2. Measurement
• Measurement is an act or the result of a quantitative
comparison between an unknown magnitude and the
predefined standard.
• The result is expressed in numerical values.
• Internationally accepted standard :
• Mass : Kg
• distance : km
3. Which quantities do we need to
measure?
• temperature
• wind speed and direction
• pressure
• humidity
• visibility
• cloud distribution
• cloud type
• type and amount of
precipitation
4. INSTRUMENT ?
• An instrument is a device that measures a physical
quantity such as flow, temperature, level, distance, angle,
or pressure.
• For example : Instruments may be as simple as direct
reading thermometers or may be complex multi-
variable process analyzers. Instruments are often part of
a control system in refineries, factories, and vehicles.
• Instruments can be classified as : mechanical, electrical,
electronics instruments.
6. What is transducer?
• A transducer is a device, usually electrical,
electronic, electro-mechanical, electromagnetic,
photonic, or photovoltaic that converts
"one type of energy or physical attribute to another for
various purposes including measurement or
information transfer".
For example :
Light Level Light Dependant Resistor (LDR)
Photodiode LED's & Displays
Temperature : Thermocouple
8. sensor :
• A sensor (also called detector) is a converter that
measures a physical quantity and converts it into a signal
which can be read by an observer or by an instrument.
For example, a mercury-in-glass thermometer converts
the measured temperature into expansion and
contraction of a liquid which can be read on a calibrated
glass tube.
• A thermocouple converts temperature to an output
voltage which can be read by a voltmeter.
• The audio loudspeaker, which converts electrical voltage
variations representing music or speech, to mechanical
cone vibration and hence vibrates air molecules creating
acoustical energy.
• traffic lights etc.
9. Difference between transducer and sensor.
• Transducers and sensors are physical devices that are
used in electrical, electronic and many other types of
gadgets and appliances.
• Transducers are used to convert one energy type into
another while sensors measure energy levels and convert
them into electrical signals that can be measured digitally.
10. Characteristics and choice of Transducers:
1. Operating Principle: The transducer are many times selected on the basis
of operating principle used by them. The operating principle used may be
resistive, inductive, capacitive ,optoelectronic, piezo electric etc.
2. Sensitivity: The transducer must be sensitive enough to produce
detectable output.
3. Operating Range: The transducer should maintain the range requirement
and have a good resolution over the entire range.
4. Accuracy: High accuracy is assured.
5. Cross sensitivity: It has to be taken into account when measuring
mechanical quantities. There are situation where the actual quantity is being
measured is in one plane and the transducer is subjected to variation in
another plan.
6. Errors: The transducer should maintain the expected input-output
relationship as described by the transfer function so as to avoid errors.
11. 7 Transient and frequency response : The transducer should meet the
desired time domain specification like peak overshoot, rise time, setting
time and small dynamic error.
8. Loading Effects: The transducer should have a high input impedance
and low output impedance to avoid loading effects.
9. Environmental Compatibility: It should be assured that the transducer
selected to work under specified environmental conditions maintains its
input- output relationship and does not break down.
10. Insensitivity to unwanted signals: The transducer should be minimally
sensitive to unwanted signals and highly sensitive to desired signals.
11. Usage and Ruggedness : ruggedness both electrical as well as
mechanical intensities of a transducer must be considered.
12. Electrical aspects : length and type of cable, signal to noise ratio,
frequency response.
13. Stability and reliability : high degree of stability to be operated during
its operation and storage life.
14. Static characteristics : low static errors, low non-linearity, low hystersis,
high resolution and a high degree of repeatability.
12. Classification of transducer
Transducers can be classified :
1. on the basis of transduction form used
2. as primary and secondary transducers.
3. as passive and active transducers
4. as analog and digital tranducers
5. as transducers and inverse transducers.
15. Stress
• Stress is a measure of the average amount of
force exerted per unit area. It is a measure of
the intensity of the total internal forces acting
within a body across imaginary internal surfaces,
as a reaction to external applied forces and body
forces. It was introduced into the theory of
elasticity by Cauchy around 1822. Stress is a
concept that is based on the concept of
continuum.
16. Stress
In general, stress is expressed as
is the average stress, also called
engineering or nominal stress
and is the force acting over the area .
17. Strain
Strain is the geometrical expression of
deformation caused by the action of stress on a
physical body. Strain is calculated by first
assuming a change between two body states: the
beginning state and the final state. Then the
difference in placement of two points in this
body in those two states expresses the numerical
value of strain. Strain therefore expresses itself
as a change in size and/or shape.
18. Strain
• The strain is defined as the fractional change in
length
• Strain is thus a unitless quantity
l
l
strain
∆
=
19. Strain gauge
L – increase
A – decrease
From the equation of resistance,
R – increase
A
L
R
ρ
=
20. Strain gauge – the gauge factor
LL
RR
FG
/
/
..
∆
∆
=
K = the gauge factor
R = the initial resistance in ohms (without strain)
ΔR = the change of initial resistance in ohms
L = the initial length in meters (without strain)
ΔL = the change of initial length in meters
ε
RR
FG
/
..
∆
=
21. Classification of strain gauge
1. wire strain gauge
(a) bonded wire strain gauge
(b) Unbonded metal strain gauge
(c) bonded metal foil strain gauge
2. semiconductor strain gauge.
22. BONDED WIRE STRAIN GAUGE
A resistance wire strain gauge consist of a grid of fine resistance wire. The grid is cemented to carrier
which may be a thin sheet of paper bakelite or teflon.
The wire is covered on top with a thin sheet of material so as to prevent it from any mechanical demage.
The carrier is bonded with an adhesive material to the specimen which permit a good transfer of strain
from carrier to grid of wires.
23. UNBONDED METAL STRAIN GAUGE
The unbonded meter wire gauges
employ preloaded resistance wire
connected in Wheatstone bridge as
shown in fig.
At initial preload the strain and
resistance of the four arms are
nominally equal with the result the
output voltage of the bridge is equal
to zero.
Application of pressure produces
a small displacement , the
displacement increases a tension in
two wire and decreases it in the
other two thereby increase the
resistance of two wire which are in
tension and decreasing the
resistance of the remaining two
wire .
This causes an unbalance of the
bridge producing an output voltage
which is proportional to the input
displacement and hence to the
24. Bonded metal foil strain gauge
This class of strain gauge is only an
extension of the bonded metal wire strain
gauges.
Base (carrier) Materials: several types of
base material are used to support the wires.
Adhesive: The adhesive acts as bonding
materials. successful strain gauge bonding
depends upon careful surface preparation and
use of the correct bonding agent.
strain be faithfully transferred on to the
strain gauge, the bond has to be formed
between the surface to be strained and the
plastic backing material on which the gauge is
mounted .
.Leads: The leads should be of materials
which have low and stable resistivity and also
a low resistance temperature coefficent.
grid pattern is formed with a thin foil.
larger surface area, therefore higher heat
dissipation capability and better bonding
property.
25. Semiconductor strain gauges
Semiconductor gauge are
used in application where a
high gauge factor is desired.
A high gauge factor means
relatively higher change in
resistance that can be
measured with good
accuracy.
The resistance of the
semiconductor gauge
change as strain is applied to
it. The semiconductor gauge
depends for their action upon
the piezo-resistive effect i.e.
change in value of resistance
due to change in resistivity.
Silicon and germanium are
used as resistive material for
semiconductor gauges
26.
27. Pressure sensors (transducers)
pressure = force
area
pressure may be considered as stress.
pressure sensors - measurement of pressure.
Types of pressure transducers :
LVDT based Diaphragm
Piezoelectric
28. Diaphragm
(a) flat diaphragm; (b) corrugated diaphragm
A diaphragm usually is designed so that the deflection-versus-pressure
characteristics are as linear as possible over a specified pressure range,
and with a minimum of hysteresis and minimum shift in the zero point.
29. Diaphragm
Uses the elastic deformation of a flexible membrane that separates two
different pressures.
The deformation of the diaphragm is dependent on the difference in pressure
between the two faces.
The diaphragm expands when very small pressures are applied.
31. Electromagnetic Flowmeters
• Magnetic flowmeters have been widely used in industry for
many years.
• Unlike many other types of flowmeters, they offer true
noninvasive measurements.
• They are easy to install and use to the extent that existing
pipes in a process can be turned into meters simply by
adding external electrodes and suitable magnets.
• They can measure reverse flows and are insensitive to
viscosity, density, and flow disturbances.
• Electromagnetic flowmeters can rapidly respond to flow
changes and they are linear devices for a wide range of
measurements.
• As in the case of many electric devices, the underlying
principle of the electromagnetic flowmeter is Faraday’s law
of electromagnetic induction.
• The induced voltages in an electromagnetic flowmeter are
linearly proportional to the mean velocity of liquids or to
the volumetric flow rates.
32. • As is the case in many applications, if the pipe walls are
made from nonconducting elements, then the induced
voltage is independent of the properties of the fluid.
• The accuracy of these meters can be as low as 0.25% and, in
most applications, an accuracy of 1% is used.
• At worst, 5% accuracy is obtained in some difficult
applications where impurities of liquids and the contact
resistances of the electrodes are inferior as in the case of
low-purity sodium liquid solutions.
• Faraday’s Law of Induction
• This law states that if a conductor of length l (m) is moving
with a velocity v (m/s–1
), perpendicular to a magnetic field of
flux density B (Tesla), then the induced voltage e across the
ends of conductor can be expressed by:
Blve =
36. Performance Considerations
Reynolds number constraints
Entrained gas or particles for doppler
Clean liquids for time of flight
Installed without process shut down
Straight upstream piping requirements
V
ADVANTAGES
No Moving Parts
Unobstructed Flow Passage
Wide Rangeability
DISADVANTAGES
For Liquids Only (limited gas)
Flow Profile Dependent
Errors Due To Deposits
37.
38.
39. Signal conditioning
• In electronics, signal conditioning means
manipulating an analogue signal in such a
way that it meets the requirements of the next
stage for further processing. For example, the
output of an electronic temperature sensor,
which is probably in the millivolts range is
probably too low for an Analog-to-digital
converter (ADC) to process directly. In this
case the signal conditioning is the
amplification necessary to bring the voltage
level up to that required by the ADC.
40. Signal conditioning
• Types of devices that use signal conditioning include
signal filters, instrument amplifiers, sample-
and-hold amplifiers, isolation amplifiers,
signal isolators, multiplexers, bridge
conditioners, analog-to-digital converters,
digital-to-analog converters, frequency
converters or translators, voltage converters or
inverters, frequency-to-voltage converters,
voltage-to-frequency converters, current-to-
voltage converters, current loop converters,
and charge converters.
41. Signal conditioning
• Signal inputs accepted by signal conditioners
include DC voltage and current, AC voltage
and current, frequency and electric charge
• Outputs for signal conditioning equipment can
be voltage, current, frequency, timer or counter,
relay, resistance or potentiometer, and other
specialized outputs
43. Thermocouple
• In 1821, T.J. Seebeck discovered that an electric
potential occurs when 2 different metals are joined
into a loop and the two junctions are held at
different temperatures.
• Seebeck emf – a voltage difference between the two
ends of the conductor that depends on the
temperature difference of the ends and a material
property.
• If the ends of the wire have the same temperature,
no emf occurs, even if the middle of the wire is
hotter or colder.
47. Thermocouples
• Type K : Chromel-Alumel
• Type J : Iron-Constantan
• Type E : Chromel-Constantan
• Type N : Nicros-Nisil
• Type T : Copper-Constantan
• It is important to note that thermocouples
measure the temperature difference between two
points, not absolute temperature.
48.
49. Magnitude of thermal EMF
where
c and k = constants of the thermocouple materials
T1 = the temperature of the ‘hot’ junction
T2 = the temperature of the ‘cold’ or ‘reference’ junction
)()( 2
2
2
121 TTkTTcE −+−=
50. Problem
A thermocouple was found to have linear calibration
between 0⁰C and 400⁰C with emf at maximum
temperature (reference junction temperature 0⁰C) equal
to 20.68 mV.
a) Determine the correction which must be
made to the indicated emf if the cold junction
temperature is 25⁰C.
b) If the indicated emf is 8.82 mV in the
thermocouple circuit, determine the temperature of
the hot junction.
51. Solution
(a) Sensitivity of the thermocouple
= 20.68/(400-0)
= 0.0517 mV/⁰C
Since the thermocouple is calibrated at the reference
junction of 0⁰C and is being used at 25⁰C, then the
correction which must be made, Ecorr between 0⁰C
and 25⁰C
Ecorr = 0.0517 x 25
Ecorr = 1.293 mV
52. Solution
(b) Indicated emf between the hot junction and
reference junction at 25⁰C = 8.92 mV
Difference of temperature between hot and cold
junctions = 8.92/0.0517 = 172.53⁰C
Since the reference junction temperature is 25⁰C,
hot junction temperature = 172.53 + 25 = 197.53⁰C.
53. Thermocouple - applications
• Thermocouples are most suitable for measuring
over a large temperature range, up to 1800 K.
Example:
Type K : Chromel-Alumel (-190⁰C to 1260⁰C)
Type J : Iron-Constantan (-190⁰C to 760⁰C)
Type E : Chromel-Constantan
(-100⁰C to 1260⁰C)
54. Thermocouple - applications
• Thermocouples are most suitable for measuring
over a large temperature range, up to 1800 K.
• They are less suitable for applications where smaller
temperature differences need to be measured with
high accuracy, for example the range 0–100 °C with
0.1 °C accuracy. For such applications, thermistors
and RTDs are more suitable.
55. Resistance temperature detector
(RTD)
Resistance temperature detectors (RTDs),
also called resistance thermometers, are
temperature sensors that exploit the predictable
change in electrical resistance of some materials
with changing temperature.
Temperature Metal Resistance
The resistance ideally varies linearly with
temperature.
57. Resistance vs Temperature
Approximations
• A straight line has been drawn between the
points of the curve that represent temperature,
T1 and T2, and T0 represent the midpoint
temperature.
58. Resistance vs Temperature
Approximations
Straight line equation
R(T) = approximation of resistance at
temperature T
R(T0) = resistance at temperature T0
αo = fractional change in resistance per
degree of temperature at T0
ΔT = T - T0
21]1)[()( TTTTTRTR oo <<∆+= α
59. Resistance vs Temperature Linear
Approximations
Straight line equation
R2 = resistance at T2
R1 = resistance at T1
)(
)(
1
12
12
0 TT
RR
TR
o
−
−
=α
61. RTD – quadratic approximation
• More accurate representation of R-T curve over
some span of temperatures.
62. RTD – quadratic approximation
R(T) = quadratic approximation of
resistance at temperature T
R(T0) = resistance at temperature T0
α1 = linear fractional change in resistance
with temperature
α2 = quadratic fractional change in
resistance with temperature
ΔT = T - T0
21
2
21 ])(1)[()( TTTTTTRTR o <<∆+∆+= αα
67. RTD - sensitivity
• Sensitivity is shown by the value αo
▫ Platinum – 0.004/ °C
▫ Nickel – 0.005/ °C
• Thus, for a 100Ω platinum RTD, a change of
only 0.4 Ω would be expected if the temperature
is changed by 1°C
68. RTD – response time
• Generally 0.5 to 5 seconds or more
• The slowness of response is due principally to
the slowness of thermal conductivity in bringing
the device into thermal equilibrium with its
environment.
70. Construction of a platinum resistance
thermometer
Wire is in a coil to achieve small size and improve thermal conductivity
to decrease response time.
71. Construction of a platinum resistance
thermometer
Protect from the environment
72. Thermistor
• Semiconductor resistance sensors
• Unlike metals, thermistors respond negatively to
temperature and their coefficient of resistance is
of the order of 10 times higher than that of
platinum or copper.
• Temperature semiconductor resistance
• Symbol
84. Capacitive transducers
• The capacitance of a parallel-plate capacitor is
given by
ε = dielectric constant
εo = 8.854 x 1o-12
, in farad per meter
A = the area of the plate, in square meter
d = the plate spacing in meters
d
A
C oεε
=
90. Variable Inductance Transducers
• Principle: modulation of the excitation signal.
• Consist of a primary winding and two secondary
windings, wound over a hollow tube and
positioned so that the primary is between two
secondary.
96. Variable Inductance Transducers –
operation
When the core is in the center, the voltage induced
in the two secondaries is equal.
When the core is moved in one direction from the
center, the voltage induced in one winding is
increased and that in the others is decreased.
Movement in the opposite direction reverse the
effect.
104. Strain
• The strain is defined as the fractional change in
length
• Strain is thus a unitless quantity
l
l
strain
∆
=
105. Strain gauge
From the equation of resistance,
R = resistance
ρ = specific resistance of the conductor material
L = the length of the conductor in meters
A = the area of the conductor in square meters
A
L
R
ρ
=
106. Strain gauge – the gauge factor
LL
RR
K
/
/
∆
∆
=
K = the gauge factor
R = the initial resistance in ohms (without strain)
ΔR = the change of initial resistance in ohms
L = the initial length in meters (without strain)
ΔL = the change of initial length in meters
