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

The correct answer is: 1) have a fast response time and high sensitivity

Concept:

Thermistors (thermal resistors) are widely preferred in temperature sensing applications due to these key advantages:

Primary Benefits of Thermistors

• High Sensitivity:

  • Thermistors exhibit large resistance changes with small temperature variations (typically negative temperature coefficient, i.e., NTC thermistors).

  • Far more sensitive than RTDs or thermocouples.

• Fast Response Time:

  • Small size and low thermal mass allow rapid reaction to temperature changes (milliseconds to seconds).

• Cost-Effective:

  • Cheaper than RTDs and more accurate than thermocouples for limited ranges.

Comparison with Other Transducers

Explanation:

When subjected to mechanical stress, a piezoelectric transducer exhibits the characteristic of voltage generation. Let's delve into the details of how this phenomenon occurs and analyze why the other options are not correct.

Piezoelectric Effect and Voltage Generation:

Piezoelectric transducers operate based on the piezoelectric effect, a property of certain materials (such as quartz, ceramics, and some polymers) to generate an electric charge in response to applied mechanical stress. When a piezoelectric material is deformed or stressed, the internal distribution of electric charges within the material changes. This results in the generation of an electric potential, or voltage, across the material. This voltage can then be measured and utilized in various applications, such as sensors, actuators, and energy harvesting devices.

The process can be summarized as follows:

• Mechanical stress is applied to the piezoelectric material.
• The stress induces a deformation in the crystal lattice structure of the material.
• This deformation causes a shift in the positions of the positive and negative charge centers within the material, creating an electric dipole moment.
• The accumulation of these dipole moments generates a measurable electric potential (voltage) across the material.

This phenomenon is the cornerstone of piezoelectric transducers, making them highly effective for converting mechanical energy into electrical energy and vice versa. The generated voltage is directly proportional to the applied mechanical stress, allowing for precise measurements and control in various applications.

Concept

The force on point charge (Q₂) at point (x₂,y₂,z₂) due to point charge (Q₁) at point (x₁,y₁,z₁) is given by:

\(F_2=k{Q_1Q_2\over r^2}\space \widehat{a_r}\)

\(r={\sqrt{(x_2-x_1)^2+(y_2-y_1)^2+(z_2-z_1)^2}}\)

\(\widehat{a_r}={\vec{r}\over r}\)

Calculation

Given,  Q1 = 4 μC is at P1 (3, 11, 8)

Q2 = -5 μC is at P2 (6, 15, 8)

\(r={\sqrt{(6-3)^2+(15-11)^2+(8-8)^2}}=5\)

\(\widehat{a_r}={3U_x+4U_y\over 5}\)

\(F_2={9\times 10^9\times 4\times 10^{-6}\times (-5)\times 10^{-6}\over (5)^2}\space \times {3U_x+4U_y\over 5}\)

\(F_2={9\times 4\times (-5)\over (5)^2}\space \times {3U_x+4U_y\over 5}\times 10^{-3}\)

F₂ = - (4.32 Ux + 5.76 Uy) mN

Active Transducers:

• An active transducer is a device that converts one form of energy directly into electrical energy without the need for an external power source.
• They are also known as self-generating type transducers.
• Active transducers generate an electrical signal in response to a physical phenomenon.

Examples of Active Transducers:

• Thermocouple:
  • A thermocouple is a type of active transducer that converts thermal energy directly into electrical energy.
  • It consists of two dissimilar metals joined together at one end. When this junction is heated or cooled, it produces a voltage that can be correlated to temperature.
  • Thermocouples are widely used for temperature measurement and control.

Explanation of Other Options:

• Strain Gauge:
  • A strain gauge is a passive transducer that converts mechanical deformation into a change in electrical resistance.
  • It requires an external power source to measure the change in resistance.
• Thermistor:
  • A thermistor is a passive transducer that changes its resistance with temperature.
  • It also requires an external power source to measure the change in resistance.
• LVDT (Linear Variable Differential Transformer):
  • An LVDT is a passive transducer that converts linear displacement into an electrical signal.
  • It operates on the principle of mutual induction and requires an external AC excitation source.

Hence, the Thermocouple is an active transducer as it generates an electrical signal directly from thermal energy without the need for an external power source.

Concept

The change in output voltage is the sensitivity multiplied by the displacement.

ΔV = S⋅Δx

where, ΔV = Change in output voltage

S = Sensitivity

Δx = Displacement

Calculation

The sensitivity (S) of the LVDT is the output voltage per unit displacement, calculated as:

\(S={Output \space voltage \space at \space full \space range \over Range}\)

\(S={5.2\over 0.5}=10.4\space V/M\)

The core movement is from + 0.45 m to - 0.30 m.

Δx = −0.30 − (+0.45) = −0.75 m

ΔV = 10.4 × (-0.75) = -7.8 V

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).

​

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.

​ 

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.

​