Measurement of Temperature MCQ Quiz - Objective Question with Answer for Measurement of Temperature - 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

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:

Thermocouple Neutral and Inversion Temperature

Definition: A thermocouple is a device used to measure temperature, consisting of two different types of metals joined together at one end. The junction of the two metals produces a voltage that is related to the temperature difference between the "hot" and "cold" junctions. Two important temperatures associated with thermocouples are:

• Neutral Temperature: The temperature at which the thermoelectric voltage reaches its maximum value for a given cold junction temperature.
• Inversion Temperature: The temperature beyond the neutral temperature at which the thermoelectric voltage begins to decrease and eventually becomes zero.

Given Data:

• Cold junction temperature = 20°C
• Neutral temperature = 250°C

Formula: The relationship between the neutral temperature (T_n), inversion temperature (T_i), and cold junction temperature (T_c) is given by:

T_i = 2 × T_n - T_c

Here:

• T_n is the neutral temperature (250°C).
• T_c is the cold junction temperature (20°C).
• T_i is the inversion temperature.

Calculation:

Substitute the given values into the formula:

T_i = 2 × T_n - T_c

T_i = 2 × 250 - 20

T_i = 500 - 20

T_i = 480°C

Correct Option: Option 1 (480°C)

This is the correct answer because the inversion temperature is calculated accurately using the formula and the given data.

Additional Information

Important Information:

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

Option 2: 500°C

This option assumes that the inversion temperature is simply double the neutral temperature, without considering the cold junction temperature. However, the formula explicitly accounts for the cold junction temperature, and neglecting it leads to an incorrect result.

Option 3: 520°C

This option is incorrect because it exceeds the calculated inversion temperature. There is no basis for this value within the context of the formula provided.

Option 4: 460°C

This option underestimates the inversion temperature. It might result from an incorrect application of the formula or a misunderstanding of the relationship between the neutral and inversion temperatures.

Conclusion:

The inversion temperature is a critical parameter in understanding the behavior of thermocouples. It is calculated using the relationship between the neutral temperature and the cold junction temperature. In this case, the correct inversion temperature is 480°C, as determined by the formula T_i = 2 × T_n - T_c. This temperature represents the point at which the thermoelectric voltage begins to decrease after reaching its maximum value at the neutral temperature.

Explanation:

EMF of a Thermocouple

Definition: The EMF (Electromotive Force) of a thermocouple is the voltage generated due to the temperature difference between two different metals or semiconductors joined at two junctions. This phenomenon is based on the Seebeck effect, which states that a voltage (EMF) is induced in a circuit made of two different metals when there is a temperature gradient between the junctions.

Working Principle: When two dissimilar metals are joined at two junctions and there is a temperature difference between these junctions, a small voltage is generated. This voltage can be measured and is directly related to the temperature difference between the hot and cold junctions. The relationship between the temperature difference and the EMF is primarily linear within a certain temperature range. However, at higher temperatures, the relationship may become non-linear.

Applications: Thermocouples are widely used in various industries for temperature measurement in processes such as metal processing, power generation, chemical manufacturing, and HVAC systems. They are also used in scientific research and in household appliances like ovens and water heaters.

The correct option is: Linear

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

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

Comparison between RTD's and thermistor are as follows:

Range:

1) Thermistors can only track a narrow temperature range. They can only measure up to 130°C

2) While some RTDs can exceed 600°C

Sensitivity:

1) Thermistors have a sensitivity of around 10 Ω/°C

2) While RTD sensors change resistance <<0 Ω/°C 

3) Therefore Thermistors are faster than RTD's

Accuracy:

1) Thermistors due to their high sensitivity are more accurate.

2) RTD's are comparatively less accurate than Thermistors.

Material:

1) Thermistors are made of semiconductors, ceramic, polymer material, etc.

2) RTD's are made of platinum, nickel, copper, etc.

Important Points

Applications:

1) Thermistors are widely used in home appliances.

2) RTD's are used for industrial purposes.

Cost:

1) Are comparatively inexpensive than RTD's.

2) Thermistors with a wider temperature range are often more costly than RTDs.

3) Temperature changes cause predictable resistance changes in both thermistors and RTDs.

Thermistor:

• It is a kind of resistor whose resistivity depends on surrounding temperature.
  It is a temperature sensitive device.
• The word thermistor is derived from the word, thermally sensitive resistor.
• The thermistor is made of the semiconductor material that means their resistance lies between the conductor and the insulator.
• The variation in the thermistor resistance shows that either conduction or power dissipation occurs in the thermistor.

The thermistor is classified into types.

• Negative Temperature Coefficient (NTC) Thermistor –
  • In this type of thermistor the temperature increases with the decrease of the resistance or resistance decreases with increasing temperature.
  • The resistance of the negative temperature coefficient thermistor is very large due to which it detects the small variation in temperature.
• Positive Temperature Coefficient (PTC) Thermistor –
  • The resistance of the thermistor increases with the increases in temperature.

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

The correct answer is option (3)

Concept:

Thermistor:

• A thermistor is made with the combination of two words Thermal + Resistor.
• Thermistors are generally made up of semiconductor material.
• Thermistors have a negative temperature coefficient of resistance.
• In other words, the resistance decreases with the increase in temperature.
• Thermistors can measure the temperature in the range of -100^∘ C to 300^∘ C.
• Thermistor have a sensitivity of around 10 Ω/ ^∘C 

Fig: Basic circuit diagram of a thyristor

Advantages of thermistor:

• It is very cheap.
• It is rugged and compact.
• It has good stability.
• It requires very simple circuitry to measure temperature.
• The response time of a thermistor can vary from a fraction of a second to minutes, depending upon the size of the sample and the capacity of the thermistor.

Platinum Resistance Thermometer:

• A (PRT) is a piece of platinum wire which determines the temperature by measuring its electrical resistance.
• The electrical resistance of many metals (e.g. copper, silver, aluminium, platinum) increases approximately linearly with absolute temperature and this feature makes them useful as temperature sensors.
• The resistance of a wire of the material is measured by passing a current through it and measuring the voltage across it with a suitable voltmeter. The reading is converted to temperature using a calibration equation, and the PRTs work on this principle.
• It is used for accurate and precise measurement of temperature.

Important Points:

• The Thermistor is a solid-state temperature sensing device that acts a bit like an electrical resistor but is temperature-sensitive. Thermistors can be used to produce an analog output voltage with variations in ambient temperature and as such can be referred to as a transducer.
• A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires 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.
• Semiconductor temperature sensors commonly use a bandgap element which measures variations in the forward voltage of a diode to determine temperature. To achieve reasonable accuracy, these are calibrated at a single temperature point, typically 25 °C.

Thermocouple:

• The thermocouple is an electrical device containing junctions of two dissimilar metal joints. It is used as temperature sensors.
• It works on the principle of the thermoelectric effect or the Seebeck effect which means which states that the temperature difference between two dissimilar electric conductors produces a voltage difference between them.
• This potential difference is used to measure temperatures.

• Thermocouple types of instruments can be used for dc and ac applications. 
• They can be used for measurements of currents and voltages at high frequencies. 
• These instruments are very accurate well above a frequency of 50 MHz.

Additional Information

Hall effect:

Whenever we place a current-carrying conductor in a magnetic field, there is a deflection of the charge carriers due to the influence of the magnetic field in the conductor body. This phenomenon is known as the Hall effect.

Mainly Lorentz force is responsible for the Hall effect. When we place a current-carrying conductor inside a magnetic field, the conductor experiences a mechanical force to a direction depending upon the direction of the magnetic field and the direction of current in the conductor.

In metal, it is entirely due to the flow of electrons whereas in semiconductor it is due to the flow of free electrons as well as holes.

The Hall Effect satisfies the Lorentz’s Force which is E = V x B

So, the direction of the velocity, electric field, and magnetic field should be perpendicular to each other.

This is explained with the help of the following diagram:

Hall Voltage is given by:

\({V_H} = Ed = \frac{{BI}}{{\rho W}}\)

Applications:

Hall effect is a very useful phenomenon and helps to

• Determine the type of semiconductor
• Calculate the carrier concentration
• Determine the mobility (hall mobility)
• Measure magnetic flux density
• Linear angular displacement

Concept:

RTD’s are based on the principle that the resistance of a metal increases with temperature.

the relationship between resistance and temperature, may then be expressed as-

R_t = R₀(1+α₀t)

where: Rt = resistance of rtd at temperature t (ohm),

Ro = resistance of rtd at 0 °C (ohm), and

αo = temperature coefficient of resistance (TCR) at 0 °C (per °C)

Calculation:

Given:

 18 Ω when t = 20°C assume temperature coefficient is α.

18 = R₀(1 + α20) ____(1)

20 = R₀(1 + α50)_____(2)

divide equation (1) by equation (2)

\(\frac{{18}}{{20}} = \frac{{1 + 20\alpha }}{{1 + 50\alpha }}\)

10 + 200α = 9 + 450α

\(\alpha = \frac{1}{{250}}\)

\({R_0}\left( {1 + \frac{{20}}{{250}}} \right) = 18\)

\({R_0} = \frac{{18 \times 50}}{{270}} = \frac{{50}}{3}\;{\rm{\Omega }}\)

Now assume temperature is t°C

21 = 12.85 (1 + α.t)

\(\frac{{21}}{{12.85}} - 1 = \frac{t}{{50}}\)

\(21 = \frac{{50}}{3}\left\{ {1 + \frac{1}{{250}}t} \right\}\)

\(\frac{{63}}{{50}} - 1 = \frac{t}{{250}}\)

\(t = \frac{{13}}{{50}} \times 250\)

t = 65°

Since surrounding temperature is 15°C so rise in

Mean temperature = 65° - 15°

= 50°C