Sensors and Industrial Instrumentation MCQ Quiz - Objective Question with Answer for Sensors and Industrial Instrumentation - Download Free PDF

Concept:

An RTD (Resistance Temperature Detector) changes resistance with temperature. The power dissipated in a resistor is given by:

\(P = \frac{V^2}{R} \)

For pulsed operation at 50% duty cycle, the average power is:

\(P_{avg} = \frac{V_{peak}^2}{R} \times Duty~Cycle \)

Given:

• Base resistance at 25°C: R₂₅ = 100 Ω
• Sensitivity = 1 Ω/°C
• Final temperature: 125°C
• Voltage peak: V = 2.5 V
• Duty cycle = 50% = 0.5

Step 1: Find resistance at 125°C

\(R_{125} = R_{25} + \Delta R = 100 + (125 - 25) \times 1 = 200~\Omega \)

Step 2: Use the power formula

\( P_{avg} = \frac{(2.5)^2}{200} \times 0.5 = \frac{6.25}{200} \times 0.5 = 0.03125 \times 0.5 = 0.0625~W = 62.5~mW\)

Hence,  the correct answer is 62.5 mW

Capacitive Accelerometer

Definition: A capacitive accelerometer is a type of accelerometer that measures acceleration by detecting changes in capacitance due to the relative displacement of its plates. The system typically consists of a movable plate and a fixed plate, forming a parallel plate capacitor. The distance between these plates changes with acceleration, causing a corresponding change in capacitance.

Working Principle: The capacitance (C) of a parallel plate capacitor is given by:

C = ε × A / d

where:

• ε: Permittivity of the medium between the plates
• A: Area of the plates
• d: Distance between the plates

When the accelerometer experiences acceleration, the movable plate shifts, altering the distance d between the plates. This change in d modifies the capacitance, which is then measured to determine the corresponding acceleration.

Scenario: In the given problem, the accelerometer is configured such that the inter-plate gap increases with acceleration. This implies that when acceleration decreases (e.g., halved), the distance between the plates will also decrease. According to the capacitance formula, a decrease in d will increase the capacitance, as C is inversely proportional to d.

Correct Option Analysis:

The correct option is:

Option 4: The distance between plates is d/2 and capacitance doubled.

Explanation:

When the input acceleration is halved, the inter-plate gap decreases to half of its original value, i.e., the new distance between the plates becomes d/2. Substituting this into the capacitance formula:

Initial Capacitance, C₁ = ε × A / d

New Capacitance, C₂ = ε × A / (d/2) = 2 × (ε × A / d) = 2 × C₁

This shows that when the distance between the plates is halved, the capacitance doubles. Therefore, the correct answer is Option 4.

The correct answer is: 4) Linear Variable Differential Transformer (LVDT)

Explanation:

• LVDT is a type of displacement transducer used to measure linear displacement accurately. It converts linear motion into an electrical signal.

• RTD (Resistance Temperature Detector) measures temperature.

• Opto-coupler is used for isolating different parts of an electrical system, typically not for displacement.

• Microphone converts sound (acoustic energy) into electrical signals.

Explanation:

Thermistors for Temperature Measurement

Definition: A thermistor is a type of temperature-sensitive resistor that exhibits a significant change in resistance with a change in temperature. It is widely used for temperature measurement, control, and compensation in various applications due to its high sensitivity and precision.

Correct Option Analysis:

The correct answer is:

Option 2: Negative temperature coefficient

This option correctly describes the behavior of commonly used thermistors for temperature measurement. Negative Temperature Coefficient (NTC) thermistors are characterized by a decrease in resistance as the temperature increases. This property makes them ideal for precise temperature measurement applications.

Working Principle:

NTC thermistors operate based on the principle that their resistive material exhibits a negative temperature coefficient. As the temperature rises, the thermistor's resistance decreases exponentially. This behavior occurs due to the increase in the number of charge carriers (electrons and holes) available in the material, which enhances conductivity. The relationship between resistance (R) and temperature (T) is often expressed mathematically as:

R(T) = R₀ × e^(β/T)

Where:

• R₀: Resistance at a reference temperature
• β: Material constant
• T: Temperature in Kelvin

Advantages of NTC Thermistors:

• High sensitivity to temperature changes, making them suitable for precise measurements.
• Compact size and ease of integration into electronic circuits.
• Cost-effective solution for temperature sensing and monitoring.
• Wide range of operating temperatures, enabling diverse applications.

Applications:

• Temperature measurement in electronic devices, such as thermostats and temperature sensors.
• Overcurrent protection in circuits by monitoring temperature rise.
• Compensation for temperature-induced variations in electronic components.
• Used in automotive, medical, and industrial equipment for temperature control.

Explanation:

Measurement of the Highest Temperature Range

Definition: The measurement of temperature in various industrial and scientific applications requires specific devices that are designed to operate within particular temperature ranges. The highest temperature ranges can only be measured by devices that are constructed to withstand extreme heat while maintaining accuracy and reliability. Among the listed options, the device that can measure the highest temperature range is the Noble Metal Thermocouple.

Correct Option Analysis:

The correct answer is:

Option 2: Noble Metal Thermocouple

Noble Metal Thermocouple: Noble metal thermocouples are made using noble metals such as platinum and rhodium. These thermocouples are specifically designed to measure high temperatures, often up to 1800°C or more. Some of the most commonly used noble metal thermocouples are Type R, Type S, and Type B thermocouples.

Key Features of Noble Metal Thermocouples:

• Material Composition: They are made from a combination of noble metals like platinum and rhodium, which are highly resistant to oxidation and corrosion at elevated temperatures.
• High-Temperature Range: Noble metal thermocouples can measure temperatures up to approximately 1800°C, making them suitable for applications like metal smelting, glass manufacturing, and high-temperature furnaces.
• Accuracy and Stability: These thermocouples offer excellent accuracy and stability over a wide temperature range, ensuring reliable readings even in extreme conditions.

Applications of Noble Metal Thermocouples:

• Used in industries such as metal processing, ceramics, and glass manufacturing, where extremely high temperatures are present.
• Found in laboratories and research facilities for precise high-temperature measurements.
• Commonly used in aerospace and power generation industries to monitor turbine and exhaust temperatures.

The noble metal thermocouple is the most suitable device for measuring the highest temperature range due to its unique combination of material properties, accuracy, and ability to withstand extreme environments.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Base Metal Thermocouple

Base metal thermocouples, such as Type K, J, T, and E, are constructed from less expensive metals like nickel and copper. These thermocouples are widely used in industrial applications for temperature measurements up to around 1000°C. While they are cost-effective and versatile, they cannot measure the extremely high temperatures that noble metal thermocouples can handle. Therefore, this option is not suitable for measuring the highest temperature range.

Option 3: Resistance Thermometer

Resistance thermometers, also known as Resistance Temperature Detectors (RTDs), measure temperature by correlating the resistance of a material (usually platinum) with temperature. These devices are highly accurate and stable but are generally limited to a temperature range of approximately -200°C to 850°C. Due to this limitation, resistance thermometers are not suitable for measuring the highest temperature ranges.

Option 4: Platinum Resistance Thermometer

Platinum resistance thermometers are a specific type of RTD that use platinum as the sensing element. They offer excellent accuracy and stability and are commonly used in laboratory and industrial applications. However, their temperature range is typically limited to around -200°C to 850°C, similar to standard RTDs. As a result, they are not capable of measuring the extremely high temperatures that noble metal thermocouples can handle.

Conclusion:

The correct choice for measuring the highest temperature range is the Noble Metal Thermocouple (Option 2). These thermocouples are specifically designed for extreme temperature conditions and are widely used in industries requiring precise high-temperature measurements. While other devices like base metal thermocouples, resistance thermometers, and platinum resistance thermometers have their unique advantages, they are not suitable for the highest temperature ranges, making noble metal thermocouples the optimal choice.

Explanation

The correct answer is Pyrometer.

Key Points

• Pyrometer 
  • The device for measuring relatively high temperatures, such as are encountered in furnaces. Hence, Option 2 is correct.
  • Most pyrometers work by measuring radiation from the body whose temperature is to be measured.
  • Radiation devices have the advantage of not having to touch the material being measured.
  • Radiation Pyrometers are used to measure the temperature of red hot metals up to 3000°C.
  • It is also known as an Infrared thermometer or Radiation thermometer or non-contact thermometer used to detect the temperature of an object’s surface temperature, which depends on the radiation (infrared or visible) emitted from the object.
  • It acts as a photodetector because of the property of absorbing energy and measuring EM wave intensity at any wavelength.
  • These are used to measure high-temperature furnaces.
  • These devices can measure the temperature very accurately, precisely, pure visually, and quickly.
  • Pyrometers are available in different spectral ranges ( since metals – short wave ranges and non-metals-long wave ranges).

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Additional Information

The linear variable differential transformer (LVDT) is a type of electrical transformer used for measuring linear displacement (position).

Sensitivity is defined as the ratio between the output signal and the measured property.

Here the output of LVDT is a voltage signal and the measured quantity is displacement.

Sensitivity(S) = output voltage/displacement measured

Given, RMS output voltage = 2.6 V

Displacement = 0.4 μm

S = 2.6 / 0.4

Sensitivity = 6.5 V / μm

Piezoelectric materials are materials that can produce electricity due to mechanical stress, such as compression. These materials can also deform when a voltage is applied.

All piezoelectric materials are non-conductive for the piezoelectric effect to occur and work. They can be separated into two groups:

1) Crystals

2) Ceramics

Extra Information:

• The piezoelectric effect is generally carried out in crystal oscillators, crystal filters, production and detection of sound, piezoelectric inkjet printing, generation of high voltages, electronic frequency generation, microbalances, to drive an ultrasonic nozzle, and ultrafine focusing of optical assemblies.
• On the application of a force resulting in the deformation of the crystal, the charges on the crystal will tend to separate such that one end will be positively charged and the other end of the crystal will be negatively charged.
• If the crystals are oriented accordingly, we can cause efficient charge separation which can be tapped by connecting the arrangement through a closed circuit and this will thus cause flow of current through the circuit.

A transducer is an electronic device that converts energy from one form to another. Common examples include microphones, loudspeakers, thermometers, i.e.

Transducers can be classified into the following types:

• Active or Passive Transducers
• Analog or Digital Transducers

Analog transducers:

• These transducers convert the input quantity into an analog output which is a continuous function of time
• Thus, a strain gauge, an L.V.D.T., a thermocouple or a thermistor may be called as Analog Transducers as they give an output which is a continuous function of time

Digital Transducers:

• These transducers convert the input quantity into an electrical output which is in the form of pulses and its, output is represented by 0 and 1

Explanation:

Thermocouple: A thermocouple is a sensor used to measure temperature. Generally, temperature up to 1400° is measured through this device.

• It consists of two wire legs made from different metals.
• The wire's legs are welded together at one end, creating a junction.
• This junction is where the temperature is measured.
• When the junction experiences a change in temperature, a voltage is created.
• The voltage can then be interpreted using thermocouple reference tables to calculate the temperature.

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Types of Thermocouple:

1. Type K Thermocouple (Nickel-Chromium / Nickel-Alumel): The type K is the most common type of thermocouple. It’s inexpensive, accurate, reliable, and has a wide temperature range.

• Temperature Range: - 270 to 1260°C

2. ​​Type J Thermocouple (Iron/Constantan): The type J is also very common. It has a smaller temperature range and a shorter lifespan at higher temperatures than Type K. It is equivalent to the Type K in terms of expense and reliability.

• Temperature Range: - 210 to 760°C

3. Type T Thermocouple (Copper/Constantan): The Type T is a very stable thermocouple and is often used in extremely low-temperature applications such as cryogenics or ultra-low freezers.

• Temperature Range: - 270 to 370°C

4. Type E Thermocouple (Nickel-Chromium/Constantan): Type E has a stronger signal & higher accuracy than Type K or Type J at moderate temperature ranges of 1,000F and lower. See temperature chart (linked) for details

• Temperature Range: - 270 to 870°C

5.  Type N Thermocouple (Nicrosil / Nisil): The Type N shares the same accuracy and temperature limits as Type K. The type N is slightly more expensive.

• Temperature Range: ​​- 270 to 392°C

6. Type S Thermocouple (Platinum Rhodium - 10% / Platinum): The Type S is used in very high-temperature applications. It is commonly found in the BioTech and Pharmaceutical industries. It is sometimes used in lower temperature applications because of its high accuracy and stability.​

• ​Temperature Range: - 50 to 1480°C

7. ​Type R Thermocouple (Platinum Rhodium -13% / Platinum): The Type R is used in very high-temperature applications. It has a higher percentage of Rhodium than Type S, which makes it more expensive. The Type R is very similar to the Type S in terms of performance. It is sometimes used in lower temperature applications because of its high accuracy and stability

• Temperature Range: - 50 to 1480°C

8. Type B Thermocouple (Platinum Rhodium – 30% / Platinum Rhodium – 6%): The Type B thermocouple is used in extremely high-temperature applications. It has the highest temperature limit of all of the thermocouples listed above. It maintains a high level of accuracy and stability at very high temperatures.

• Temperature Range: 0 to 1700°C

Operation of flowmeter according to the direction:

Important Points

Ultrasonic Flowmeter:

• Basically, an ultrasonic flowmeter consists of two piezoelectric crystals in liquid or gas separated by a distance d.
• One of the crystal act as a transmitter (T) and other act as a receiver (R) as shown.

• The transmitter emits an ultrasonic pulse which received at the receiver a time Δt later.

This transmitted time (Δt) is given as,

\(\Delta t=\frac{d}{c+v}\)

Where, C is the velocity of sound propagation in a given medium and V is the flow velocity.

• By means of this principle, an ultrasonic flowmeter works.
• It has no moving part and allows bidirectionally flow.
• It has linear relation between input and output.

Turbine Flowmeter:

• It is a type of volumetric flowmeter and available for a wide range.
• The output for this flowmeter is in the form of a digital signal whose frequency is directly proportional to the flow rate.
• When fluid passed through the flowmeter then its rotor rotates which produced EMF and later this EMF is converted into DC analog voltage by means of a D/A converter.
• It also allows only the unidirectional flow.

Electromagnetic Flowmeter:

• Electromagnetic flowmeters are basically used for the flow measurement of slurries, sludge, and an electrically conducting liquid.
• The basic arrangement of the Electromagnetic flow meter is shown,

• It consists of basic pair of the insulated electrode on the opposite side of a non-conducting, non-magnetic pipe which carries the fluid for fluid measurement.
• The pipe is surrounded by an electromagnet which produced a magnetic field around it.
• Its basic principle is that conductor moving across the magnetic field just like MHD.

The EMF (E) induced is given by,

E = Blv

Where B is the magnetic flux density
l is the length of conductor and,
v is velocity

• By means of this principle velocity or flow is measured.
• It also used for the measurement of bidirectional flow.

Coriolis mass Flowmeter:

• These are more effective in mass-related processes as they measure the force that results from the acceleration of the mass.
• More specifically, the force is measured as the mass moving per unit of time, instead of the volume per unit of time.
• Mass flow meters include Coriolis mass meters and thermal dispersion meters.
• It also can measure bidirectional flow.

Explanation:

A transducer is a device that is used to convert a physical quantity into its corresponding electrical signal i.e. the input is non-electrical and is converted into its corresponding electrical signal.

Input can be: Resistance, capacitance Inductance, stress, strain Heat.

Output can be: Force, Displacement, Pressure, sound, voltage, current

Active transducers:

• Active transducers (self-generating) are those which do not require any power source for their operation.
• They work on the energy conversion principle. They produce an electrical signal proportional to the input (physical quantity).
• Piezoelectric, thermocouple, and photovoltaic cell transducers are some examples of active transducers.

Passive transducers:

• Transducers which require an external power source for their operation is called a passive transducer.
• They produce an output signal in the form of some variation in resistance, capacitance, or any other electrical parameter, which then has to be converted to an equivalent current or voltage signal.
• LVDT is an example of a passive transducer. LVDT is used as an inductive transducer that converts motion into the electrical signal.

​Inverse transducers:

• The inverse transducer is defined as a device which converts an electrical quantity into a non-electrical quantity.
• A piezoelectric crystal acts as an inverse transducer because when a voltage is applied across its surfaces, it changes its dimensions causing a mechanical displacement.

Primary and Secondary transducer:

• Sometimes, the transducer measures one phenomenon in order to measure another variable.
• The primary transducer senses the preliminary data and converts it into another form, which is again converted into some useable form by the secondary transducer.
• Ex. Measurement of force is performed using a spring element and the resulting displacement of the spring is measured using another electrical transducer.

Concept:

Piezoelectric material:

• The piezoelectric material is the one that possesses the property to convert mechanical energy into electric energy and vice versa. 
• A commonly known piezoelectric material is quartz. 
• The mechanism involves the development of electric charge due to the movement of electrons upon application of stress.
• Piezoelectric materials have a low value of dielectric constant.

Ferroelectric material:

• Ferroelectric materials are materials that exhibit Ferroelectricity.
• Ferro electricity is the ability of the material to have a spontaneous electric polarization. 
• All the ferroelectric materials exhibit a piezoelectric effect due to a lack of symmetry.

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