1) What is instrument?
It is a device for determining the value or magnitude of a quantity or variable.
2) List three sources of possible errors in instruments.
Gross Error, Systematic & Random errors.
3) Define Instrumental error
These are the errors inherent in measuring instrument because of their mechanical
structure.
4) Define limiting error
Components are guaranteed to be with in a certain percentage of rated value. Thus
the manufacturer has to specify the deviations from the nominal value of a particular
quantity.
5) Define probable error
It is defined as r = ± 0.6745s where s is standard deviation.
Probable error has been used in experimental work to some extent in past, but standard
deviation is more convenient in statistical work.
6) Define Environmental error
These are due to conditions in the measuring device, including conditions in the
area surrounding the instrument, such as the effects of changes in temperature,
humidity.
7) Define units.
It is necessary to define a physical quantity both in kind and magnitude in order to
use this information for further proceedings. The standard measure of each kind of
physical quantity is named as the unit.
8) Define Standards
The physical embodiment of a unit of measurement is a standard. For example,
the fundamental unit of mass in the international system is the kilogram and defined as
the mass of a cubic decimeter of water at its temperature of maximum density of 4°c.
9) Mention the purpose of the measurement.
· To understand an event or an operation.
· To monitor an event or an operation.
· To control an event or an operation.
· To collect data for future analysis
· To validate an engineer design.
10) What are the methods of measurement?
· Direct comparison method
· Indirect comparison method
11) Define ODDs
The specification of limiting error is in itself uncertain because the manufacture
himself is not sure about the accuracy because of the presence of random errors.
12) Define transducer and give an example.
Transducer is a device which convert one form of energy into electrical energy.A
thermocouple converts heat energy into electrical voltage.
13) What is primary transducer?
Bourdon tube acting as a primary transducer, senses the pressure and convert the
pressure into displacement. No output is given to the input of the bourdon tube.
So it is called primary transducer. Mechanical device can act as a primary transducer.
14) What is secondary transducer?
The output of the Bourdon tube is given to the input of the LVDT. There are two
stages of trasduction, firstly the pressure is converted into a displacement by the
Bourdon tube then the displacement is converted into analog voltage by LVDT. Here
LVDT is called secondary transducer. Electrical device can act as a secondary
transducer.
15) What is passive transducer?
In the absence of external power, transducer cannot work and it is called a passive
transducer. Example capacitive , inductive, resistance transducers.
16) What is active transducer?
In the absence of external power, transducer can work and it is called active
transducer. Example velocity, temperature, light can be transdued with the help of
active transducer.
17) What is analog transducer?
These transducers convert the input quantity into an analog output which is a
continuous function of time. Thus a strain gauge, an LVDT, a thermocouple or a
thermistors may be called analog transducer, as they give an output which is a
continuous function of time.
18.Give the classification of units.
1.Absolute units
2.Fundamental and derived units
3.Electromagnetic units
4.Electrostatic units
19.Define Primary fundamental and auxiliary fundamental units.
Fundamental units in mechanics are measures of length,mass and time and those
are fundamental to most other physical quantities and hencee they are called Primary
fundamental units
Measures of certain physical quantities in thermal ,electrical and illumination
fields are also represented by fundamental units and are used only where those
disciplines are involved and hence called auxiliary fundamental units.
20.Define the unit of mass preserved at International Bureau of weights and measures at
Severes ,Near Paris.
The unit of mass is represented by a material standard;the mass of
International prototype kilogramme consisting of platinum Iridium hollow cyclinder.
21.Define static calibration.
It refers to a process in which all the inputs(desired,modifying,interfering)except
one are kept at some constant values.
22.Define Traceability.
The ability to trace the accuracy of the standard back to its ultimate source in
fundamental standards of National Institute of Science and Technology is termed
“Traceability”
23.What are random errors or residual errors.
The happenings or disturbances about which we are unaware and lumped together
are called random errors or residual errors.Since these errors remain even after the
systematic errors are taken care of,they are called residual errors.
24.Give one property of piezo-electric crystal.
When a force is applied to piezo-electric crystals ,they produce an output voltage
25.Define an Inverse transducer.Give an example.
A device which converts an electrical quantity into an non-electrical quantity.A
piezo-electric crystal acts as a inverse transducer because whwn a voltage is applied
across its surfaces ,it changes its dimensions causing a mechanical displacement
.
26.List the factors responsible in selection of a transducer.
1.Operating principle
2.Sensitivity
3.Operating range.
4.Accuracy.
27) Define static characteristics.
static characteristics of a measurement system are, in general, those that must be
considered when the system or instrument is used to measure a condition not varying
with time.
28) Mention different types of static characteristics.
(i) accuracy (ii) sensitivity (iii) Reproducibility (iv) Drift (v) Static error and (vi)
Dead zone .
29) What is dynamic characteristics?
Many measurements are concerned with rapidly varying quantities and , therefore,
for such cases we must examine the dynamic relations which exist between the output
and the input . This is normally done with the help of differential equations . Performance
criteria based upon dynamic relations constitute the Dynamic Characteristics.
30) Mention different types dynamic characteristics?
Zero- order transducers
First - order transducers
Second-order transducers
Higher-order transducers
31) What are the test inputs of the transducer?
Impulse input
Step input
Ramp input
Parabolic input
Sinusoidal input
32) Define- zero order transducer.
The input- output relationship of a zero- order transducer is given by
Y(t) = K r(t)
Where r(t) is the input, Y(t) is the output and K is the static – sensitivity of the transducer.
Example for zero- order transducer is a potentiometer.
33) What is frequency response of ZOT.
Frequency response is thus defined as the steady – state output of a transducer
When it is excited with sinusoidal input . the frequency response is represented with the
help of two plots namely amplitude radio verses frequency and phase angle shift versus
frequency.
34) What is damping ratio?
The damping ratioV is an important parameter which decides the nature of
oscillation in the transducer output . when V =0 , the second – order system is said to be
un damped and the system behaves like an oscillator . when V =1 , the second – order
system is said to be critical damped and when V >1 , the second – order system is said to
be over damped.
35) Define static sensitivity
Sensitivity should be taken depending on the operating point. The sensitivity is
expressed in output unit/ input unit.
36) Define linearity.
Linearity is a measure of the maximum deviation of the plotted transducer
response from a specified straight line .
37) Compare accuracy and precision.
Accuracy is the closeness to true value whereas precision is the closeness amongst
the readings .precision is the degree of closeness with which a given value may be
repeatedly measured.
38) What is Threshold?
When the input to a transducer is increased from zero, there is a minimum value
below which no output can be detected .This minimum value of the input is defined as
the threshold of the transducer.
39) Define resolution.
When the input to a transducer is increased slowly from some non-zero arbitary
value, the change in output is not detected at all until a certain input increment is
exceeded. This increment is defined as the resolution.
40) Define hysteresis.
When the input to a transducer which is initially at rest is increased from zero to
full-scale and then decreased back to zero, there may be two output values for the same
input. Hysteresis effects can be minimized by taking readings corresponding to ascending
and descending values of the input and then taking their arithmetic average.
41) What is range and span?
The range of the transducer is specified as from the lower value of input to higher
value of input.
The span of the transducer is specified as the difference between the higher and
lower limits of recommended input values.
42) What is rise time ?
It is defined as time required for the system to rise from 0 to 100 per cent of its
final value .
43) What is potentiometer?
Basically a resistance potentiometer, or simply a POT, (a resistive potentiometer
used for the purposes of voltage division is called a POT) consists of a resistive element
provided with a sliding contact. The POT is a passive transducer.
44) What are the advantages and disadvantages of potentiometer?
Advantages :
Inexpensive
Useful for measurement of large amplitudes
Efficiency is very high
Frequency response of wire wound potentiometers is limited
Disadvantages :
Require a large force to move
45) What is gauge factor?
The gauge factor is unit resistance change per unit strain, which is due to three
factors as revealed by the above equation.
46) What are the different types of strain gauge?
Unbonded metal strain gauges
Bonded metal wire strain gauges
Bonded metal foil strain gauges
Vacuum deposited thin metal film strain gauges
Sputter deposited thin metal strain gauges
Bonded semiconductor strain gauges
Diffused metal strain gauges.
47) What are the factors to be considered for bonded strain gauge?
Filament construction
Material of the filament wire
Base carrier material or backing material
Cement used to bond the filament to the carrier
Lead wire connections.
48) What is strain?
It is a ratio of changing length to original length.
49) What is young’s modulus ?
It is a ratio of stress and strain , dR/R / Dl/l
50) What is resistance thermometers?
A resistance thermometer consists of a resistive element which is exposed to the
temperature to be measured. If the conductors or metals are used to measure the
temperature, they are known as resistance thermometers and if semiconductors are used
then they known as thermistors.
51) What are the different approximation method of resistance thermometer? ¾ Linear approximation: ¾ Quadratic approximation
52) What is self heating error of thermometer?
Resistance thermometer bridges may be excited with either d.c.or a.c .The direct
or rms alternating current through the thermometer is usually in the range of 2 to 20 mA.
This current causes an I2R heating which raises the temperature of the thermometer above
its surrounding, causing the so called self-heating error.
53) What are the advantages and disadvantages of resistance thermometers?
Advantages :
They are suitable for measuring large temperature differences and high
temperatures.
They are very accurate which makes them suitable for small temperature
measurement.
Well designed resistance thermometers have excellent stability.
Unlike thermocouples, they do not need a reference junction and this
favors them in many aerospace and industrial applications.
Disadvantages :
Their relatively large volume compared to thermocouples results in
monitoring an average temperature over the length of the resistor rather
than a point temperature.
They need auxiliary apparatus and power supply.
The resistance element is usually more expensive than a thermocouple.
There are errors due to self heating and thermoelectric effect of the
resistive element and connecting leads (dissimilar metal junctions).
54) What is the principle of hotwire anemometer?
Another resistance variation type transducers is hot wire anemometer. In general
anemometers are devices used for measurement of velocity of flow.
55) Why dynamic compensation is required for hotwire anemometer?
It is used avoid the fluctuation, we need dynamic compensation circuits for the
hotwire anemometer.
56) What are the applications of thermistors? Measurement of power at high frequencies
Measurement of thermal conductivity
Measurement of level, flow and pressure of liquids
Measurement of composition of gases
Vacuum measurements
Providing time delay.
57) Mention the features of thermistors.
Compact, rugged and inexpensive.
Good stability.
The response time of thermistors can vary from a fraction of a second to minutes.
Self heating of thermistors is avoided.
Thermistors can be installed at a distance from their associated measuring circuits.
58) Mention the materials used for thermistors.
Mixture of metallic oxides such as manganese, nickel, cobalt, copper, iron and
uranium.
59) Mention the applications of strain gauge.
Used to measure pressure
Used to measure torque
Used to measure acceleration
Used to measure force
60) What is inductance transducer?
Transducers based on the variation of inductance are another group of importance
devices used in many application. In these transducers self inductance or the mutual of a
couple of coils is changed when the quantity to be measured is varied.
61) Mention three principles of inductance transducer.
Change of self inductance
Change of mutual inductance
Production of eddy currents.
62) What is LVDT?
The Linear Variable Differential Transformer (LVDT) is the most common
mutual inductance element. This can be considered to be a versatile transducer element
for most of the electromechanical measuring systems with regards to resolution,
hysteresis, dynamic response, temperature characteristics, linearity and life.
63) What are the advantages and disadvantages of LVDT?
Advantages :
High range
Friction and electrical Isolation
Immunity from external effects
High input and high sensitivity
Ruggedness
Low hysteresis
Low power consumption
Disadvantages :
Relatively large displacements are required for appreciable differential output
They are sensitive to stray magnetic fields but shielding is possible
Many a times, the transducer performance is affected by vibrations.
The receiving instrument must be selected to operate on a.c The dynamic response is limited
Temperature affects the performance of the transducer
.
64) What are the applications of LVDT?
Displacement measurement and
LVDT Gageheads
LVDT pneumatic servofollower
LVDT Loadcells
LVDT Pressure Transducer
65) What is null voltage?
Ideally the output voltage at the null position should be equal to zero. However, in
actual practice there exists a small voltage at the null position
.
66) Explain the principle of Induction Potentiometer?
The primary is excited with alternating current. This induces a voltage in to the
secondary. The amplitude of this output voltage varies with the mutual inductance
between the two coils and this varies with the angle of rotation.
67) Explain the principle of Variable Reluctance Accelerometer?
Another common version of the variable reluctance principle. This is an
accelerometer for measurement of accelerometer for measurement of acceleration in the
range ± 4g. Since the force required to accelerate a mass is proportional to the
acceleration.
68) What is the principle of capacitive transducer?
Many industrial variables like displacement, pressure, level, moisture, thickness
etc can be transduced into an electrical variation using capacitance variation as the
primary sensing principle.
70) What are the desirable feature of capacitive transducer?
Its force requirements are very small.
As the moving plates have very little mass, design of transducer with fast
response characteristics is possible.
There is no physical between moving and stationary parts.
Does not depends the conductivity of the metal electrode.
Shielded against the effect of external electric stray fields.
71) What are the different practical capacitance pickup’s ?
Equibar differential pressure transducer.
Feedback type capacitance proximity pickup.
Condenser microphone.
72) What is Microphone?
It is also a transducer which convert sound energy into electrical energy.
Example is condenser microphone.
73) What is the principle of change of capacitance?
Change in overlapping area A,
Change in the distance between the plates, d
Change in dielectric constant.
74) What are the advantages of capacitive transducers?
They require only small force to operate.
Have a good frequency response.
Extremely sensitive.
High input impedance
75) What are the disadvantages of capacitive transducers?
The metallic parts of the capacitive transducers must be insulated from
each other.
Non-linear behaviour
This leads loading effects.
The cable may be source of loading resulting loss of sensitivity.
76) What are the uses of capacitive transducer?
it Can be used for measurement of linear and angular displacement. Can be used for measurement of force and pressure. It can be used as pressure transducer. Measurement of humidity in gases. Commonly used for measurement of level, density, weight.
77) What is analog transducer?
These transducer convert input quantity into an analog output which is a
continuous function of time. Thus a strain gauge, LVDT, thermocouple, thermistors,
may be called as analog trasnducer.
78) What is digital transducer?
These transducer convert input quantity into an electrical output which is in the
form of pulses.
79) What is piezo electric transducer?
They convert pressure or force into electrical charge. These transducers are based
upon the natural phenomenon of certain non-metal and di-electric components.
80) What are the suitable materials for piezo electric transducer?
Primary quartz, Rochelle salt, ammonium dihydrogen phosphate (ADP), and
ceramics with barium titanate, dipottasium tartrate, potassium dihydrogen phosphate and
lithium sulfate.
81) What is ‘d’ coefficient ?
Gives the charge output per unit force input (or charge density per unit pressure)
under short circuit condition, it is measured in
Columbs / newton
82) What is ‘g’ coefficient ?
g-coefficient representing the generated e.m.f gradient per unit pressure input.
83) What is ‘h’ coefficient ?
It is obtained by multiplying the g-coefficient by youngs modulus valid for the
appropriate crystal orientation of the material, and thus measures the e.m.f gradient per
unit mechanical deformation, or ( V/m) / (m/m )
84) What are the suitable materials for magnetostrictive transducer?
Iron , nickel, 68 permalloy, ferroxcube ect
85) What is magnetostrictive transducer?
The permeability can increase or decrease depending upon the material, type of
stress , and the magnetic flux density in the sample.
86) What is fibre optic transducer?
Fibre optic cable consists of outer core and inner cladding.Data is
transmitted in the form of light.It is used for measuring displacement,torsion.
87) Compare digital transducer with analog .
Digital transducer give digital outputs. Analog transducers outputs are continuous
functions of time. If these analog transducers are to be interfaced with digital devices,
then one has to use analog to digital converters.
88) what is piezo electric effect?
A piezo electric material is one in which an electric potential appears across
certain surfaces of a crystal if the dimensions of the crystal are changed by the
application of the mechanical force.
89) What id digitiser?
Digital encoding transducer or digitiser, enable a linear or rotary displacement to
be directly converted into digital form without intermediate form of analog to digital
(A/D) conversion.
90) What is zero error of the transducer?
In this case output deviates from the correct value by a constant factor over the
entire range of transducer.
It is a device for determining the value or magnitude of a quantity or variable.
2) List three sources of possible errors in instruments.
Gross Error, Systematic & Random errors.
3) Define Instrumental error
These are the errors inherent in measuring instrument because of their mechanical
structure.
4) Define limiting error
Components are guaranteed to be with in a certain percentage of rated value. Thus
the manufacturer has to specify the deviations from the nominal value of a particular
quantity.
5) Define probable error
It is defined as r = ± 0.6745s where s is standard deviation.
Probable error has been used in experimental work to some extent in past, but standard
deviation is more convenient in statistical work.
6) Define Environmental error
These are due to conditions in the measuring device, including conditions in the
area surrounding the instrument, such as the effects of changes in temperature,
humidity.
7) Define units.
It is necessary to define a physical quantity both in kind and magnitude in order to
use this information for further proceedings. The standard measure of each kind of
physical quantity is named as the unit.
8) Define Standards
The physical embodiment of a unit of measurement is a standard. For example,
the fundamental unit of mass in the international system is the kilogram and defined as
the mass of a cubic decimeter of water at its temperature of maximum density of 4°c.
9) Mention the purpose of the measurement.
· To understand an event or an operation.
· To monitor an event or an operation.
· To control an event or an operation.
· To collect data for future analysis
· To validate an engineer design.
10) What are the methods of measurement?
· Direct comparison method
· Indirect comparison method
11) Define ODDs
The specification of limiting error is in itself uncertain because the manufacture
himself is not sure about the accuracy because of the presence of random errors.
12) Define transducer and give an example.
Transducer is a device which convert one form of energy into electrical energy.A
thermocouple converts heat energy into electrical voltage.
13) What is primary transducer?
Bourdon tube acting as a primary transducer, senses the pressure and convert the
pressure into displacement. No output is given to the input of the bourdon tube.
So it is called primary transducer. Mechanical device can act as a primary transducer.
14) What is secondary transducer?
The output of the Bourdon tube is given to the input of the LVDT. There are two
stages of trasduction, firstly the pressure is converted into a displacement by the
Bourdon tube then the displacement is converted into analog voltage by LVDT. Here
LVDT is called secondary transducer. Electrical device can act as a secondary
transducer.
15) What is passive transducer?
In the absence of external power, transducer cannot work and it is called a passive
transducer. Example capacitive , inductive, resistance transducers.
16) What is active transducer?
In the absence of external power, transducer can work and it is called active
transducer. Example velocity, temperature, light can be transdued with the help of
active transducer.
17) What is analog transducer?
These transducers convert the input quantity into an analog output which is a
continuous function of time. Thus a strain gauge, an LVDT, a thermocouple or a
thermistors may be called analog transducer, as they give an output which is a
continuous function of time.
18.Give the classification of units.
1.Absolute units
2.Fundamental and derived units
3.Electromagnetic units
4.Electrostatic units
19.Define Primary fundamental and auxiliary fundamental units.
Fundamental units in mechanics are measures of length,mass and time and those
are fundamental to most other physical quantities and hencee they are called Primary
fundamental units
Measures of certain physical quantities in thermal ,electrical and illumination
fields are also represented by fundamental units and are used only where those
disciplines are involved and hence called auxiliary fundamental units.
20.Define the unit of mass preserved at International Bureau of weights and measures at
Severes ,Near Paris.
The unit of mass is represented by a material standard;the mass of
International prototype kilogramme consisting of platinum Iridium hollow cyclinder.
21.Define static calibration.
It refers to a process in which all the inputs(desired,modifying,interfering)except
one are kept at some constant values.
22.Define Traceability.
The ability to trace the accuracy of the standard back to its ultimate source in
fundamental standards of National Institute of Science and Technology is termed
“Traceability”
23.What are random errors or residual errors.
The happenings or disturbances about which we are unaware and lumped together
are called random errors or residual errors.Since these errors remain even after the
systematic errors are taken care of,they are called residual errors.
24.Give one property of piezo-electric crystal.
When a force is applied to piezo-electric crystals ,they produce an output voltage
25.Define an Inverse transducer.Give an example.
A device which converts an electrical quantity into an non-electrical quantity.A
piezo-electric crystal acts as a inverse transducer because whwn a voltage is applied
across its surfaces ,it changes its dimensions causing a mechanical displacement
.
26.List the factors responsible in selection of a transducer.
1.Operating principle
2.Sensitivity
3.Operating range.
4.Accuracy.
27) Define static characteristics.
static characteristics of a measurement system are, in general, those that must be
considered when the system or instrument is used to measure a condition not varying
with time.
28) Mention different types of static characteristics.
(i) accuracy (ii) sensitivity (iii) Reproducibility (iv) Drift (v) Static error and (vi)
Dead zone .
29) What is dynamic characteristics?
Many measurements are concerned with rapidly varying quantities and , therefore,
for such cases we must examine the dynamic relations which exist between the output
and the input . This is normally done with the help of differential equations . Performance
criteria based upon dynamic relations constitute the Dynamic Characteristics.
30) Mention different types dynamic characteristics?
Zero- order transducers
First - order transducers
Second-order transducers
Higher-order transducers
31) What are the test inputs of the transducer?
Impulse input
Step input
Ramp input
Parabolic input
Sinusoidal input
32) Define- zero order transducer.
The input- output relationship of a zero- order transducer is given by
Y(t) = K r(t)
Where r(t) is the input, Y(t) is the output and K is the static – sensitivity of the transducer.
Example for zero- order transducer is a potentiometer.
33) What is frequency response of ZOT.
Frequency response is thus defined as the steady – state output of a transducer
When it is excited with sinusoidal input . the frequency response is represented with the
help of two plots namely amplitude radio verses frequency and phase angle shift versus
frequency.
34) What is damping ratio?
The damping ratioV is an important parameter which decides the nature of
oscillation in the transducer output . when V =0 , the second – order system is said to be
un damped and the system behaves like an oscillator . when V =1 , the second – order
system is said to be critical damped and when V >1 , the second – order system is said to
be over damped.
35) Define static sensitivity
Sensitivity should be taken depending on the operating point. The sensitivity is
expressed in output unit/ input unit.
36) Define linearity.
Linearity is a measure of the maximum deviation of the plotted transducer
response from a specified straight line .
37) Compare accuracy and precision.
Accuracy is the closeness to true value whereas precision is the closeness amongst
the readings .precision is the degree of closeness with which a given value may be
repeatedly measured.
38) What is Threshold?
When the input to a transducer is increased from zero, there is a minimum value
below which no output can be detected .This minimum value of the input is defined as
the threshold of the transducer.
39) Define resolution.
When the input to a transducer is increased slowly from some non-zero arbitary
value, the change in output is not detected at all until a certain input increment is
exceeded. This increment is defined as the resolution.
40) Define hysteresis.
When the input to a transducer which is initially at rest is increased from zero to
full-scale and then decreased back to zero, there may be two output values for the same
input. Hysteresis effects can be minimized by taking readings corresponding to ascending
and descending values of the input and then taking their arithmetic average.
41) What is range and span?
The range of the transducer is specified as from the lower value of input to higher
value of input.
The span of the transducer is specified as the difference between the higher and
lower limits of recommended input values.
42) What is rise time ?
It is defined as time required for the system to rise from 0 to 100 per cent of its
final value .
43) What is potentiometer?
Basically a resistance potentiometer, or simply a POT, (a resistive potentiometer
used for the purposes of voltage division is called a POT) consists of a resistive element
provided with a sliding contact. The POT is a passive transducer.
44) What are the advantages and disadvantages of potentiometer?
Advantages :
Inexpensive
Useful for measurement of large amplitudes
Efficiency is very high
Frequency response of wire wound potentiometers is limited
Disadvantages :
Require a large force to move
45) What is gauge factor?
The gauge factor is unit resistance change per unit strain, which is due to three
factors as revealed by the above equation.
46) What are the different types of strain gauge?
Unbonded metal strain gauges
Bonded metal wire strain gauges
Bonded metal foil strain gauges
Vacuum deposited thin metal film strain gauges
Sputter deposited thin metal strain gauges
Bonded semiconductor strain gauges
Diffused metal strain gauges.
47) What are the factors to be considered for bonded strain gauge?
Filament construction
Material of the filament wire
Base carrier material or backing material
Cement used to bond the filament to the carrier
Lead wire connections.
48) What is strain?
It is a ratio of changing length to original length.
49) What is young’s modulus ?
It is a ratio of stress and strain , dR/R / Dl/l
50) What is resistance thermometers?
A resistance thermometer consists of a resistive element which is exposed to the
temperature to be measured. If the conductors or metals are used to measure the
temperature, they are known as resistance thermometers and if semiconductors are used
then they known as thermistors.
51) What are the different approximation method of resistance thermometer? ¾ Linear approximation: ¾ Quadratic approximation
52) What is self heating error of thermometer?
Resistance thermometer bridges may be excited with either d.c.or a.c .The direct
or rms alternating current through the thermometer is usually in the range of 2 to 20 mA.
This current causes an I2R heating which raises the temperature of the thermometer above
its surrounding, causing the so called self-heating error.
53) What are the advantages and disadvantages of resistance thermometers?
Advantages :
They are suitable for measuring large temperature differences and high
temperatures.
They are very accurate which makes them suitable for small temperature
measurement.
Well designed resistance thermometers have excellent stability.
Unlike thermocouples, they do not need a reference junction and this
favors them in many aerospace and industrial applications.
Disadvantages :
Their relatively large volume compared to thermocouples results in
monitoring an average temperature over the length of the resistor rather
than a point temperature.
They need auxiliary apparatus and power supply.
The resistance element is usually more expensive than a thermocouple.
There are errors due to self heating and thermoelectric effect of the
resistive element and connecting leads (dissimilar metal junctions).
54) What is the principle of hotwire anemometer?
Another resistance variation type transducers is hot wire anemometer. In general
anemometers are devices used for measurement of velocity of flow.
55) Why dynamic compensation is required for hotwire anemometer?
It is used avoid the fluctuation, we need dynamic compensation circuits for the
hotwire anemometer.
56) What are the applications of thermistors? Measurement of power at high frequencies
Measurement of thermal conductivity
Measurement of level, flow and pressure of liquids
Measurement of composition of gases
Vacuum measurements
Providing time delay.
57) Mention the features of thermistors.
Compact, rugged and inexpensive.
Good stability.
The response time of thermistors can vary from a fraction of a second to minutes.
Self heating of thermistors is avoided.
Thermistors can be installed at a distance from their associated measuring circuits.
58) Mention the materials used for thermistors.
Mixture of metallic oxides such as manganese, nickel, cobalt, copper, iron and
uranium.
59) Mention the applications of strain gauge.
Used to measure pressure
Used to measure torque
Used to measure acceleration
Used to measure force
60) What is inductance transducer?
Transducers based on the variation of inductance are another group of importance
devices used in many application. In these transducers self inductance or the mutual of a
couple of coils is changed when the quantity to be measured is varied.
61) Mention three principles of inductance transducer.
Change of self inductance
Change of mutual inductance
Production of eddy currents.
62) What is LVDT?
The Linear Variable Differential Transformer (LVDT) is the most common
mutual inductance element. This can be considered to be a versatile transducer element
for most of the electromechanical measuring systems with regards to resolution,
hysteresis, dynamic response, temperature characteristics, linearity and life.
63) What are the advantages and disadvantages of LVDT?
Advantages :
High range
Friction and electrical Isolation
Immunity from external effects
High input and high sensitivity
Ruggedness
Low hysteresis
Low power consumption
Disadvantages :
Relatively large displacements are required for appreciable differential output
They are sensitive to stray magnetic fields but shielding is possible
Many a times, the transducer performance is affected by vibrations.
The receiving instrument must be selected to operate on a.c The dynamic response is limited
Temperature affects the performance of the transducer
.
64) What are the applications of LVDT?
Displacement measurement and
LVDT Gageheads
LVDT pneumatic servofollower
LVDT Loadcells
LVDT Pressure Transducer
65) What is null voltage?
Ideally the output voltage at the null position should be equal to zero. However, in
actual practice there exists a small voltage at the null position
.
66) Explain the principle of Induction Potentiometer?
The primary is excited with alternating current. This induces a voltage in to the
secondary. The amplitude of this output voltage varies with the mutual inductance
between the two coils and this varies with the angle of rotation.
67) Explain the principle of Variable Reluctance Accelerometer?
Another common version of the variable reluctance principle. This is an
accelerometer for measurement of accelerometer for measurement of acceleration in the
range ± 4g. Since the force required to accelerate a mass is proportional to the
acceleration.
68) What is the principle of capacitive transducer?
Many industrial variables like displacement, pressure, level, moisture, thickness
etc can be transduced into an electrical variation using capacitance variation as the
primary sensing principle.
70) What are the desirable feature of capacitive transducer?
Its force requirements are very small.
As the moving plates have very little mass, design of transducer with fast
response characteristics is possible.
There is no physical between moving and stationary parts.
Does not depends the conductivity of the metal electrode.
Shielded against the effect of external electric stray fields.
71) What are the different practical capacitance pickup’s ?
Equibar differential pressure transducer.
Feedback type capacitance proximity pickup.
Condenser microphone.
72) What is Microphone?
It is also a transducer which convert sound energy into electrical energy.
Example is condenser microphone.
73) What is the principle of change of capacitance?
Change in overlapping area A,
Change in the distance between the plates, d
Change in dielectric constant.
74) What are the advantages of capacitive transducers?
They require only small force to operate.
Have a good frequency response.
Extremely sensitive.
High input impedance
75) What are the disadvantages of capacitive transducers?
The metallic parts of the capacitive transducers must be insulated from
each other.
Non-linear behaviour
This leads loading effects.
The cable may be source of loading resulting loss of sensitivity.
76) What are the uses of capacitive transducer?
it Can be used for measurement of linear and angular displacement. Can be used for measurement of force and pressure. It can be used as pressure transducer. Measurement of humidity in gases. Commonly used for measurement of level, density, weight.
77) What is analog transducer?
These transducer convert input quantity into an analog output which is a
continuous function of time. Thus a strain gauge, LVDT, thermocouple, thermistors,
may be called as analog trasnducer.
78) What is digital transducer?
These transducer convert input quantity into an electrical output which is in the
form of pulses.
79) What is piezo electric transducer?
They convert pressure or force into electrical charge. These transducers are based
upon the natural phenomenon of certain non-metal and di-electric components.
80) What are the suitable materials for piezo electric transducer?
Primary quartz, Rochelle salt, ammonium dihydrogen phosphate (ADP), and
ceramics with barium titanate, dipottasium tartrate, potassium dihydrogen phosphate and
lithium sulfate.
81) What is ‘d’ coefficient ?
Gives the charge output per unit force input (or charge density per unit pressure)
under short circuit condition, it is measured in
Columbs / newton
82) What is ‘g’ coefficient ?
g-coefficient representing the generated e.m.f gradient per unit pressure input.
83) What is ‘h’ coefficient ?
It is obtained by multiplying the g-coefficient by youngs modulus valid for the
appropriate crystal orientation of the material, and thus measures the e.m.f gradient per
unit mechanical deformation, or ( V/m) / (m/m )
84) What are the suitable materials for magnetostrictive transducer?
Iron , nickel, 68 permalloy, ferroxcube ect
85) What is magnetostrictive transducer?
The permeability can increase or decrease depending upon the material, type of
stress , and the magnetic flux density in the sample.
86) What is fibre optic transducer?
Fibre optic cable consists of outer core and inner cladding.Data is
transmitted in the form of light.It is used for measuring displacement,torsion.
87) Compare digital transducer with analog .
Digital transducer give digital outputs. Analog transducers outputs are continuous
functions of time. If these analog transducers are to be interfaced with digital devices,
then one has to use analog to digital converters.
88) what is piezo electric effect?
A piezo electric material is one in which an electric potential appears across
certain surfaces of a crystal if the dimensions of the crystal are changed by the
application of the mechanical force.
89) What id digitiser?
Digital encoding transducer or digitiser, enable a linear or rotary displacement to
be directly converted into digital form without intermediate form of analog to digital
(A/D) conversion.
90) What is zero error of the transducer?
In this case output deviates from the correct value by a constant factor over the
entire range of transducer.
hi! very nice blogs your site is good.have shared useful information ......Thankq for Aircraft weighing
ReplyDeleteWeight transducer