KTU - EET 203 Measurements and Instrumentation Module 4.pdf

Module 4
Magnetic, Lumen and Temperature Measurements
Magnetic Measurements: Measurement of flux and permeability - flux meter, BH
curve and permeability measurement - hysteresis measurement- ballistic
galvanometer – principle- determination of BH curve - hysteresis loop. Lloyd Fisher
square — measurement of iron losses.
Measurement luminous intensity-Photoconductive Transducers-Photovoltaic cells
Temperature sensors- Resistance temperature detectors-negative temperature
coefficient Thermistors-thermocouples- silicon temperature sensors.

Measurement of Flux Density – Method of Reversal

Determination of BH curve – Method of reversal
 A ring shaped specimen whose dimensions are known is used for the purpose.
 A layer of thin tape is put on the ring and a search coil insulated by paraffined wax is
wound over the tape.
 Another layer of tape is put over the search coil and the magnetising winding is uniformly
wound over this tape.
 The test is started by setting the magnetising current to its lowest test value.
 With galvanometer key K closed, the reversing switch is reversed.
 The flux density corresponding this H is calculated by dividing flux by cross sectional area.
 The current is set to another value and the procedure is repeated till the maximum
current.

Step-by-step Method
 The magnetizing winding is
supplied through a potential
divider having a large number of
tappings.
 The tappings are arranged so
that the magnetising force H
may be increased, in a number
of suitable steps, up to the
desired maximum value.
 The specimen before being
tested is demagnetised.

 The tapping switch S2 is set on tapping 1 and the· switch S1 is closed.
 The throw of galvanometer corresponding to this increase in flux density in the specimen,
from zero to some value B1 is observed.
 The value of Bi can be calcu1ated from the throw of the galvanometer, . The value of
corresponding magnetising force H1 may be calculated from the value of current flowing in
the magnetising winding at tapping 1.
 The magnetising force is then increased to H2 by switching S2 suddenly to tapping 2, and
the corresponding increase in flux density ΔB is determined from the throw of the
galvanometer.
 Then flux density B2 corresponding to magnetising force H2 is given by B1+ ΔB.
 This process is repeated for other values of H up to the maximum point, and the complete
B-H curve is thus obtained as shown in Fig.

Hysteresis loop- step by step Method
 The determination of hysteresis loop by this method is done by simply continuing the
procedure just described for the determination of B·H curve .
 After reaching the point of maximum H i.e., when switch S2 is at tapping 10, the
magnetising current is next reduced in steps to zero by moving switch S2 down
through the tapping points 9, 8, 7 ........ :, 3, 2, 1.
 After reduction of magnetising force to zero, negative values of H are obtained by
reversing the supply to potential divider and then moving the switch S2 up again in
order I, 2, 3, ....... :., 7, 8, 9, 10.

Method of Reversal
 This test is done by means of a
number of steps, but the change
in flux density measured at each
step is the change from the
maximum value + Bm down to
some lower value.
 But before the next step is
commenced the iron specimen is
passed through the remainder of
the cycle of magnetization back
to the flux density + Bm. Thus
the cyclic state of magnetization
is preserved.

 R4, R2 and R1 are resistances in the magnetising winding and galvanometer circuits.
 R3 is a variable shunting resistance, which is connected across the magnetising winding
by moving over the switch S2.
 Thus the current in this winding can be reduced from its maximum value down to any
desired value by adjusting the value of R3.
 The value of magnetising force Hm required to produce flux density Bm to be used during
the test is obtained from the previously determined BH curve of the specimen.
 The resistances R2 and R4 are then adjusted so that the magnetising current is such that
this value of H (i e., Hm) is obtained when switch S2 is in off position
 R1 is adjusted to get a convenient deflection in galvanometer corresponding to Hm.
 Resistance R3 is adjusted to a value that will give a certain reduction in current when it is
included to the circuit.
 Switch RS2 is placed on contacts 1, 1' and key K is opened. Since the maximum value of
current is flowing in the magnetising winding, the magnetisaiion of the specimen
corresponds to point A on the hysteresis loop shown in Fig.
 Now switch S2 is quickly thrown over from off position to contact b, thus shunting the
magnetising winding with resistance R3.

 The magnetising force is reduced to Hc say
 The corresponding ΔB can be detected from galvanometer throw and point C can be
located.
 The key K is now closed, and switch RS2 reversed on to contacts 2, 2'. Switch S2 is then
opened and switch RS2 moved back again to contacts I, l'. This procedure passes the
specimen through the cycle of magnetisation and back to the point A.
 The step is repeated up to point D.
 After this to get the part DEF the switch with K closed and S2 in OFF position RS2 is
reversed to take the value to –Hm.
 The process is then repeated’
 FGLA is drawn identical to ACDEF.

Ballistic Galvanometer
 A ballistic galvanometer is a type of sensitive galvanometer.
 Unlike the current measuring galvanometer the moving part has large moment of inertia
thus giving it a long oscillation period.
 It measure the charge discharge through it.
 It can be either moving coil or moving magnet type.

Construction:
 The ballistic galvanometer consists of copper wire which is wound on non conducting frame
of galvanometer.
 For increasing the magnetic flux the iron core is placed
within the coil.
 The lower portion of the coil connected through spring
which provides the restoring torque to the coil.
 When charge passes through the galvanometer it gets an
impulse.
 The impulse of the coil is proportional to the charge passing
through it.
 The coil has a high moment of inertia, the their oscillations are large and hence accurate
reading is obtained.

Working:
 It works on the concept of Lorentz Force Law, according to which whenever a current-
carrying coil is placed under the influence of a magnetic field, a force is exerted on the
coil.
 In this case, when the current flows through the coil and impulsive force is exerted on
the coil.
 The force is proportional to the magnitude of current passing through the coil.
 To settle the torque to its steady-state position damping torque is used in other
moving coil instruments. But in the case of the ballistic galvanometer, damping torque
is zero. It’s damping constant is zero. That’s the reason the word ballistic is used.

Calibration:
 Calibration is a process in which changes are made in the instrument so that the
instrument can maintain accuracy.
 It is the process of configuring the instrument so that it produces accurate results.
 In this process, the instrument constants are calculated.
 For a ballistic galvanometer, different methods like calibration using a capacitor,
calibration using mutual inductance, etc. are used to calculate the meter constants.

Consider a rectangular coil having N numbers of turn placed in a uniform magnetic field. Let 𝑙
be the length and 𝑏 be the breadth of the coil
the area of the coil is given by 𝐴 = 𝑙 × 𝑏 → (𝑖)
The magnitude of torque developed is 𝜏 = 𝑁𝑖𝐵𝐴 → 𝑖𝑖
Let the current flow through the coil for the very short duration (𝑑𝑡) is 𝜏𝑑𝑡 = 𝑁𝑖𝐵𝐴𝑑𝑡 → 𝑖𝑖𝑖
Theory:

Here 𝑞 be the total charge passes through the coil. The moment of inertia is given by 𝐼 and
angular velocity by 𝜔 . By definition angular momentum is given by
𝐴𝑛𝑔𝑢𝑙𝑎𝑟 𝑀𝑜𝑚𝑒𝑛𝑡𝑢𝑚 = 𝐼𝜔 → (𝑣)
The angular momentum of the coil is equal the force acting on the coil. Thus from equation (𝑖
𝑣) and (𝑣) we get
𝐼𝜔 = 𝑁𝐴𝐵𝑞 → (𝑣𝑖)
The kinetic energy (𝐾) deflects the coil through the angle 𝜃and this defection restored
through the spring.

Charge and Current sensitivity:
The charge sensitivity of a ballistic galvanometer is defined as the deflection per unit
charge. It is denoted by Qs
.

On the other hand the current sensitivity of the ballistic galvanometer is the
deflection per unit current. It is denoted by 𝐼𝑠
.

Advantages
 Linear Scale. The scale of the galvanometer is linear.
 It is highly sensitive.
 It is accurate and precise
 The toque to weight ratio is high. (This avoids errors)
 It is not affected by stray magnetic fields
Disadvantages
 The disadvantages of the ballistic galvanometer include the following.
 Since it works on the principle of PMMC, it can be used only of DC measurements.
 Due to components such as springs, permanent magnets, etc. it develops errors due
to aging.

Applications
 Used in Wheatstone bridge, to detect the presence of current in the loop
 Can be used to measure current by connecting a low resistance in parallel to it.
 Can be used to measure voltage by connecting a high resistance in series to it.
 Detecting errors in communication cables
 Positioning the pen in analog strip chart recorders, electrocardiographic, etc.

Flux meter:
 The meter which is used for measuring the flux of the permanent magnet such type of
meter is known as the flux meter.
 The flux meter is the advanced form of the ballistic galvanometer which has certain
advantages like the meter has low controlling torque and heavy electromagnetic damping.
Construction:
 The fluxmeter has a coil which is freely
suspended by the help of the spring and the
single silk thread. The coil moves freely between
the poles of the permanent magnet.
 The current enters into the coil with the help of
the helices which is very thin and made from the
annealed silver strips.
 This current reduces the controlling torque to
the minimum value. The air friction damping of
the coil is negligible.

Operation:
 The terminals of the flux meter are connected
across the search coil as shown in the figure.
 The flux linking with the coil is varied by either
removing it from the magnetic field or by reversing
the field of the magnet. The change of the flux
induces the electromotive force in the coil.
 This emf induces the current in the search coil and
send it through the flux meter.
 Because of the current, the pointer of the flux meter deflects, and their deflection is
directly proportional to the change in the value of flux linkages.
 As, the variation of the flux linkages reduces, coil stop moving because of their high
electromagnetic damping
 The high electromagnetic damping is because of the low resistance circuit between the
flux meter and the search coil.

Advantages of Flux meter:
 The flux meter is portable.
 The scale of the flux meter is calibrated in Weber meters.
 The deflection of the coil is free from the time taken by the flux to change.
Disadvantages:
 The only disadvantage of the flux meter is that it is less sensitive and accurate as
compared to the flux meter.

Lloyd-Fisher Square:
 The strips are usually 0.25 m long and 50 to 60 mm wide.
 These strips are built up in to four stacks and each stack is
made up of two types of strips one cut in direction of
rolling and another perpendicular.
 The stacks of strips are placed inside four similar
magnetising coils of larger cross section area.
 These four coils are connected in series to form primary
winding.
 Each magnetising coil has two similar single layer coil beneath it, adding up to eight coils.
 They are called secondary coil and are in eight numbers.
 These are connected in series in groups of four, one from each core to form two separate
winding.
 The ends of the strips project beyond the magnetising coil.
 The plane of strips are perpendicular to plane of square.
 The magnetic circuit is completed by joining with corner pieces.

 The corner pieces are of the same material as that of strips or material with same
properties.
 There is an overlapping of corner pieces and strips due to which cross sectional area is
doubled at the corners.
 And hence a correction must be applied for this.
 Advantages are
Gives more reliable results.
Corner pieces make it suitable for testing anisotropic materials.

Wattmeter method for iron loss measurement
The test specimen is weighed before assembly
and its effective cross-section- is determined
The primary winding is connected to a
sinusoidal voltage supply .
The primary winding contains the current coil
of the wattmeter.
The pressure coil of the wattmeter is supplied
from one of the secondary windings.
The wattmeter is designed for low .Power
factor operation as the power factor is usually
about 0.2.
The other secondary is connected to an electrostatic voltmeter.
The voltage applied to the primary winding is adjusted, preferably with the help of a
variable ratio transformer, till the magnetising current is adjusted to give the required
value of Bm.

 The frequency of supply is adjusted to the correct value.
 The wattmeter and voltmeter readings are observed.
 The electrostatic voltmeter connected across secondary winding S2 measures the
rms value of induced voltage.

Correction must be applied to this value of flux density as the secondary winding S2
encloses the flux in the air space. between specimen and the coil in addition to the flux
in the specimen.
 The wattmeter reading includes the iron loss of specimen and the copper loss of
secondary winding ckt.
 The copper loss of secondary ckt can be calculated and subtracted from the
wattmeter reading

 Specific iron loss can be calculated by dividing the total iron loss by weight of the
specimen

 luminous intensity is a measure of the wavelength-weighted power emitted by a light
source in a particular direction per unit solid angle,
 The SI unit of luminous intensity is the candela (cd).
 Two lamps S and T are placed at a distance apart with a screen in between them.
 S is a standard lamp whose intensity is known and T is the test lamp.
 The screen is moved in between the lamps till the illuminance(illumination) on both sides
of the screen is equal·
Measurement of Luminous Intensity:

 The screen used for determining the point of equal luminance is called "Photometer
Head".
 The graduated bench on which the pbotometer head slides is called "Photometer
Bench".
 Different photometer heads used are Bunsen Photometer Head, Lummer-Brodhun
Photometer Head and Flicker Photometer Head.

Photoconductive Transducers
 The working is based upon the change in conductivity of a semi-conducting material
with change in radiation intensity.
 The change in conductivity appears as change in resistance and therefore these devices
are photo resistive cells.
 In a semi-conductor an energy gap exists between conduction electrons and valence
electrons.
 In a semi-conductor photoconductive transducer, a photon is absorbed and thereby
excites an electron from valence band to conduction band.
 As electrons are excited from valence band to conduction band, the resistance
decreases, making the resistance an inverse function of radiation intensity.

Photoconductive cell and circuit
materials are cadmium sulphide (CdS) with a band gap of 2.42 eV and
cadmium selenide (CdSe) with 1.74 eV band gap.

Photovoltaic Cell:
 This is an important class of photo detectors.
 They generate a voltage which is proportional to EM radiation intensity.
 They are called photovoltaic ce!ls because of their voltage generating characteristics.
 They in fact convert the EM energy in to electrical energy.
 They are passive transducers. i e., they do not need an external source to power them.
 The cell is a giant diode, constructing a pn junction between appropriately doped
semiconductors.
 Photons striking the cell pass through the thin p-doped upper layer and are absorbed by
electrons in the lower n layer, causing formation of conduction electrons and holes.
 The depletion zone potential of the pn junction then separates these conduction holes
and electrons causing a difference of potential to develop across the junction .

 The photovoltaic cells can operate satisfactorily in
the temperature range of -100 to 1250C.
 The temperature changes have little effect on short-
circuit current but affect the open circuit voltage
considerably.
 These changes may be of the order of a few mV/0C
in output voltage.
 The response of these cells is very fast ·and
therefore photovoltaic cells are widely used in light
exposure meter in photographic work.

Temperature Measurement:
Resistance Thermometer or Resistance Temperature Detector(RTD):
 It uses the change in electrical resistance of conductor with temperature.
 The requirements of a conductor material to be used in resistance thermometers
are :
(i) the change in resistance of the material per °C should be as large as possible, and
(ii} the resistance of the material should have a continuous and stable relationship
with temperature.
(iii) The material should have a high value of resistivity so that minimum volume of
material is used

 Gold and silver are rarely used due to their low resistivity.
 Tungsten has relatively high resistivity , but used for high temperature applications.
 The most commonly used material for metallic resistance thermometer is platinum.
 The resistance temperature characteristics of pure platinum are very well defined
and show a high degree of repeatability.
 It can be used for a wide range.
 Sensing elements in the form of wires.
 Helical coil wound as double layer to reduce L.
 In industrial type the former being of grooved
ceramic and the wire being protected by a glass
coating or by a stainless steel tube.

Semiconductor thermometers:
 Semi-conductor materials like germanium crystals with controlled doping can be used
for measurement of cryogenic temperatures especially below 25 K ( -248 °C).
 They are also usable at 100 K (-17 3 °C).
 Semi-conductor materials used in resistance temperature thermometers have large
resistance temperature coefficients and hence they are very sensitive in the above
mentioned range.
 They are repeatable within 0·01°C, but the units must be individually calibrated.
 Silicon crystals are now being used in the range -48 0 C to 250 C.
 In this range, their resistance increases with increasing temperature, and the
relationship is sufficiently linear.
 These thermometers must be individually calibrated.

The advantages are :
(i) They are readily available and are inexpensive.
(ii) They are rugged and sensitive.
(iii) Their sizes are small.
iv) They have a good frequency response characteristics.
Their disadvantages are :
(i) They are not so reproducible as germanium.
(ii) They have a large negative resistance temperature co-efficient below -213°C.

 A thermistor (or thermal resistor) is defined as a type of resistor whose electrical
resistance varies with changes in temperature.
 Although all resistors’ resistance will fluctuate slightly with temperature, a thermistor is
particularly sensitive to temperature changes.
 Thermistors act as a passive component in a circuit. They are an accurate, cheap, and
robust way to measure temperature.
 While thermistors do not work well in extremely hot or cold temperatures, they are the
sensor of choice for many different applications.
 Thermistors are ideal when a precise temperature reading is required.
Thermistor:

 The working principle of a thermistor is that its resistance is dependent on its
temperature and we can measure the resistance of a thermistor using an ohmmeter.
 If we know the exact relationship between how changes in the temperature will affect
the resistance of the thermistor – then by measuring the thermistor’s resistance we can
derive its temperature.
 If we had a thermistor with the below temperature graph, we could simply line up the
resistance measured by the ohmmeter with the temperature indicated on the graph.
There are two types of thermistors:
Negative Temperature Coefficient (NTC) Thermistor
Positive Temperature Coefficient (PTC) Thermistor

NTC Thermistor
 In an NTC thermistor, when the temperature increases, resistance decreases. And when
temperature decreases, resistance increases.
 Hence in an NTC thermistor temperature and resistance are inversely proportional.
 These are the most common type of thermistor.
If the value of β is high, then the resistor–temperature
relationship will be very good

Thermocouple
 The thermocouple can be defined as a kind of temperature sensor that is used to measure
the temperature at one specific point in the form of the EMF or an electric current.
 This sensor comprises two dissimilar metal wires that are connected together at one
junction.
 The temperature can be measured at this junction, and the change in temperature of the
metal wire stimulates the voltages.
 The amount of EMF generated in the device is very minute (millivolts), so very sensitive
devices must be utilized for calculating the e.m.f produced in the circuit.
 The common devices used to calculate the e.m.f are voltage balancing potentiometer and
the ordinary galvanometer.

Working:
 The thermocouple principle mainly depends on the three effects namely Seebeck, Peltier,
and Thompson.
 See beck-effect
This type of effect occurs among two dissimilar metals. When the heat offers to any one of
the metal wires, then the flow of electrons supplies from hot metal wire to cold metal wire.
Therefore, direct current stimulates the circuit.
 Peltier-effect
This Peltier effect is opposite to the Seebeck effect. This effect states that the difference of
the temperature can be formed among any two dissimilar conductors by applying the
potential variation among them.
 Thompson effect
This effect states that as two disparate metals fix together & if they form two joints then the
voltage induces the total conductor’s length due to the gradient of temperature. This is a
physical word that demonstrates the change in rate and direction of temperature at an
exact position

Construction:
 It comprises two different metal wires and that are connected together at the junction
end.
 The end of the junction is classified into three type’s namely ungrounded, grounded,
and exposed junction.
 Ungrounded-Junction - In this type of junction, the conductors are totally separated
from the protecting cover. The applications of this junction mainly include high-pressure
application works. The main benefit of using this function is to decrease the stray
magnetic field effect.
 Grounded-Junction - In this type of junction, the metal wires, as well as the protection
cover, are connected together. This function is used to measure the temperature in the
acidic atmosphere, and it supplies resistance to the noise.
 Exposed-Junction - The exposed junction is applicable in the areas where a quick
response is required. This type of junction is used to measure the gas temperature. The
metal used to make the temperature sensor basically depends on the calculating range
of temperature.

Generally, a thermocouple is designed with two different metal wires namely iron and
constantan that makes in detecting element by connecting at one junction that is named as
a hot junction.

 Digital thermometers (thermostats)
 Automotive applications (to measure oil and coolant temperatures in cars &
trucks)
 Household appliances (like microwaves, fridges, and ovens)
 Circuit protection (i.e. surge protection)
 Rechargeable batteries (ensure the correct battery temperature is maintained)
 To measure the thermal conductivity of electrical materials
 Useful in many basic electronic circuits (e.g. as part of a beginner Arduino starter
kit)
 Temperature compensation (i.e. maintain resistance to compensate for effects
caused by changes in temperature in another part of the circuit)
 Used in wheatstone bridge circuits
Applications:

KTU - EET 203 Measurements and Instrumentation Module 4.pdf

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