6.2 Dynamometer Wattmeter.
The dynamometer wattmeteris most commonly used to measure power in ac circuits.
It works on the dynamometer principle i.e mechanical force exists between two current
carrying conductors or coils.
6.2.1 Construction:
When dynamometer instrument is used as a wattmeter, the fixed coil are connected in
series with the load and carry the load current( I1) while the moving coil is connected across
the load through a series multiplier Rand carries a current (I2 ) proportional to the load
voltage as shown in fig 6.1. The fixed coil (or coils) is called the current coil and the movable
coil is known as
21
I1
Fixed coil
Fixed coil
Movable coil
I2
Multiplier
Load
R
Fig 1.3 Dynamometer wattmeter
potential coil. The controlling coil is provided by two spiral spring which also serve the
additional purpose of leading current into out of the moving coil. Air friction damping is
provided in such instruments. A pointer is attached to the movable coil.
6.2.2 Operation:
When the wattmeter is connected in the circuit to measure power (see fig 6.1), the
current coil carries the load current and potential coil carries current proportional to the load
voltage. Due to currents in the coils, mechanical force exists between them. The result is that
movable coil moves the pointer over the scale. The pointer comes to rest at a position where
deflecting torque is equal to controlling torque. Reversal of current reverses currents in both
the fixed coils and the movable coil so that the direction of deflecting torque remains
unchanged . Hence, such instruments can be used for the measurement of d.c as well as a.c
power.
6.3 Deflecting Torque:
We shall now prove that deflecting torque is proportional to load power in a d.c as
well as a.c circuit.
(i)Consider that the wattmeter is connected in a d.c circuit to measure power as shown
in fig (6.1b). the power taken by the load is VI1.
Deflecting torque , Td I1I2
22
Since I2 is directly proportional to v, therefore deflecting torque, Td VI1 load power.
(ii)Consider that the wattmeter is connected in a.c circuit to measure power . Suppose
at any instant , current through the load is I and voltage across the load is V.
I1
Current
coil
Potential
coil
V
R
Load
I2
. V = VM Sin
I =IM sin( -)
Instantaneous deflecting torque VI
The pointer cannot follow the rapid changes in the instantaneous power owing to the
large inertia of the moving system. Hence the instruments indicates the mean
oType equation here.r average power.
Therefore, average deflecting torque , Td average of VI over a cycle.
56
Td + 1/2π,7 -./ 0 1./2 3 4
56 8.1./ sin2
1/2π,7
3 4 56
VmIm/2π,7 / sin2 3 4 56
VmIm/2π,7 3 22 3 4
VmIm/4π; 3 sin 22 3 4/2=
23
Vm/√2 · Im/√2 COS
?VI COS
@Td load power
Thus weather the instrument is used to measure dc or ac power, deflecting torque is
proportional to load power (true power).
Since the instrument is spring controlled, Tc In the steady position of deflection Td = Tc
@ load power. Hence such instruments have uniform scale.
6.4 Advantages of Dynamometer Wattmeters
(i)Such instruments can be made to give a very high degree of accuracy . Hence, they
are used as standard for calibration purposes.
(ii)They are equally accurate on dc as well as ac measurements.
(iii)Scales are more or less uniform because the deflection is proportional to the
average power.
(iv)It can be used for both ac and dc supply, for any waveform of voltage and current,
and is not restricted to sinusoidal waveforms.
6.4.1 Disadvantages:
(i)At low power factors, the inductance of the potential coils causes serious errors
unless special precautions are taken to reduce this effect.
(ii)The readings of the instrument may be affected by strong magnetic fields. In order
to prevent it, the instrument is shielded from the external magnetic fields by enclosing it with
a soft-iron case.
6.4.2 Errors:
Following the errors of dynamometer wattmeter.
(i)The inductance of the moving (or voltage)coil causes errors, which cam be avoided
to some extent by connecting a high non-inductive resistance in series with the coil.
(ii)Errors due to voltage drop in the circuit.
(iii)Errors due to current taken by the voltage coil.
(iv)Errors due to capacitance in potential coil circuit.
(v)Errors due to stray fields.
24
(vi)Errors due to eddy currents.
Example 1:
A dynamometer type wattmeter with its voltage coil connected across the load sides reads
192w. The load voltage is 208v and the resistance of the potential coil circuit is 3825Ω,
calculate (i)True load power (ii) percentage errorto wattmeter connection.
Solution:
Wattmeter reading =192w
Power taken by potential circuit =V2/R
=2082/3825 = 11.3w
(i)True load power = 192 – 11.3 = 180. 7w
(ii)percentage error = 192 – 180.7/180.7 x 100 ? 6.25%
Example 2:
A dynamometer type wattmeter with its voltage coil connected across the load side of the
instrument reads 250w. If the load voltage is 200v, what power is being taken by load? The
voltage coil branch has a resistance of 2000Ω.
Solution:
Power consumed by voltage coil is =V2/R
2002/2000
@Power being taken by load = 250 – 20 = 230w
Example 3:
The resistance of the two coils of a wattmeter are 0.01Ωand 1000Ω respectively and both are
non-inductive. The load is taken a current of 20A at 200V and 0.8 p.f lagging. Show the two
ways in which the voltage coil can connected and find the error in the reading of the meter in
each case.
Solution:
Load power = IVCOS = 200 X 20 X 0.8 = 3200W
(I)Consider the connection shown below
Power loss in current coil = I2 RC = 202 X 0.01 = 4W
@ Wattmeter reading = 3200 + = 3204w
25
Percentage error = 40/3200 x 100 = 1.25%
The figure below shows the two ways of connecting the voltage coil of the wattmeter.
QUESTIONS
1. What do you understand by wattmeter.
2. Mention the two measure types of wattmeter.
3. Draw and explain the construction of dynamometer wattmeter.
26
POWER MEASUREMENT IN THREE PHASE Week 7
CIRCUIT
7.1 Introduction:
The following methods are available for measuring power in a 3-phase load (STAR
OR DELTA CONNECTED).
7.2 THREE- WATTMETER METHOD
In this method , three watt meters connected in such away that each has its current
coil in one line and its potential coil between that line and some common point. The algebraic
sum of the readings of the three wattmeters give the total power consumed whether the load
is balanced or not. If neutral wire is available, the common point should be at the neutral
wire.
L3
L1
W2
0Y
L2
0B
27
W1
R
L1
N
L1
L2
L3
W2
B
Y
W3
It can be shown mathematically that algebraic sum of their readings gives the total power
consumed whether the load is balanced or not i.e Total power = w1 + w2 +w3
7.3 TWO-WATTMETER METHOD
In this method, the current coils of the two watt wattmeters are connected in any two
lines and the potential coil of each joined to the third line. The algebraic sum of their readings
gives the total power consumed whether the load is balance or not. If the neutral wire is
available, it should carry no current or else the neutral of the load should be isolated from the
neutral of the source.
0R
R
IR
L1
L1
L3
IB
B
Iy
Y
0Y
0B
28
0R
R
IR
L1
L1
L3
IB
Iy
Y
B
0Y
0B
It can be proved that the sum of the instantaneous power indicated by W1 and W2
gives the instantaneous power absorbed by the three loads L1, L2 and L3. The star connected
load is considered in the following discussion , although it can equally applied to a delta
connection load because a delta connected load can always be replaced by an equivalent Yconnected load.
Now before we consider the current through and p.d across each wattmeter, it may be
pointed out that it is important to take the direction of the voltage through the circuit is the
same as that taken for the current when establishing the reading of the two wattmeter.
Instantaneous current through WL = Ir
Instantaneous p.d across W1 = eRB = eR - eB
Instantaneous power read by w1 = Ir(eR - eB )
Instantaneous current through w2 = iY
Instantaneous p.d across w2 = eYB = eY – eB
Instantaneous power read by w2 = Iy(eY –eB)
@ W1 + W2 = iR(eR – eB) + iY(eY – eB)
=iReR – iReB + iYey - iyeB
IReR + iYeY – Eb (iR B Iy)
Now iR B iY B iB = 0 …………………………………………………………kirchoff’s
point law
@ iR B iY ? 3Ib
W1 B W2 ? iReR B iYeY B iBeB ? P1 B P2 B P3
29
Where p1 is the power absorbed by load L1
P2 is that absorbed by load L2
P3 is absorbed by load L3
W1 B W2 ? Total power absorbed
This proof is true whether the load is balanced or unbalanced. If the load is Y
connected, then it should have no neutral connection (i.e.3- , 3-wire connected) and if it has
a neutral connection (i.e. 3- , 4-wire connected), then it should be exactly balanced so that
in each case there is no neutral current iN, otherwise kirchoff’s point law will give iN B iR B
iY B iB ? 0.
7.3.1 Two-Wattmeter Method – Balanced load
If the 3- phase load (C ∆) is balanced, power factor of the load can also be found
from two wattmeter readings. The C- connected load in fig. above will be assumed inductive.
The phasor diagram of such a balanced C-connected load is shown in fig. below. We shall
now consider the problem in terms of r.m.s values instead of instantaneous values.
Let VR, VY and VB be the r.m.s value of the three phase voltages and IR, IY and IB be
the r.m.s values of the currents. Since these voltages and currents are assumed sinusoidal,
they can be represented by vectors, the currents lagging behind their phase voltages by as
shown in the phasor diagram.
Current through current coil of Wi ? IR
p.d across potential coil of W1 is VRB ? VR - VB (vectorially)
Thus VRB is found by compounding VR and VB as shown in fig. below. It is seen that phase
difference between VR and IR ? (30 3 )
Reading of W1 ? VRBIRCOS (30O3 )
Current through W2 ? IY
p.d across
w2 ? VYB? VY3VB…………………vectorially
Again VYB is found by compounding VY with VB reversed as shown in fig. below.
The angle between IY and VYB is (30o B )
Reading of W2 ? VYBIYCOS (30o B )
Since the load is balanced
VRB ? 8YB? Line voltage VL
IR ? IY ? line current IL
@ W1 ? VLILCOS (303 )
W2 ? VLILCOS(30 B )
@ W1 B W2 ? VLILCOS(30 3 ) B VLILCOS(30 B )
30
=VLIL(2COS30oCOS)? √3 VLILCOS ? the total power in the three phase
load
Hence the sum of the two wattmeter readings give the total power consumption in the
three phase load.
Example
A three phase generator has 15000V and 400Aat 0.9 power factor. Find the power in
kilowatt if it is star Y connected.
Solution
P? √3 VLILCOS
P ? √3 x 15000 X 400 X 0.9 ? 9353074W
P ? 9353074/1000 ? 9353KW
QUESTIONS
1. Draw a simplified diagram for a three phase wattmeter two elements. Label the parts.
2. Three phase induction motor operating at 415V and draw a current of 8A at power
factor of 0.85. Find
a. The power consumed in kilowatts.
b. The apparent power in KVA
3. Find the line current of a three phase star connected balanced load, if the operating
voltage is 2400Vwith 400KW at 0.9 power factor.
31
ONE WATTMETER METHOD
Week 8
8.1 Introduction:
The method can be used only when load is balanced, the power in any phase can be
measured by a single wattmeter. The total circuit power can be measured by a single
wattmeter. The total circuit power is given by multiplying the wattmeter reading by three.
This method can only be used if the load is balanced. For the shown in fig. below, the current
coil is connected in one of the lines and one two lines. The phasor diagram is shown in fig,
below.
Fig.8.1 One wattmeter method.
32
Fig. 8.1.1 phasor diagram of one wattmeter method.
8.2 Reading of wattmeter
How can we tell which wattmeter reads higher and which reads lower?
W1 ? VLILCOS (30O 3 )
W2 ? VLILCOS (30OB )
Since the value of the load P.F. can vary from 0 to 1 (i.e. can vary from 90o to 0o), it
is clear that wattmeter whose deflection is proportional to (30o3 ) is always positive and
always higher reading wattmeter (i.e. w1 in this case). Except for the case when load P.F. is
unity, (i.e. ? F4 at, which the two wattmeter have equal readings.
8.2.1 Balanced Load Lagging Power Factor
In case the load is balanced (currents and voltages are sinusoidal) and for a lagging
power factor,
W1 B W2 ? VLILCOS (30° 3 4 B VLILCOS (30° B 4
? √3VLILCOS …………………………………….(1)
Similarly, W 1 – W2 ? VLILCOS(30° 3 ) – VLILCOS(30° B 4
? VLIL (2sin H ½) ? V LIL Sin ………………(2)
Dividing (ii) by (i), we have
IJKI5
tan ? √3
IJL I5
Knowing tan and hence , the value of power factor COS can be found.
It should how ever , be kept in mind that if W2 reading has been taken after reversing the
pressure coil i.e. if W2Type equation here. is negative, then the above relationship becomes,
Tan ? M3
IJK2KI54
? √3
IJL2KI54
IJLI5
IJKI5
33
8.2.2 Balanced Load Leading Power Factor
In the above discussion, lagging angles are taken positive. Now we will see how
wattmeter reading are changed if the power factor becomes leading.
For ? B 60(lag), W2 is zero. But for ? 360°(lead). W1 is zero so, we find that for
angles of lead, the reading of the two wattmeter are inter-changed. Hence, for a leading
power factor
W1 ? VLIL COS(30° B 4
W2 ?VLILCOS(30° 3 )
@W1 B W2 ?VLILCOS
W1 –W2 ? -VLIL sin
Then tan ? 3 √3
IJ –I5
IJLI5
Example 1
A 220V has a full load output of 10hp, the power factor being 0.8. Full load efficiency
82%. Find the reading on each of the two wattmeter connected to measure the input.
Solution
Output = 10 x 746 = 7460W
Input = 7460/0.82 = 9100W
COS = 0.83 = ? cos KJ 0.83 ? 33.9°
tan ? 0.672
TJKT5
TJKT5
0.672 = √3
? √3
TJLT5
U.J
7.VW5 HU.J
W1 – W2?
? 3.53Z[
√X
W1 +W2 = 9.10KW
W1 = 6.315KW, W2 = 2.785KW
Example 2
Two wattmeter connected to measure the power input to a three phase balanced load
give the following readings. 10KW, AND 20KW, the latter reading after reversal of the
current coil connections. Calculate the power and the power factor of the load.
Solution
Power = 10 – 1.2 =8.8KW
TJKT5
J7LJ.5
Tan = √3
= √3
TJLT5
J7KJ.5
? 65.6°
QUESTIONS
1. Draw a simplified diagram of one wattmeter method.
2. Derive an expression for the balanced load lagging power factor meter
34
INSTRUMENTS SELECTION AND SPECIFICATION
Week 9
9.1 Introduction:
The considerations for selecting an instrument may be regarded as falling into two
categories: either an engineer is selecting the most suitable instrument from those within a
department or establishment to perform a particular measurement, or he is undertaking the
purchase of a new instrument to perform a particular measurement and possibly at the same
time extend the measurement capabilities of the department or establishment in which he
works. Many of the criteria in selecting an instrument are the same, whether an engineer is
selecting an instrument off the shelf or purchasing new equipment.
The general criteria for selecting an instrument may be summarized by the following
factors, which although it will be more suitable when considering a moderately sophisticated
instrument, could prove valuable as a guide in selecting the right instrument on every
occasion.
9.2 FACTORS OF AN INSTRUMENT SELECTION
(a)Range
(b)Accuracy
(c)Response
(d)Stability
(e)Reliability
(f)Sensitivity
The important factors of selecting measurement instruments are explain as follows.
(a)Range:
The difference between the greatest and the least values of data.
(i)what are the maximum and minimum magnitude of the values to be measured.
(ii)Will a single range or multirange instruments be the most suitable?
(iii)Is a linear scale required.
35
(b)Accuracy:
This is the closeness in which an instrument approaches the true of the quantity being
measured.
(i)what is the accuracy required in the measurement?
(ii)Is the same accuracy required over the entire range of measurement?
(iii)What is the maximum tolerance acceptable?
(iv)Is the resolution of the instrument consistent with its specified errors?
(c)Response:
This is when the quantity being measured changes with time.
(i)What response time is required?
(ii)What bandwidth is required?
(III)For a.c instruments, to what aspect of the waveform should the instrument respond i.e
peak, mean or r.m.s values?
(iv)For auto range (for example d.v.m) instruments must include the time for range and
polarity changes.
(d)Stability:
(i)what is the maximum acceptable time between calibration?
(ii)Is the instrument to be operated unattended for a long period.
(iii)Is there a built in calibration system?
(e)Reliability:
(i)What is the required reliability?
(ii)What will be the consequences of failure , and will a standby instrument be required?
(iii)What are the maintenance requirements and will any special equipment be required?
(iv)Are there any cost limitations on the choice of instrument?
(f)Sensitivity:
(i)Is the quantity being measured floating or has it one side earthed?
(ii)Are there likely to be stray electromagnetic or electrostatic fields?
(iii)What are the required common mode and normal mode rejection ration ratio?
On the completion of the above factors for a particular application, the derived specifications
for the desired instrument may not be possible in practical terms, and a compromise between
that which is available within an organization, or can be afforded, will have to be adopted.
If a new instrument is to be purchased , it is essentially to ensure that a “right” instrument is
being purchased. This is particularly relevant, if the instrument is for a permanent installation
although it may be considered as good practice to purchased to a slightly higher
36
specifications if proposed instrument is for used in a laboratory where the measurement
requirement may change with experience and time.
QUESTIONs
1. List the important factors in selecting an instruments
2. Explain the following,
(i)
Range
(ii)
Accuracy
(iii)
Response
(iv)
Sensitivity
(v)
reliability
37
IMPORTANCE OF INSTRUMENTS IN INDUSTRIES
Week 10
10.1 INTRODUCTION:
Measurement can be defined as finding the size, unit, standard, device or system used for
obtaining action taken for a purpose. Many instruments have been invented to measure the
following importance of instrument listed below, with different advantages and
disadvantages. Instruments are of different ranges and cost all vary by several orders of
magnitude from one instrument design to the next.
1 Instrumentation plays a significant role in both gathering information from the field
and changing field parameters, and as such are a key part of control loops.
2
Instrumentation can be used to measure certain field parameters (physical values):
such as pressure, either differential or static, flow.
3
In addition to measuring field parameters, information is also responsible for
providing the ability to modify some field parameters.
10.2 IMPORTANCE OF MEASUREMENT IN INDUSTRY
1.
Measurement is important because is the estimating of the magnitude of some
attribute of an object, such as its length or weight relative to a unit of measurement.
Measurement involve using a measuring instrument such as a ruler or scale, which is
calibrated to compare the object to some extent.
2. Measurement is important because is the process of estimating the magnitude of some
attribute of an object such as its length or weight, relative to some standard(unit of
measurement), such as a meter or a kilogram.
3. Measurement is another process use to indicate the number that results from the
process . Metrology is the scientific study of measurement. The act of measuring
usually involves using a measuring instrument, such as a ruler weighing scale,
thermometer, speedometer or voltmeter, which is calibrated to compare the measured
attribute to a measurement unit. Any kind of attributes can be measured, including
physical quantities such as distance, velocity energy, temperature or time.
4. Measurement is important because it is the assessment of attitudes or perception in
surveys or the testing of aptitudes of individuals are also considered to be
measurements. Indeed, surveys and tests are considered to be “measurement
instructions”.
5. Measurement is important because is fundamental in science; it is one of the things
that distinguish science from pseudoscience. It is easy to come up with a theory that
38
predicts measurement with great accuracy. Measurement is also essential in industry,
commerce, engineering, construction, manufacturing, pharmaceutical production, and
electronics. The following factors are also important of measurement in an industry.
(a) Pressure
(b)Temperature
(C)Level
(d)Flow rate
(e)Density
(f)Viscosity
(g)Humidity
10.3 (a)Pressure Measurement:
Many techniques have been developed for the measurement of pressure and vacuum
instruments used to measure pressure are called PRESSURE GAUGES or VACUUM
GAUGES.
A MANOMETER could also be referred to a pressure measuring instrument, usually limited
to measuring pressures near to atmospheric pressures. The term manometer is often used to
refer specifically to liquid column hydrostatic instruments. A VACUUM GAUGE is used to
measure pressure in a vacuum--- which is further divided into two subcategories: high and
low vacuum (and sometimes ultra-high vacuum). The applicable pressure range of many of
the techniques used to measure vacuums have an overlap. Hence by combining several
different types of gauge, it is possible to measure system pressure continuously from 10mbar
down to 10-11mbar.
10.3.1 IMPORTANCE OF PRESSURE MEASUREMENT
Hydrostatic gauges (such as the mercury column manometer) compare pressure to the
hydrostatic force per unit area at the base of a column of fluid. Hydrostatic gauge
measurements are independent of the type of gas being measured, and can be designed to
have a very linear calibration. They have poor dynamic response.
Piston type gauge counter balance the pressure of fluid with a solid weight or a spring.
For example dead-weight testers used for calibration and tire pressure gauges.
39
QUESTIONS
1. Explain the importance of instruments in industries.
2. List the important measurement in industries.
40
TEMPERATURE MEASUREMENT
Week 11
11.1 Introduction:
Temperature is a physical quantity which determines the direction of flow of heat
when two bodies are placed in contact. It can be defined as that which measures the degree of
hotness or coldness of a body. A thermometer is an instrument used to measure the
temperature of a body. For it to be used as temperature measuring instrument:
(i) It must posses certain physical quantities that will be to change continuously with
temperature and
(ii) Its physical nature must not change as a result of an increase in temperature.
11.2 Types of thermometers:
The existence of different types of thermometer comes from taking advantage of the
responsiveness of certain physical properties to change in temperature. Areas where use is
made of such physical properties to slight change in temperature are in:
i.
Expansion of liquid due to increase in temperature e.g. liquid in glass
thermometer.
ii.
Expansion of gas volume at constant pressure e.g. gas thermometer.
iii.
Change in electric resistance of a metal such as platinum e.g. resistance
thermometer.
iv.
Change in electric current generated by a thermocouple e.g. thermoelectric
thermometer.
v.
Difference in expansion of the two metals that make up a bimetallic strip, e.g.
bimetallic thermometer.
11.3 Classification of thermometers
11.3.1 Practical Thermometers:
These are thermometers used for temperature measurement, e.g. all liquid-in-glass
thermometer, resistance thermometer, gas thermometer etc.
11.3.2Common Thermometers:
These are thermometers that use the expansion of a liquid to indicate temperature
rise. Example are liquid-in-glass thermometers: mercury-in-glass thermometer, maximum
and minimum thermometers.
41
11.4 Importance of temperature measurement
The existence of the different types of thermometer comes from taking advantage of the
responsiveness of certain physical properties to change in temperature. Areas used of such
physical properties to slight change in temperature are in:
(i)
Expansion of liquid due to increase in temperature e.g. liquid in glass
thermometer.
(ii)
Expansion of gas volume at constant pressure e.g. gas thermometer.
(iii)
Change in electric resistance of a metal such as platinum e.g. resistance
thermometer.
(iv)
Change in electric current generated by a thermocouple e.g. thermoelectric
thermometer.
(v)
Difference in expansion of the two metals that make up a bimetallic strip, e.g.
bimetallic thermometer.
11.5 LEVEL MEASUREMENT
Integral to process measurement sensors fall into two main types. Point level
measurement sensors are used to mark a single discrete liquid height-a preset level condition.
Generally, this type of sensor functions as a high alarm, signaling an overfill condition, or as
a marker for a low alarm condition. Continuous level sensors are more sophisticated and can
provide level monitoring of an entire system. They measure fluid level within a range, rather
than at one point, producing an analog output that directly correlates to the level in the vessel.
To create a level management system, the output signal linked to a process control loop and
to a visual indicator.
11.5.1 HOW TO OBTAIN LEVEL MEASUREMENT
These sensors incorporate an analog signal processor, a microprocessor, binary coded
decimal (BCD) range switches, and an output driver circuit. Transmit pulses and a gate signal
from the microprocessor route through the analog signal processor to the sensor, which sends
an ultrasonic beam to the liquid surface. The sensor detects the echo from the surface and
routes it back to the microprocessor for a digital representation of the distance between the
42
sensor and the surface level. Through constant updating of received signals, the
microprocessor calculate average values to measure liquid level.
With a continuous sensor, the microprocessor converts the average value to an analog
4 to 20MA signal linear with the liquid level. When the echo from the level does not return to
the sensor within 8 seconds, the output signal from the system drops below 4MA, indicating a
low level condition or empty pipe. With a point sensor, the microprocessor compares the
average value with BCD switch setting and energizes an output relay for either high or low
level indication. A signal loss exceeding 8 second de-energizes the relays and restores their
original states. The electronics incorporate a half second delay that minimizes surface
turbulence effects.
With the wide variety of approaches to level measurement and as many as 163
suppliers offering one or more types of level measuring instrument, identifying the right one
for your application can be difficult. In recent years, technologist that capitalized on
microprocessor developments have stood out from the pack. For example, the tried true
technique of measuring the head of a liquid has gained new life thanks to “smart” differential
pressure (DP) transmitters.
11.5.2 Importance of level measurement
Today’s local level measuring instruments can include diagnostic as well as
configuration and process data that can be communicated over a network to remote
monitoring and control instrumentation. One model even provides local PID control. Some of
the most commonly used liquid- level measurement methods are:
-RF capacitance
-Conductance (conductivity)
-Hydrostatic head/tank gauging
-Radar
-Ultrasonic
QUESTIONS
1. What do you understand by temperature measurement?
2. State the classes of thermometers.
3. How can we obtain level measurement?
43
FLOWRATE MEASUREMENT
Week 12
12.1 Introduction:
Increasing industrialization and population growth has caused tremendous
contamination of surface water during the last decades. As a consequence, the laws regarding
environmental protection have been tightened in many countries. To fulfill this regulations
means increasing the number and the efficiency of waste water treatment plants. Today is
necessary to use ongoing treatment processes and control them by means of measurement and
control systems. In service monitoring has become more extensive and diverse. Waste water
treatment not only includes operating. Supervising, servicing and repairing equipment, it also
involves the key topics of operational flow, monitoring and controlling, measuring,
analyzing, recording and evaluating. This ensures that the key processes, the reduction of
hydrocarbons, of nitrate, ammonium and phosphates and carried out in a most efficient way
in a large as well as in small treatment plants.
The driving factor in today’s investment in waste water treatment most be cost
reduction. The only way to maintain high standards while reducing costs is to invest in highly
sophisticated measurement and process control technology, ideally from a supplier offering
the whole basket of instrument.
12. 1.1 Importance of flow rate
The key is obtaining is obtaining representative measurements as closely as possible
to the true process control conditions. Filtering and sampling systems used in conjunction
with smart analyzers and plant supervision tools can aid improving the efficiency of the waste
water treatment process control.
12.2 MEASUREMENT OF DENSITY
The density of a fluid is its weight per unit volume. Both temperature and pressure
affect the density of fluids and can alter the accuracy of measurements especially for gases
and vapors. High temperatures or lower pressure cause the fluid expand so that molecules
move farther apart, which causes the weight of given volume to be less than it will be at a
lower temperature of higher pressure.
44
12.3 Viscosity measurement
The viscosity of the coating fluid is important rheological property, which will be
accurately measured, and controlled during the development of the product and in
manufacturing. This will insure optimum quality and a reproducible cost effective
manufacturing process.
A fluid is defined as substance that undergoes irreversible deformation when subject
to a steady state shear or tensile stress. The viscosity is a parameter to characterize this
deformation and is a measure of the resistance of the solution to flow under mechanical
stress. High viscosity solution will deform and flow rapidly with a minimum of force. For
example, water has a low viscosity and will flow readily out of glass under normal gravity
forces, whereas molasses, a high viscosity material, will take a long time to flow from a glass.
Why is it important to understand viscosity? Because viscosity is central to the
performance of lubricated machinery, such as your car.
•
If you use high- viscosity engine oil in your car, the oil puddles in your driveway will
be smaller, but your engine will run hotter and it probably won’t start on a cold
winter morning.
•
If you use low viscosity engine oil in your car, the position rings may wear out in a
few thousand miles, the crankshaft bearings may seize and the puddles in your drive
will amaze your neighbors!
As we said, the simplest definition of viscosity is resistance to flow. Sir Isaac Newton
defined it as “the resistance that arises from lack of slipperiness in a fluid.” Cold maple syrup
is thick and not slippery, but cold water is thin and slippery.
The importance of characterizing the viscosity behavior over a wider range of shear rates is
that there are a wide variety of shear rates in the coating and ancillary processes process.
Therefore, the behavior needs to determined to insure compatibility.
12.3.1 Importance of viscosity
The fluid viscosity is key variable in the following properties:
-Determine the best coating method to be used.
-Coating weight control in roll coating methods.
-Coating quality level and reproducibility from applicator.
-Coating quality in dryer.
45
-Level of coating after application.
Routing measurements of viscosity prior to coating should be part of the normal
quality control determine the source of the problem.
QUESTIONS
1. What do you understand by density measurement?
2. What are the importance of viscosity in industry?
46
HUMIDITY MEASUREMENT
Week 13
13.1 INTRODUCTION:
Humidity measurement instruments and transducers test absolute humidity, relative
humidity, or dew point in air. The range humidity measurement instruments operator in is
typically from 0 to 100% humidity. They are sometimes combined with other sensing devices
such as temperature sensors.
Humidity measurement instruments and sensors can sense a number of different
factors. Absolute humidity, expressed as grams of water vapor per cubic meter volume of air,
regardless of the air’s temperature. Relative humidity (RH), express as percent, also measures
water vapor, but also relative to the temperature of the air. The dew point temperature , which
provides a measure of the actual amount of water vapor in the air, is the temperature to which
the air must be cooled in for that air to be saturated and dew to form. Because of the
intertwining of atmospheric measurements, humidity measurement instruments are
sometimes equipped with pressure and temperature sensors as well. Three main application s
for humidity measurement are judging moisture in gas or air, bulk solids or powders, or else
if fuels or other liquids.
There are many technologies for humidity measurement instruments. Capacitive or
dielectric instruments have a material that absorbs moisture, which changes it dielectric
properties and enhance its capacitance. Chilled mirror technology uses a mirror that is chilled
to the point that moisture starts to condense on it. The temperature is the dew point. With
electrolytic technology, moisture is proportional to the current needed to electrolyze it from a
desiccant. For resistivity or impedance style sensors, a material absorbs moisture , which
changes its resistivity or impedance. In strain gauge instruments, a material absorbs water,
expands and is measured with a strain gauge. Psychrometers, often called wet/dry bulbs,
measure relative humidity by gauging the temperature difference between two thermometers,
one wet and one dry.
One critical specification for these devices is the humidity for moisture range to be
measured or else the dew point range. Humidity and moisture accuracy is expressed in terms
of percentage of measurement. The dew point accuracy, since this is a temperature reading,
express as a variance in temperature output.
Outputs from humidity measurement instruments can be analog current, voltage, or
frequency ; digital, including computer signals; or a switch of alarm. They can have analog,
47
digital or video type displays and can have a number of different form factors. They can be
PCB-amount devices, standard sensors or transducers, or a simple gauge or indicator. They
can also be various types of instruments, whether handheld, bench top or mounted.
In addition to pressure and temperature compensation, humidity measurement
instruments can have a number of features to make them more useful or easier to use. These
can include data logging, event triggering, self testing, self calibration, and battery power.
QUESTION
Explain the importance of humidity measurement in an industry.
48
Week 14
ELECTRICAL/ELECT.INSTRUMENTS
14.1 Introduction:
Electrical energy is being used in the manufacture of many commodities. In order to
ensure quality and efficiency, it is important that we should be able to measure accurately the
electrical quantities involved. The instruments used to measure electrical quantities (e.g.
current, voltage power, energy etc.) are called electrical instruments. These instruments are
generally made after the electrical quantity to be measured. Thus the instruments which
measure which measure current, voltage, power and energy are called ammeter, voltmeter,
wattmeter and energy meter respectively. The accuracy, convenience and reliability of
electrical instruments.
14.2 Types of Electrical Instruments
Electrical measuring instruments may be classified according to their functions as
(a)Indicating instruments
(b)Recording instruments
(c)Controlling instruments
14.2.1
Indicating instruments:
These are the instruments which indicate the instantaneous value of the quantity
being measured, at the time it is being measured. The indication is in the form of a pointer
deflection (analog instruments) or digital readout (digital instruments). Ammeters, voltmeters
and watt meters are example of such instruments. In analog instruments, a pointer moving
over a graduated scale directly gives the value of the electrical quantity being measured. For
example when an ammeter is connected in the circuit, the pointer of the meter directly
indicates the value of the current flowing in the circuit at that time
14.2.2 Recording
instruments:
These are instruments which gives a continuous record of the variations of the
electrical quantity being measured over a selected period of time. e.g. recording ammeters
which are used in sub-stations for registering the amount of current taking from the batteries
while recording voltmeters are used in sub-stations to record the variation of supply voltage
the day. Many of these instruments are electromechanical devices which use paper chart and
mechanical writing instruments such as an inked pen or stylus.
14.2.3 Controlling
instruments:
49
These are instruments used to control a varying system at a fixed value or range. The
instruments serve as a component of an automatic control system.
14.3 Principles of Operation of Electrical instruments
An electrical instrument essentially consists of a movable element and a scale to
indicate or register the electrical quantity being measured. The movable element is supported
on jeweled bearings and carries a pointer or sets of dials. The movement of movable elements
is caused by utilizing one or more of the following effect of current or voltage:
1. Magnetic effect
……….
Moving iron instruments
2. Electrodynamic effect
……….
(i)Permanent-magnet moving coil
(ii)Dynamometer type
3. Electromagnetic induction
……….
Induction type instruments
4. Thermal effect
……….
Hot wire instruments
5. Chemical effect
……….
Electrolytic instruments
6.
……….
Electrostatic instruments
Electrostatic effect
QUESTIONS
1. List and explain the three major classes of instruments.
2. Explain the principles of operation of an electrical instruments.
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3.
ESSENTIALS OF INDICATING INSTRUMENTS
Week 15
15.1 Introduction:
An indicating instrument essentially consists of moving system pivoted in jewel
bearings. A pointer is attached to the moving system which indicates on a graduated scale,
the value of electrical quantity being measured. In order to ensure proper operation of
indicating instruments, the following three torques are required:
(i)
Deflecting (or operating) torque
(ii)
Controlling (or restoring) torque
(iii)
Damping torque
The deflecting torque is produced by utilizing the various effects of electric current or
voltage and causes the moving system (and hence the pointer) to move from zero position.
The controlling torque is provided by spring or gravity and opposes the deflecting torque.
The pointer comes to rest at a position where these two opposing torque are equal. The
damping torque is provided by air friction or eddy currents. It ensures that the pointer comes
to the final position without oscillations, thus enabling accurate and quick readings to be
taken.
15.1.1
Deflecting torque
One important requirement in indicating instruments is the arrangement for producing
deflecting or operating torque (Td) when the instrument is connected in the circuit to measure
the giving electrical quantity. This achieved by utilizing the various effects of electric current
or voltage mentioned in Art 14.2.The deflecting torque causes the moving system (and hence
the pointer attached to it) to move position to indicate on a graduated scale, the value of
electrical quantity being measured. The actual method of producing the deflecting torque
depends upon the type of instrument.
15.1.2 Controlling Torque
If deflecting torque were acting alone, the pointer will continue to move indefinitely and
would swing over to the maximum deflected position irrespective of the magnitude of current
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(or voltage or power) to be measured. This necessitates to provide some form of controlling
or opposing torque (Tc). This controlling torque should oppose the deflecting torque and
should increase with the deflection of the moving system. The pointer will be brought to rest
at a position where the two opposing torque are equal i.e. Td = Tc. The controlling torque
performs two functions:
(i)
It increase with the deflection of the moving system so that the final position of
the pointer on the scale will be according to the magnitude of current (or voltage
or power) to be measured.
(ii)
It brings the pointer back to zero position when the deflecting torque is removed.
If it were not provided, the pointer once deflected would not return to zero
position removing the deflecting torque.
15.1.3 Damping Torque
If the moving system is acted upon by deflecting and controlling torques alone, then
the pointer, due to inertia, will oscillate about its final deflected position for quite sometimes
before coming to rest. This often undesirable because it makes difficult to obtain quick and
accurate readings. In order to avoid these oscillations of the pointer and to bring it quickly to
its final deflected position, a damping torque is provided in the indicating instruments. This
damping torque acts only when pointer is in motion and always opposes the motion. The
position of the pointer when stationary is therefore not affected by damping.
The degree of damping decides the behavior of the moving system. If the instruments
are under damped, the pointer will oscillate about the final position for sometime before
coming to rest. On the other hand if the instrument is over-damped, the pointer will become
slow and lethargic. However, if the degree of damping is adjusted to such a value that the
pointer comes to the correct reading quickly without passing beyond it or oscillating about it,
the instrument is said to be dead-beat or critical damped. Fig. below shows graph for under
damping, over damping and critical damping (dead-beat).
QUESTIONS
1. What are the essentials of indicating instruments? Hence explain at least two.
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