Transducers and Analyzers: Synchros, Gauges, and More

Posted on Mar 19, 2025 in Electromechanical Engineering

Synchros

A synchro is an electromagnetic transducer commonly used to convert the angular position of a shaft into an electrical signal. Synchros are also known as Telsyn, Autosyn, and Selsyn. A classical synchro system consists of two units: (a) a synchro transmitter and (b) a synchro receiver. The basic synchro unit is called a “Synchro Transmitter.” Its construction is similar to that of a three-phase alternator. The stator is the stationary part, slotted and made of laminated steel to reduce eddy current losses. The axes of the stator windings are kept 120° apart, and the coils are connected in a star shape. The rotor has a salient pole construction, is dumbbell-shaped, and wound with a concentric coil. An AC voltage is applied to the rotor through two slip rings. The constructional feature of a synchro transmitter is shown below:

Photoelectric Transducers

The photoelectric transducer converts light energy into electrical energy. It is made of semiconductor material and uses a photosensitive element that ejects electrons when a light beam is absorbed. The discharge of electrons varies the property of the photosensitive element, inducing a current in the device. The magnitude of the current is equal to the total light absorbed by the photosensitive element. Photoelectric transducers are classified as:

• Photoemissive Cell
• Photoconductive Cell
• Photovoltaic Cell

Photoemissive Cell

The photoemissive cell converts photons into electric energy. It consists of an anode rod and a cathode plate, both coated with a photoemissive material called caesium antimony. When light radiation falls on the cathode plates, electrons start flowing from anode to cathode. Both the anode and the cathode are sealed in a closed, opaque evacuated tube. When light radiation falls on the sealed tube, electrons emit from the cathode and move towards the anode, which is kept at a positive potential. Thus, a photoelectric current starts flowing through the anode. The magnitude of the current is directly proportional to the intensity of light passing through it.

Photoconductive Cell

The photoconductive cell converts light energy into an electric current. It uses semiconductor materials like cadmium selenide, Ge, Se as a photo-sensing element. When a light beam falls on the semiconductor material, its conductivity increases, and the material works like a closed switch. The current starts flowing into the material and deflects the pointer of the meter.

Photovoltaic Cell

The photovoltaic cell is a type of active transducer. Current starts flowing into the photovoltaic cell when a load is connected to it. Silicon and selenium are used as semiconductor materials. When the semiconductor material absorbs heat, the free electrons of the material start moving. This phenomenon is known as the photovoltaic effect. The movement of electrons develops the current in the cell, and the current is known as the photoelectric current.

Piezoelectric Transducers

A piezoelectric crystal generates an electric charge in response to mechanical stress, such as pressure or vibration. When subjected to mechanical deformation, it produces a small electric current.

Working Principle

The piezoelectric crystal works on the principle of the piezoelectric effect. When mechanical stress or forces are applied to some materials, the piezoelectric crystal gets compressed, producing a charge and leading to voltage development. This electric voltage can be measured easily by voltage measuring instruments, which can be used to measure the stress or force. Various materials exhibit the piezoelectric effect. Materials used for measurement purposes should possess desirable properties like stability, high output, and insensitivity to extreme temperature and humidity. Examples of piezoelectric materials are: Barium Titanate, Lead Zirconate Titanate (PZT), Quartz, Rochelle salt, etc.

Pirani Vacuum Gauge

The Pirani Gauge is a type of Thermal Conductivity Gauge. It consists of a metal filament (usually platinum) suspended in a tube connected to the system whose vacuum is to be measured. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken. A conducting wire (platinum filament) gets heated when electric current flows through it. This wire suspended in a gas will lose heat to the gas as its molecules collide with the wire and remove heat. As the gas pressure is reduced (by the vacuum pumps), the number of molecules present will fall proportionately, the conductivity of the surrounding media will fall, and the wire will lose heat more slowly. Measuring the heat loss is an indirect indication of pressure. The electrical resistance of the wire varies with its temperature, so the measurement of resistance also indicates the temperature of the wire. The change in resistance of the filament is determined using a bridge. This change in resistance of the pirani gauge filament becomes a measure of the applied pressure when calibrated.

Ionization Gauge

For very low pressure, or high vacuum, measurement, some form of ionization gauge is used. The ionization gauge operates by using a stream of electrons to ionize a sample of the remaining gas in the space in which the pressure is being measured. The positive gas ions are then attracted to a negatively charged electrode, and the amount of current carried by these ions is measured. Since the number of ions per unit volume depends on the number of atoms per unit volume, and this figure depends on pressure, the reading of ion current should be reasonably proportional to gas pressure. The proportionality is fairly constant for a fixed geometry of the gauge and for a constant level of electron emission. The range of the gauge is to about 10^7 mm (0.013 Pa), which is about the pressure used in pumping transmitting radio valves and specialized cathode ray tubes. The most serious problem in using an ionization gauge is that it requires electron emission into a space that is not a perfect vacuum. The type of electron emitter that is used in the hot-cathode or Bayard-Alpert gauge is invariably a tungsten filament.

Bourdon Tube Pressure Gauges

It can be defined as an instrument for measuring the pressure of gases or liquids, consisting of a semicircular or coiled, flexible metal tube attached to a gauge that records the degree to which the tube is straightened by the pressure of the gas or liquid inside. The Bourdon pressure gauge operates on the principle that, when pressurized, a flattened tube tends to straighten or regain its circular form in cross-section. The Bourdon tube comes in C, helical, and spiral shapes—although most gauges employ the C shape, which is the type of Bourdon pictured here.

Moving Magnet Type Velocity Transducer

Basic principle: When a permanent magnet moves inside a coil, the change in the length of the air gap varies the reluctance. Hence, the output voltage is directly proportional to the rate of change of the length of the air gap (change in length produced by velocity). Thus, the output voltage becomes a measure of the velocity when calibrated.

Description: The sensing element, which is a rod, is a permanent magnet. The rod is rigidly coupled to the device whose velocity is being measured. There is a coil surrounding the permanent magnet. The permanent magnet is movable, that is, it can move in and out of the coil.

Operation

1. The instrument is fixed to the device whose velocity is to be measured.
2. Due to the application of the velocity, the permanent magnet moves in or out of the coil. Due to its motion, the length of the air gap varies.
3. The output voltage also varies due to the motion of the magnet, and the amplitude of the voltage is directly proportional to velocity.
4. The polarity of the output voltages determines the direction of the velocity.

Moving Coil Type Velocity Transducer

A PMMC meter places a coil of wire (i.e., a conductor) between two permanent magnets to create a stationary magnetic field. The coil is the current-carrying part of the instruments, which is freely moved between the stationary field of the permanent magnet. The current passes through the coil deflects it, due to which the magnitude of the current or voltage is determined. The coil is mounted on the rectangular former, which is made up of aluminum.

The PMMC instrument uses the permanent magnet for creating the stationary magnets. The Alcomax and Alnico material are used for creating the permanent magnet because this magnet has the high coercive force (The coercive force changes the magnetization property of the magnet). In PMMC instrument, the controlling torque is because of the springs. The springs are made up of phosphorous bronze and placed between the two jewel bearings.

According to Faraday’s Laws of electromagnetic induction, a current-carrying conductor placed in a magnetic field will experience a force in the direction determined by Fleming’s left-hand rule. The magnitude (strength) of this force will be proportional to the amount of current through the wire. A pointer is attached to the end of the wire, and it is put along a scale. When the torques are balanced, the moving coil will stop, and its angular deflection can be measured by the scale. If the permanent magnet field is uniform and the spring linear, then the pointer deflection is also linear. Hence, we can use this linear relationship to determine the amount of electrical current passing through the wire.

Tachometer

The tachometer is used for measuring the rotational speed or angular velocity of the machine which is coupled to it. The electrical tachometer converts the angular velocity into an electrical voltage. The electrical tachometer has more advantages over the mechanical tachometer. Thus, it is mostly used for measuring the rotational speed of the shaft. Depending on the natures of the induced voltage, the electrical tachometer is categorized into two types:

• AC Tachometer Generator
• DC Tachometer Generator

DC Tachometer Generator

Permanent magnet, armature, commutator, brushes, variable resistor, and the moving coil voltmeter are the main parts of the DC tachometer generator. The machine whose speed is to be measured is coupled with the shaft of the DC tachometer generator. The DC tachometer works on the principle that when the closed conductor moves in the magnetic field, EMF induces in the conductor. The magnitude of the induced EMF depends on the flux link with the conductor and the speed of the shaft. The armature of the DC generator revolves between the constant field of the permanent magnet. The rotation induces the EMF in the coil. The magnitude of the induced EMF is proportional to the shaft speed. The commutator converts the alternating current of the armature coil to the direct current with the help of the brushes. The moving coil voltmeter measures the induced EMF. The polarity of the induced voltage determines the direction of motion of the shaft. The resistance is connected in series with the voltmeter for controlling the heavy current of the armature. The EMF induces in the DC tachometer generator is given as:

Where:

• E – generated voltage
• Φ – flux per poles in Weber
• P – number of poles
• N – speed in revolution per minutes
• Z – the number of the conductor in armature windings
• a – number of the parallel path in the armature windings

AC Tachometer Generator

The DC tachometer generator uses the commutator and brushes which have many disadvantages. The AC tachometer generator designs for reducing the problems. The AC tachometer has stationary armature and rotating magnetic field. Thus, the commutator and brushes are absent in AC tachometer generator. The rotating magnetic field induces the EMF in the stationary coil of the stator. The amplitude and frequency of the induced EMF are equivalent to the speed of the shaft. Thus, either amplitude or frequency is used for measuring the angular velocity. The below mention circuit is used for measuring the speed of the rotor by considering the amplitude of the induced voltage. The induces voltages are rectified and then passes to the capacitor filter for smoothening the ripples of rectified voltages. It works on the principle of relative motion between the magnetic field and shaft of the coupled device. The relative motion induces the EMF in the coil which is placed between the constant magnetic field of the permanent magnet. The developed EMF is directly proportional to the speed of the shaft.

Drag-cup tachometer/Eddy current tachometer

It is a contact type of electrical tachometer, i.e., physical contact is made with the shaft, whose speed is to be measured. In this type of tachometer, the transducer produces an analog signal in the form of continuous drag due to eddy current, induced in the cup. The current, which is indicated by an analog indicator on a scale, is proportional to the speed. The machine shaft, whose speed is to be measured, is engaged with the tachometer shaft. This tachometer shaft is connected to a permanent magnet. The magnet is surrounded by a cup made up of nonmagnetic, but conductive material, such as aluminum. The other end of non-magnetic aluminum cup is connected to the pointer through a helical spring.

Working of Eddy Current Tachometer

Due to rotation of machine shaft, the connected tachometer shaft and hence, the magnet starts rotating. This rotation of magnet induces eddy currents and hence, EMF in the non-magnetic aluminum cup. The induced eddy currents produce a twisting moment or torque in the cup, which acts against the torque of helical spring. Due to the induced torque, the cup turns in the direction of rotating magnet, till the torque induced (developed) becomes equal to torque on helical spring. The pointer attached to the cup moves over a calibrated scale (dial), indicating the angular speed of shaft.

Photoelectric Tachometer

The tachometer which uses the light for measuring the speed of rotation of shaft or disc of machines is known as the photoelectric tachometer. The opaque disc with holes on its periphery, light source and laser are the essential parts of the photoelectric tachometer. 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.

Capacitive tachometer

A capacitive tachometer is an instrument used to measure the rotational speed (RPM) of an object, typically a rotating shaft or motor. It operates based on the principle of capacitance change due to the motion of the rotating object. The device consists of a vane attached to one end of the rotating machine shaft. When the shaft rotates between the fixed capacitive plates, there occurs a change in the capacitance. As a result of this, voltage is developed. The pulses thus produced are amplified and may then be fed to frequency measuring unit or to a digital counter so as to provide a digital analog of the shaft rotation.

Stroboscope tachometer

The instrument operates on the principle that if a repeating event is only viewed when at one particular point in it’s cycle it appears to be stationary. A mark is made on rotating shaft, and a flashing light is subjected on the shaft. The frequency of the flashing is one very short flash per revolution.

• To determine the shaft speed we increases the frequency of flashing gradually from small value until the rotating shaft appears to be stationary, then note the frequency. This frequency is proportional to the rotational speed.

Accelerometer

An accelerometer is an electromechanical device used to measure acceleration forces. Such forces may be static, like the continuous force of gravity or, as is the case with many mobile devices, dynamic to sense movement or vibrations. Acceleration is the measurement of the change in velocity, or speed divided by time.

Potentiometric accelerometer

Potentiometric accelerometer is a type of an accelerometer which bases its working principles on the spring mass system. The potentiometric accelerometer employs a mass (seismic mass), a spring, a dashpot, and a resistive element. The seismic mass is connected between a spring and a dashpot. The wiper of the potentiometer is connected to the mass. The way it works is simple. It measures the motion of the seismic mass by attaching the wiper arm to the spring mass system. When the mass is moving, the position of the wiper changes according, thus changing the resistance of the resistive element. Since the natural frequency fN of the potentiometer accelerometer is generally less then 30Hz, this type of accelerometer should be used in low frequency vibration measurements.

Piezo-electric accelerometer

In a piezo-electric accelerometer a mass is attached to a piezo- electric crystal which is in turn mounted to the case of the accelerometer. When the body of the accelerometer is subjected to vibration the mass mounted on the crystal wants to stay still in space due to inertia and so compresses and stretches the piezo electric crystal. This force causes a charge to be generated and due to Newton law F=ma this force is in turn proportional to acceleration. The charge output is either is converted to a low impedance voltage output by the use of integral electronics or made available as a charge output (Picocoulombs /g) in a charge output piezo-electric accelerometer.

Benefits of Piezoelectric Accelerometer:

• Wide frequency range
• No moving parts
• Excellent linearity over their dynamic range
• Low output noise
• Self-generating – no external power required
• Acceleration signal can be integrated to provide velocity and displacement

Seismic accelerometer

There are two types of seismic – displacement sensing accelerometers namely:

1. Linear seismic accelerometer.
2. Rotational Seismic Accelerometer.

The main parts of a seismic accelerometer are as follows:

1. A seismic mass is suspended from the housing of the accelerometer through a spring.
2. A damper is connected between the seismic mass and the housing of the accelerometer.
3. The seismic mass is connected to an electric displacement transducer.

It’s a common spring mass-damper system which accomplishes the task of acceleration measurement through displacement. The working principle of this is based on mass-spring-damper combination and similarly acceleration calculation from displacement. The mass is supported by a spring and connected to the housing frame which is attached to a machine. Relative mass movement is sensed and indicated by an electrical displacement transducer. Correspondingly displacement and velocity are measured and further acceleration are measured.

Temperature Resistance thermometers

Resistance thermometers are usually used to measure temperatures between -200 and 500°C. Resistance thermometers work by changing resistance with a change in temperature in a repeatable manner. The resistance thermometer or resistance temperature detector (RTD) uses the resistance of electrical conductor for measuring the temperature. The resistance of the conductor varies with the time. This property of the conductor is used for measuring the temperature. The main function of the RTD is to give a positive change in resistance with temperature.

Operation of Resistance Thermometer

The tip of the resistance thermometer is placed near the measurand heat source. The heat is uniformly distributed across the resistive element. The changes in the resistance vary the temperature of the element. The final resistance is measured. The below mention equations measure the variation in temperature.

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.

Thermocouple Working Principle

The thermocouple principle mainly depends on the three effects namely Seebeck, Peltier and Thompson.

Seebeck effect

This type of effect occurs among two dissimilar metals. When the heat offers to any one of the metal wire, then the flow of electrons supplies from hot metal wire to cold metal wire. Therefore, direct current stimulates in 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 which demonstrates the change in rate and direction of temperature at an exact position.

Thermistor

A thermistor is a resistance thermometer, or a resistor whose resistance is dependent on temperature. The term is a combination of “thermal” and “resistor”. It is made of metallic oxides, pressed into a bead, disk, or cylindrical shape and then encapsulated with an impermeable material such as epoxy or glass. There are two types of thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). With an NTC thermistor, when the temperature increases, resistance decreases. Conversely, when temperature decreases, resistance increases. This type of thermistor is used the most.

A PTC thermistor works a little differently. When temperature increases, the resistance increases, and when temperature decreases, resistance decreases. This type of thermistor is generally used as a fuse. Typically, a thermistor achieves high precision within a limited temperature range of about 50°C around the target temperature. This range is dependent on the base resistance.

Semiconductor temperature sensors

Semiconductor temperature sensors are the devices which come in the form of integrated circuits i.e. … These are the electronic devices manufactured in an identical fashion to present-day electronic semiconductor devices like microprocessors. More than thousands of devices can be fabricated upon thin silicon wafers.

Pyrometer

A pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry. This device can be used to determine the temperature of an object’s surface.

Optical Pyrometer

In an optical pyrometer, a brightness comparison is made to measure the temperature. An optical pyrometer has the following components:

• An eye piece at the left side and an optical lens on the right.
• A reference lamp, which is powered with the help of a battery.
• A rheostat to change the current and hence the brightness intensity.
• An absorption screen is fitted between the optical lens and the reference bulb to increase the temperature range which is to be measured.
• A red filter placed between the eye piece and the reference bulb helps in narrowing the band of wavelength.

Working

The radiation from the source is emitted and the optical objective lens captures it. The lens helps in focusing the thermal radiation on to the reference bulb. The observer watches the process through the eye piece and corrects it in such a manner that the reference lamp filament has a sharp focus and the filament is super-imposed on the temperature source image. The observer starts changing the rheostat values and the current in the reference lamp changes. This in turn, changes its intensity. This change in current can be observed in three different ways:

1. The filament is dark. That is, cooler than the temperature source.
2. Filament is bright. That is, hotter than the temperature source.
3. Filament disappears. Thus, there is equal brightness between the filament and temperature source.

At this time, the current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated light in the temperature source, when calibrated.

Radiation Pyrometer

An Optical Pyrometer can be not only be used for temperature measurement, but also can be used to see the heat that is measured. The observer is actually able to calculate the infrared wavelength of the heat produced and also see the heat patterns by the object. But the amount of heat that the device can sense is limited to 0.65 microns. This is why the radiation pyrometer is more useful, as it can be used to measure all temperatures of wavelengths between 0.70 microns and 20 microns. The wavelengths measured by the device are known to be pure radiation wavelengths, that is, the common range for radioactive heat. This device is used in places where physical contact temperature sensors like Thermocouple, RTD, and Thermistors would fail because of the high temperature of the source. The main theory behind a radiation pyrometer is that the temperature is measured through the naturally emitted heat radiation by the body. This heat is known to be a function of its temperature. According to the application of the device, the way in which the heat is measured can be summarized into two:

Total Radiation Pyrometer

In this method, the total heat emitted from the hot source is measured at all wavelengths.

Selective Radiation Pyrometer

In this method, the heat radiated from the hot source is measured at a given wavelength.

The radiation pyrometer has an optical system, including a lens, a mirror and an adjustable eye piece. The heat energy emitted from the hot body is passed on to the optical lens, which collects it and is focused on to the detector with the help of the mirror and eye piece arrangement. The detector may either be a thermistor or photomultiplier tubes. Though the latter is known for faster detection of fast moving objects, the former may be used for small scale applications. Thus, the heat energy is converted to its corresponding electrical signal by the detector and is sent to the output temperature display device.

Electromagnetic Flow Meters

Electromagnetic Flow Meters are based on Faraday’s law of induction. These meters are also called as Magflow or Electromagnetic Flow Meters. A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is electromagnetic induction and mathematically defined as E=k*B*D*V. where, E=Induced Voltage (Linear with velocity), k=Proportionality Constant, B=Magnetic Field Strength (Coil Inductance), D=Distance between electrodes, V=Velocity of process fluids. The induced voltage (E) is directly proportional to the velocity (V) of the fluid moving through the magnetic field (B). The induced voltage is carried to the transmitter through the electrode circuit. The transmitter then converts this voltage into a quantifiable flow velocity. The volumetric flow rate of the fluid is calculated using this known velocity along with the area of the pipe.

Turbo magnetic flow meter

The Turbine flow meter (axial turbine) was invented by Reinhard Woltman and is an accurate and reliable flow meter for liquids and gases. It consists of a flow tube with end connections and a magnetic multi bladed free spinning rotor (impeller) mounted inside; in line with the flow. The rotor is supported by a shaft that rests on internally mounted supports. The Supports in Process Automatics Turbine Flow Meters are designed to also act as flow straighteners, stabilizing the flow and minimizing negative effects of turbulence. The Supports also house the unique open bearings; allowing for the measured media to lubricate the bushes – prolonging the flow meters life span. The Supports are fastened by locking rings (circlips) on each end. The rotor sits on a shaft ,which in turn is suspended in the flow by the two supports. As the media flows, a force is applied on the rotor wings. The angle and shape of the wings transform the horizontal force to a perpendicular force, creating rotation. Therefore, the rotation of the rotor is proportional to the applied force of the flow. Because of this, the rotor will immediately rotate as soon as the media induces a forward force. As the rotor cannot turn thru the media on its own, it will stop as soon as the media stops. This ensures an extremely fast response time, making the Turbine Flow Meter ideal for batching applications. A pick-up sensor is mounted above the rotor. When the magnetic blades pass by the pickup sensor, a signal is generated for each passing blade. This provides a pulsed signal proportional to the speed of the rotor and represents pulses per volumetric unit.; and as such the flow rate too.

The Hot Wire Anemometer

It is a device used for measuring the velocity and direction of the fluid. This can be done by measuring the heat loss of the wire which is placed in the fluid stream. The wire is heated by electrical current. 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. Wheatstone 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.

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.

Ultrasonic flow meter

Ultrasonic flow meter is used to measure the flow rate of fluid (gas, liquid etc). Ultrasonic flow meters are used in bigger diameter pipes. An ultrasonic flow meter utilizes ultrasound to measure the velocity of a fluid and is used in a variety of fluid applications. A typical transit-time ultrasonic liquid flow meter utilizes two ultrasonic transducers that function as both ultrasonic transmitter and receiver. The ultrasonic flow meter operates by alternately transmitting and receiving a burst of ultrasound between the two transducers by measuring the transit time that it takes for sound to travel between the two transducers in both directions.

Hygrometer

Hygrometer uses for measuring the humidity present in the surrounding environment. The term humidity means the amount of water vapour present in the gas. The physical properties of the material changes by the effect of the humidity and this principle use in hygrometer for measurement. The humidity is classified into two types:

• Absolute Humidity
• Relative Humidity

The absolute humidity shows the amount of water vapour presents per unit volume. And the relative humidity is the ratio of the actual water vapour pressure to the maximum water vapour pressure reaches in the substance at the particular temperature. The relative humidity depends on the temperature.

Classifications of Hygrometer

The following are the classification of hygrometer by the material used for measuring the humidity.

Resistive Hygrometer

The conducting film of the resistive hygrometer is made by the lithium chloride and the carbon. The conducting film places between the metal electrodes. The resistance of the conducting film varies with the change in the value of humidity present in the surrounding air. The moisture absorbs by the lithium chloride will depend on the relative humidity. If the relative humidity is high, the lithium chloride will absorb more moisture and their resistance decreases. The change in the value of resistance is measured by applying the alternating current to the bridge. The direct current is not used in the bridge as they breakdowns the layer of lithium chloride. The obstructions occur in the flows of current shows the value of resistance or the value of relative humidity.

Capacitive Hygrometer

The change in capacitance of the capacitor shows the surrounding humidity. The capacitive hygrometer gives the very accurate result. It is made by placing the hygroscopic material between the metal electrodes. The hygroscopic material can quickly absorb the water. The material absorbs water because of which the capacitance of the capacitor decreases. The electronic circuit measures the change in capacitance.

Microwave Refractometer

The microwave refractometer measures the refractive index of the moist air when their humidity is changed. A Microwave Refractometer measures the refractive index of a substance using microwaves. It typically consists of a microwave source, a waveguide, and a sensing element. Microwaves are sent through the substance, and the refractive index affects the speed of the microwaves. As the microwaves pass through the substance, changes in their speed occur due to the refractive index variations. The sensing element detects these changes and converts them into a measurable parameter, often the refractive index. This information can be used to determine the composition or concentration of the substance being analyzed. Microwave refractometers are commonly employed in applications where traditional optical refractometers might not be suitable, such as in opaque or highly absorbing materials. They find applications in industries like food processing, pharmaceuticals, and chemical analysis.

Crystal Hygrometer

The working principle of a crystal hygrometer is based on the hygroscopic nature of certain salts, typically lithium chloride. Here’s a step-by-step explanation:

Hygroscopic Salt

The hygrometer contains a salt crystal, often lithium chloride, known for its ability to absorb moisture from the air.

Absorption of Moisture

When the air around the hygrometer contains moisture (humidity), the salt crystal absorbs water vapor. This absorption is a reversible process influenced by the surrounding humidity levels.

Physical Changes

As the salt crystal absorbs moisture, it undergoes physical changes. This can include an increase in volume, weight, or a change in color. The extent of these changes correlates with the amount of moisture absorbed.

Measurement

The hygrometer is designed to measure the observable changes in the salt crystal. This measurement is then converted into a relative humidity reading. For example, a darker color or increased weight might indicate higher humidity.

Wave Analyzer

The electronic instrument used to analyze waves is called a wave analyzer. It is also called a signal analyzer, since the terms signal and wave can be interchangeably used frequently.

We can represent the periodic signal as the sum of the following two terms:

• DC component
• Series of sinusoidal harmonics

So, the analysis of a periodic signal is the analysis of the harmonics components presents in it.

Heterodyne Wave Analyzer

• • Heterodyne wave analyzers are used to analyze signal in the RF range and above (MHz range).
  • Attenuator is used to modify the amplitude of the input signal .
  • In this analyzer, the input signal is mixed with the internal signal to produce a higher IF frequency.
  • The local oscillator is tunable to get all the frequency components of the input signal.
  • The first mixer stage produces an output of 30Mhz which is a difference between the input and oscillator signal.
  • This 30MHz signal will be amplified by IF amplifier and fed to the second mixer.
  • The second mixer will produce a 0 Hz signal which is the difference between IF and crystal oscillator signal
  • This signal will then be filtered by the active filter of a bandwidth less than 1500Hz
  • The amplitude of the selected frequency component can be read from the output meter in Volt or dB.
  • This wave analyzer is operated in the RF range of 10kHz – 18MHz

SPECTRUM ANALYSER:

• • Oscilloscope is used to display and measure signal in a time domain.
  • The instrument providing this frequency domain view is the spectrum analyzer

• • A spectrum analyzer display signal on its CRT with frequency on the horizontal axis and amplitude (voltage) on the vertical axis.
  • Spectrum analyzers use either a parallel filter bank or a swept frequency technique
  • In a parallel filter bank analyzer, the frequency range is covered by a series of filters whose central frequencies and bandwidth are so selected that they overlap each other
  • For the RF or microwave signals, the swept technique is preferred

The sawtooth generator provides the sawtooth voltage which drives the horizontal movement of the scope and the frequency controlled element of the voltage tuned oscillator. The voltage tuned oscillator will sweep from fmin to fmax of its frequency band at a linear recurring rate. The frequency component and voltage tuned oscillator frequency beats together to produce a difference frequency, i.e. IF (intermediate frequency) . This IF will be amplified and displayed on the CRT screen of the spectrum analyser

DISTORTION ANALYSER:

Function of distortion analyzer : measures the total harmonic power in the test wave rather than the distortion caused by each component.

• Simplest method is to suppress the fundamental frequency of the signal with a notch filter , leaving only harmonics plus noise.

Consists of three main Parts: Input section with Impedance matcher, Notch filter and amplifier section, An output metering circuit.

• • The input is impedence -matched with the help of an attenuator and an impedance matcher.
  • This signal is then preamplified to a desired level and applied to a Wien bridge notch filter, tuned to reject the fundamental frequency and balanced for minimum output by adjusting the bridge controls.
  • A feedback loop from the bridge amplifier output to the pre-amp input helps to eliminate any remaining contribution from the fundamental frequency
  • The remaining signal after the fundamental has been suppressed, is amplified to a measurable level.

Linear Variable Differential Transformer LVDT

Definition of LVDT

The term LVDT stands for the Linear Variable Differential Transformer. It is the most widely used inductive transducer that converts the linear motion into the electrical signal.

The output across secondary of this transformer is the differential thus it is called so. It is very accurate inductive transducer as compared to other inductive transducers.

Construction of LVDT

Main Features of Construction

• The transformer consists of a primary winding P and two secondary windings S1 and S2 wound on a cylindrical former (which is hollow in nature and contains the core).
• Both the secondary windings have an equal number of turns, and we place them on either side of primary winding
• The primary winding is connected to an AC source which produces a flux in the air gap and voltages are induced in secondary windings.
• A movable soft iron core is placed inside the former and displacement to be measured is connected to the iron core.
• The iron core is generally of high permeability which helps in reducing harmonics and high sensitivity of LVDT.
• The LVDT is placed inside a stainless steel housing because it will provide electrostatic and electromagnetic shielding.
• The both the secondary windings are connected in such a way that resulted output is the difference between the voltages of two windings.

Principle of Operation and Working

As the primary is connected to an AC source so alternating current and voltages are produced in the secondary of the LVDT. The output in secondary

S1 is e1 and in the secondary S2 is e2. So the differential output is,

This equation explains the principle of Operation of LVDT.

Now three cases arise according to the locations of core which explains the working of LVDT are discussed below as,

• CASE I When the core is at null position (for no displacement) When the core is at null position then the flux linking with both the secondary windings is equal so the induced emf is equal in both the windings. So for no displacement the value of output eout is zero as e1 and e2 both are equal. So it shows that no displacement took place.
• CASE II When the core is moved to upward of null position (For displacement to the upward of reference point)

In the this case the flux linking with secondary winding S1 is more as compared to flux linking with S2. Due to this e1 will be more as that of e2. Due to this output voltage eout is positive.

• CASE III When the core is moved to downward of Null position (for displacement to the downward of the reference point). In this case magnitude of e2 will be more as that of e1. Due to this output eout will be negative and shows the output to downward of the reference point. Output VS Core Displacement A linear curve shows that output voltage varies linearly with displacement of core.

Some important points about magnitude and sign of voltage induced in LVDT

• The amount of change in voltage either negative or positive is proportional to the amount of movement of core and indicates amount of linear motion.
• By noting the output voltage increasing or decreasing the direction of motion can be determined
• The output voltage of an LVDT is linear function of core displacement .

Advantages of LVDT

• High Range – The LVDTs have a very high range for measurement of displacement.they can used for measurement of displacements ranging from 1.25 mm to 250 mm
• No Frictional Losses – As the core moves inside a hollow former so there is no loss of displacement input as frictional loss so it makes LVDT as very accurate device.
• High Input and High Sensitivity – The output of LVDT is so high that it doesn’t need any amplification. The transducer posseses a high sensitivity which is typically about 40V/mm.
• Low Hysteresis – LVDTs show a low hysteresis and hence repeatability is excellent under all conditions
• Low Power Consumption – The power is about 1W which is very as compared to other transducers.
• Direct Conversion to Electrical Signals – They convert the linear displacement to electrical voltage which are easy to process

Disadvantages of LVDT

• LVDT is sensitive to stray magnetic fields so it always requires a setup to protect them from stray magnetic fields.
• LVDT gets affected by vibrations and temperature.

It is concluded that they are advantageous as compared than any other inductive transducer.

Applications of LVDT

1. We use LVDT in the applications where displacements to be measured are ranging from a fraction of mm to few cms. The LVDT acting as a primary transducer converts the displacement to electrical signal directly.
2. The LVDT can also act as a secondary transducer. E.g. the Bourbon tube which acts as a primary transducer and it converts pressure into linear displacement and then LVDT coverts this displacement into an electrical signal which after calibration gives the readings of the pressure of fluid.

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