The electromagnetic flowmeters work on the principle that the obstruction is created in the path of the liquid and the pressure of the liquid induces the voltage across the coil.

Working Principle of Electromagnetic Flow Meter

The electromagnetic flowmeter works on the principle of Faraday’s Law of electromagnetic induction. This law states that when the conductive liquid passes through the magnetic field, the voltage induces across the conductor. The magnitude of the voltage is directly proportional to the velocity, length of the conductor and the strength of the magnetic field.

The magnetic field is generated by the coil which is mounted on the external metallic body of the pipe. The liquid acts as a conductor and when passes through the magnetic field induce the voltage across the coil. The magnitude of the voltage depends on the velocity of the liquid.

Construction of Electromagnetic Flow Meter

The electromagnetic flow meter consists the electrically insulated pipe made of fibre. Electrodes placed opposite to each other, magnetic coil placed on the pipe for generating the magnetic field etc. The insulated pipe carries the liquid whose flow needs to be measured.

The electromagnet is placed around the insulated pipe. This electromagnet induces the magnetic field around the pipe. The arrangement is similar to the conductor moving in the magnetic field. The voltage is induced across the coil because of the flow of the liquid. The induces voltage is expressed as,

Where, v – velocity of conductor (flow); m/s
l – length of conductor = diameter of pipe ;m
B = flux X density ; wb/m²

If the magnetic field around the pipe remains constant than the generating voltage is proportional to the velocity of the fluid.

Advantages of Electromagnetic Flow Meter

1. The output voltage of the electromagnetic flow meter is proportional to the flow rate of the liquid.
2. The output is uninfluenced by the varying characteristic of liquid like viscosity, pressure, temperature etc.
3. The electromagnetic flow meter can measure the flow of slurries, greasy, and can handle the corrosive fluid liquids.
4. It is used as a bidirectional meter.
5. The extremely low flow rate can also be measured by the electromagnetic flow meter.

Disadvantages of Electromagnetic Flow Meter

1. The electromagnetic flow meter has low accuracy.
2. It is heavy and extremely large in size.

The electromagnetic flow meter is also known as the magmeter.

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The synchroscope consists the two-phase wound stator and rotor. The alternators provide the two-phase supply to the synchroscope. If any of the two phases of the machine are synchronised, then the third phase was automatically being synchronized.

The existing alternator gives the supply to the stator of the synchroscope. And the incoming alternator gives the supply to the rotor. The phase shift between the supplies indicates the difference in phase and frequency to the alternators connected parallel to each other. The synchroscope also indicates the speed (i.e., running fast or slow) of the incoming alternator.

The synchroscope starts operating when the alternators of different frequencies are connected to each other. If the frequency of rotor and stator remain same then the rotor will not rotate or becomes stationary, i.e., the dial also remains stationary. When the frequency of rotor and stator supply changes then the rotor starts rotating. i.e., the dials also start deflecting.

The speed of the rotor depends on the difference of the supply frequency. If the differences are large, then the rotor will rotate at high speed and if the differences are less than the speed of the rotor will become less.

Types of Synchroscope

Electrodynamometer Type Synchroscope

The static and dynamic are the two main parts of the electrodynamometer synchroscope. The static part of the synchroscope consists the three-limbed transformer and the lamp. The bus-bar excites one of the windings of the transformer, and the other two windings are excited by the incoming machines. The lamp is connected to the central limb of the transformer.

The winding of the outer limb of the transformer induces two flux. And the flux of the central limb is the resultant of the outer two limb flux. The resultant flux induces the EMF in the central winding of the transformer. The outer limbs of the transformer are connected in such a way so that if the incoming machines are in phase with each other than maximum EMF induces in the central limb winding of the transformer. And hence the lamp glows brightly.

If the voltages of the incoming machines are out of phase to each other in that case the resultant flux of the central limb of the transformer becomes zero, and hence the lamp will not glow. If the frequencies of the incoming machines and bus bar are not similar to each other in that case, the lamp becomes flicker.

The frequency of flickering is similar to the difference in frequencies of the two. The synchronisation can be done when the brightness is maximum and flickering becomes less. The electrostatic instrument is used in the system for determining the speed of the incoming machines.

Moving Iron Synchroscope

The moving iron instrument has two fixed parts. The fixed part A is designed for the small value of current, and the resistance is connected in series with the part. The synchroscope consists two cylinders which are mounted on the spindle. The pressure coil energises these cylinders.

The pressure coil is linked with two phases of the incoming machine. The resistance is connected in series with one of the pressure coil and the inductance with another of the coil. The resistance and inductance of the pressure coil create the phase difference of 90º between the pressure coils.

When the frequency of the incoming machines and the bus-bar are matched, then the spindle rotates. The revolution per second determines the differences between the frequencies of the two.

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Methods of Measurement

The different methods of measurement are explained below in details.

Direct Method of Measurement – In this method of measurement, the unknown quantity is directly compared with the standard quantity. The result of the quantity is expressed in number. It is the most common method of measuring the physical quantities like length, temperature, pressure, etc.

Example: The physical balance directly measures the weight of the matter.

Indirect Method of Measurement – The direct measurement gives the inaccurate results in most of the cases. Hence, the direct method is rarely preferred for measurement. In indirect method of measurement, the physical parameters of the quantity are measured by the direct method, and then the numerical value of the quantity is determined by the mathematical relationship.

Example: The length, breadth and height of the substance is measured by the direct method and then by the help of the given relation the weight of the substance is known.

Weight = Length X Breadth X Height X Density

Method of Measurement without Contact – The sensor remains untouched with the object whose characters need to be measured.

Method of Combination measurement closed series – The result of the direct or indirect method of measurement are used for solving the equations.

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At high frequency, stray capacitance is induced between the bridge elements, ground and between the arms of the bridge. This stray element causes the error in the measurement. One of the way, of controlling these capacitances is too enclosed the bridge elements into the shield. Another way of eliminating these stray capacitance is to places the Wagner Earth device between the elements of the bridge.

Construction of Wagner Earthing Device

The circuit diagram of the Wagner Earth Device is shown in the figure below. Consider the Z₁, Z₂, Z₃, and Z₄ are the impedances arm of the bridge. The Z₅ and the Z₆ are the two variable impedances of the Wagner Earth Device. The centre point of the Wagner earth device is earthed. The impedance of Wagner device arms is similar to the arms of the bridge. The impedance of the arm consists the resistance and capacitances.

The Wagner impedance placed in such a way so that they make the bridge balance with Z₁, Z₃ and Z_2, Z₄. The C₁, C₂ C₃ and C₄ show the stray capacitances of the bridges. The D is the detector of the bridge.

The bridge comes in the balance condition by adjusting the impedances of arms Z₁ and Z₄. The stray capacitance prevents bridge to comes in the balanced condition. When the S is not thrown on ‘e’ then the D is connected between the point p and q. But when S is thrown on ‘e’ then the detector D is connected between the terminal b and earth.

The impedance Z₄ and Z₅ are adjusted until the minimum sound is obtained. The headphones again connected between the point b and d for obtaining the minimum sound. The headphones are reconnected between the point b and d, Z₄ and Z₅ are adjusted for obtaining the minimum sound. The process is continuously repeated for obtaining the silent sound.

The point b, d, and e all are in the same potential. And the capacitance C₁, C₂, C₃, C₄ all are eliminated from the bridge circuit along with the impedances Z₅ and Z₆.

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Consider if P is the power of the single phase circuit then the 3P is the power of the three-phase balanced ac circuit. The power of the unbalanced three-phase circuit is known by adding the power of the individual phase.

How to Calculate the Power of the AC circuit?

In dc circuit the value of voltage and current becomes constant. But in AC circuit the instantaneous value of current and voltage flows and hence the power of the supply are continuously varying with the time. So different methods are used for measuring the power of ac and dc circuit.

The instantaneous power of the ac circuit changes continuously when their voltage and current changes. The instantaneous power is the power of the circuit at the specific instant of time. Let p be the power at any instant, v is the voltage and ‘i’ be the current of the circuit.

If the sinusoidal current and voltage flow through the ac circuit and the current is lagging behind the voltage by an angle of Φ, in that case, the value of the instantaneous power is given by the expression

If the voltage and current are out of phase to each other, then the value of current and voltage becomes,

The instantaneous power of the circuit is expressed as

Let, Θ = ωt

The averages power of the circuit is 

The term cosΦ in the given expression shows that the wattmeter is required for measuring the power of the AC circuit.

How Power of the AC circuit depends on Power Factor?

The power factor determines the total useful power flows in the circuit. For understanding, the role of power factor in the ac circuit consider the three conditions.

AC Power in a Purely Resistive Circuit

The resistor is an electrical component which consumed the electrical power of the ac circuit. 

In a purely resistive circuit, the current flows through the resistor is in phase with the supply voltage, i.e., the waves form of the voltage and current is in phase with each other. The zero-degree phase difference occurs between the waveform of voltage and current.

AC power in a Purely Inductive Circuit

In the purely inductive circuit, the value of voltage and current are not in phase with each other. The voltage and current rise and fall together at the phase shift of 90º. The expression gives the power of the purely inductive circuit.The above equation shows that the inductor neither consumes nor dissipates any electrical power.

AC Power in a Purely Capacitive Circuit

In the purely capacitive ac circuit, the waveform of voltage and current are out of phase to each other. The current of the circuit is leading with their voltage by an angle of 90º. In spite of dissipating the power the capacitor stores the electrical energy.

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It is not possible to measures the temperature of the highly heated body with the help of the contact type instrument. Hence the non-contact pyrometer is used for measuring their temperature.

Construction of Optical Pyrometer

The construction of the optical pyrometer is quite simple. The pyrometer is cylindrical inside which the lens is placed on one end and the eyepiece on the other end. The lamp is kept between the eyepiece and the lens. The filter is placed in front of the eyepiece. The filter helps in getting the monochromatic light. The lamp has the filament which is connected to the battery, ammeter and the rheostat.

Working of Optical Pyrometer

The optical pyrometer is shown in the figure below. It consists the lens which focuses the radiated energy from the heated object and targets it on the electric filament lamp. The intensity of the filament depends on the current passes through it. Hence the adjustable current is passed through the lamp.

The magnitude of the current is adjusted until the brightness of the filament is similar to the brightness of the object. When the brightness of the filament and the brightness of the object are same, then the outline of the filament is completely disappeared.

The filament looks bright when their temperature is more than the temperature of the source.

The filament looks dark if their temperature is less than that required for  equal brightness

Advantages of Optical Pyrometer

• The optical pyrometer has high accuracy.
• The temperature is measured without contacting the heated body. Because of this property, the pyrometer is used for the number of applications.

Disadvantages of Optical Pyrometer

The working of the pyrometer depends on the intensity of light emitted by the heated body. Thereby, the pyrometer is used for measuring the temperature having a temperature more than 700-degree Celsius. The accuracy of the pyrometer depends on the adjustment of the filament current. Also, the pyrometer is not used for measuring the temperature of clean gases.

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The tachometer consists the opaque disc which is mounted on the shaft whose speed needs to be measured. The disc consists the equivalent holes around the periphery. The light source is placed on one side of the disc and the light sensor on the other side. They are in line with each other.

When the disc rotates their holes, and the opaque portion comes alternatively between the light source and light sensor. When the holes come in the line of the light source and the light sensor, then the light passes through the holes and collapse to the sensor. Hence the pulse is generated. These pulses are measured through the electric counter.

When the opaque portion comes in the line of light source and sensor, then the disc blocked the light source, and the output becomes zero. The production of pulses depends on the following factor.

1. The number of holes on the disc.
2. The speed of rotation of the disc.

The holes are fixed, and hence the pulse generation depends on the speed of the rotation of the disc. The electronic counter is used for measuring the pulse rate.

Advantages of Photoelectric Tachometer

1. The digital output voltage is obtained, and hence there is no need of analogue to digital conversion.
2. The pulses of constant amplitude are obtained which simplify the electronic circuitry.

Disadvantages of Photoelectric Tachometer

1. The life of the light source is approximately 50,000 hours. Hence the light source needs to be replaced timely.
2. The accuracy of this method depends on the error which is represented by the unit pulse. These errors can be minimized by using the gating period. The gating period means the meter measures the frequency by counting the input pulses.

The total number of pulses generated at one revolution is also used for minimizing the error.

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The maximum demand indicators are classified into four types.

1. Recording demand indicator
2. Average demand indicator
3. Thermal type maximum demand indicator
4. Digital Maximum Demand Indicator

Construction of Maximum Demand Indicator

The maximum demand indicator has five main parts.

• Dial Connected to the moving system
• Pointer
• Reset Device
• Fraction device
• Indicating Pin

Average Demand Indicator

The average demand indicator is inbuilt into the energy meter. The energy meter and average demand indicator together measures the total power consumes and the maximum value of specific power at particular interval of time. The average demand indicator consists the complex speed dial mechanism.

The pin drive moves the dial forward for small duration (say for half an hour). The total power consumes at that interval is shown on the dial. The instrument consists the cam which is controlled by the timing gears. The cam brings back the pointer at zero positions.

The pointer records the total power consumes by the load at that particular interval of time. For the next half an hour, the pin again moved forward. But the pointer will move forward only when the total power consumed by the load is more than the previous periods.

The formula calculates the average maximum demand,

The maximum demand meter can measure the power regarding kVarh or kVah. This can be done by adding the suitable meter which will calculate such quantities.

Advantages of Average Demand Indicator

• The average demand indicator has high accuracy.
• The instrument has uniform measuring scale.

Disadvantages of Maximum Demand Indicator

• The cost of the instrument is very high.
• Their construction is very complicated.

Nowadays, the cam is replaced by the electromagnetic relay and clutch the replaces the bell crank releasing device.

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The hot wire when placed in the stream of the fluid, in that case, the heat is transferred from wire to fluid, and hence the temperature of wire reduces. The resistance of wire measures the flow rate of the fluid.

The hot wire anemometer is used as a research tool in fluid mechanics. It works on the principle of transfer of heat from high temperature to low temperature.

Construction of Hot Wire Anemometer

The hot wire anemometer consists two main parts.

1. Conducting wire
2. Wheat stone bridge.

The conducting wire is housed inside the ceramic body. The wires are taking out from the ceramic body and connecting to the Wheatstone bridge. The wheat stone bridge measures the variation of resistance.

Constant Current Method

In the constant current method, the anemometer is placed in the stream of the fluid whose flow rate needs to be measured. The current of constant magnitude is passed through the wire. The Wheatstone bridge is also kept on the constant voltage.

When the wire is kept in the stream of liquid, in that case, the heat is transferred from the wire to the fluid. The heat is directly proportional to the resistance of the wire. If heat reduces, that means the resistance of wire also reduces. The Wheatstone bridge measures the variation in resistance which is equal to the flow rate of the liquid.

Constant Temperature Method

In this arrangement, the wire is heated by the electric current. The hot wire when placed in the fluid stream, the heat transfer from wire to the fluid. Thus, the temperature of the wire changes which also changes their resistance. It works on the principle that the temperature of the wire remains constant. The total current requires to bring the wire in the initial condition is equal to the flow rate of the gas.

Measurement of the rate of a fluid using a Hot Wire Instrument

In hot wire anemometer, the heat transferred electrically to the wire which is placed in the fluid stream. The Wheatstone bridge is used for measuring the temperature of wire regarding their resistance. The temperature of the wire remains constant for measuring the heating current. Thus, the bridge remains balanced.

The standard resistor is connected in series with the heating wire. The current across the wire is determined by knowing the voltage drop across the resistor. And the value of voltage drop is determined by the potentiometer.

The equation determines the heat loss from the heated wire

Where, v – velocity of heat flow,
ρ – the density of fluid,

The a and b are the constants. Their value depends on the dimension and the physical properties of the fluid and wire.

Suppose I, is the current of the wire and the R is their resistance. In equilibrium condition,

Heat generated = Heat Lost

The resistance and temperature of the instrument are kept constant for measuring the rate of the fluid by measuring the current I.

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The mass that connected to the damper element and spring without any other support is known as spring mass damper element. And the displacement transducer converts the displacement into the electrical quantity. The seismic transducer is used for measuring the earth vibration, volcanic eruption and other vibrations etc.

Construction of Seismic Transducer

The systematic diagram of the seismic transducer is shown in the figure below. The mass is connected by the help of the damper and spring to the housing. The housing frame is connected to the source whose vibrations need to be measured.

The arrangement is kept in such a way so that the position of the mass remains same in the space. Such type of arrangement is kept for causing the relative motion between the housing frame and the mass. The term relative motion means one of the objects remains stationary, and the other is in motion concerning the first one. The displacement that occurs between the two is sensed and represented by the transducer.

Mode of Transducer

The seismic transducer works in two different modes.

• Displacement Mode
• Acceleration Mode

The selection of the mode depends on the combinations of the mass, spring and damper combinations. The large mass and soft spring are used for the displacement mode measurement while the combination of the small mass and stiff spring is used for the acceleration mode.

Types of Seismic Transducer

Vibrometer and the accelerometer are the two type of the seismic transducer.

1. Vibrometer – The vibrometer or low-frequency meter is used for measuring the displacement of the body. It also measures the high frequency of the vibrating body. Their frequency range depends on the natural frequency and the damping system.

2. Accelerometer – The accelerometer measures the acceleration of the measuring body. The acceleration shows the total force acting on the object.

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