Temperature Measurement and Heat Transfer

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Questions and Answers

What is the process by which heat is transferred between objects that are touching each other?

Radiation

Diffusion

Conduction (correct)

Convection

Which temperature scale uses absolute zero as its starting point?

Rankine

Celsius

Kelvin (correct)

Fahrenheit

What physical state is primarily responsible for convection?

Both B and C (correct)

Liquid

Solid

Gas

Which of the following is an example of convection?

Warm air rising from a heating vent Signup and view all the answers

What is the value of 25°C in Kelvin?

298.15 K Signup and view all the answers

What is the primary method of heat transfer when warm fluid rises and cool fluid descends?

Convection Signup and view all the answers

When transferring heat internally within a solid, which method of heat transfer is primarily in effect?

Conduction Signup and view all the answers

What is heat transfer through electromagnetic waves called?

Radiation Signup and view all the answers

What type of element is used in resistive temperature devices?

A coil of wire Signup and view all the answers

Which material is commonly used to house resistive temperature devices?

Ceramic Signup and view all the answers

What is a key characteristic of thermistors?

High sensitivity Signup and view all the answers

What is a drawback of using thermistors?

They are nonlinear Signup and view all the answers

Why might thermistors be considered low cost?

They utilize simple design techniques Signup and view all the answers

What is a limitation of the temperature range for thermistors?

Limited temperature range Signup and view all the answers

Which factor is important when selecting a temperature sensor?

Resistance variable with temperature Signup and view all the answers

Which of the following is NOT a feature of thermistors?

Linear response Signup and view all the answers

What materials typically compose a thermistor?

Ceramic base material and metal oxides Signup and view all the answers

Which term refers to thermistors that increase resistance as temperature increases?

Positive Temperature Coefficient (PTC) Signup and view all the answers

How does a thermistor with a Negative Temperature Coefficient (NTC) behave as temperature decreases?

Resistance decreases Signup and view all the answers

What type of device uses a metal sensing element that changes resistance in response to temperature?

Thermistor Signup and view all the answers

What is a common feature of thermistors regarding their construction?

They have lead wires soldered to a semiconductor Signup and view all the answers

Which application would likely utilize a Positive Temperature Coefficient (PTC) thermistor?

Overcurrent protection in electrical devices Signup and view all the answers

What kind of electrical response do thermistors exhibit when temperature changes?

Variable resistance response Signup and view all the answers

Which of the following statements about thermistors is FALSE?

NTC thermistors decrease resistance with an increase in temperature. Signup and view all the answers

What is a major disadvantage of thermistors compared to RTDs and thermocouples?

Limited temperature range Signup and view all the answers

Which of the following statements is true regarding Resistance Temperature Detectors (RTDs)?

They have a wide temperature range of –200 to +650°C. Signup and view all the answers

Which characteristic of RTDs contributes to their reliability over time?

Drift of >0.1°C/year Signup and view all the answers

What is one of the primary drawbacks of using thermistors in temperature sensing applications?

Their resistance is non-linear and complicates interpretation. Signup and view all the answers

What practical advantage do RTDs offer in terms of output compared to thermocouples?

Larger voltage output Signup and view all the answers

What is a limitation of using thermistors regarding system reliability?

Increased component count can lead to higher failure rates. Signup and view all the answers

Which statement accurately reflects the durability of RTDs?

They are less durable than thermocouples in harsh conditions. Signup and view all the answers

What is one of the advantages of using RTDs over thermistors?

Wider temperature range and better accuracy Signup and view all the answers

How do the resistance changes of thermistors compare to RTDs?

RTDs provide linear resistance changes that simplify measurement. Signup and view all the answers

Which disadvantage is commonly associated with thermistors?

Self-heating can impair accuracy. Signup and view all the answers

Which of the following characteristics is an advantage of bi-metal thermostats?

Fast response with no additional circuitry Signup and view all the answers

What is a notable disadvantage of bulb and capillary thermostats?

Control can be distant from the application Signup and view all the answers

Which statement accurately describes silicon sensors?

They are less expensive than thermistors but more costly than thermocouples. Signup and view all the answers

What is an advantage of infrared (IR) pyrometry?

Suitable for hazardous material applications Signup and view all the answers

Which disadvantage of thermoelectric sensors is significant?

Costly calibration process Signup and view all the answers

In what way are thermistors advantageous as temperature sensors?

Rapid thermal response Signup and view all the answers

What is a primary benefit of using thermocouples?

Durability in high-shock environments Signup and view all the answers

What limitation do silicon sensors have compared to RTDs?

They operate within a more restricted temperature range. Signup and view all the answers

Which factor can significantly affect the accuracy of infrared (IR) sensors?

Surface finish of the target Signup and view all the answers

What is the major drawback of using thermistors as a temperature sensor?

They are more limited in usable temperature ranges. Signup and view all the answers

Which of the following is a disadvantage of bi-metal thermostats?

Less accuracy than many electronic-based systems Signup and view all the answers

Which of these sensors is best suited for high temperature applications?

Thermocouples Signup and view all the answers

Why might a user choose thermistors over other types of sensors?

Significantly lower costs and fast response times Signup and view all the answers

When considering a sensor, which factor is least likely to influence its selection?

Brand popularity Signup and view all the answers

Study Notes

Temperature Scales

Celsius (°C) is a common temperature scale.
Fahrenheit (°F) is another common temperature scale.
Kelvin (K) is the absolute temperature scale, 0K is absolute zero.
Rankine (°R) is another absolute temperature scale, often used in engineering.

Heat Transfer

Conduction is the transfer of heat between objects in contact.
Convection is the transfer of heat by moving fluids.

Thermistors

Thermistors are devices that change electrical resistance with temperature.
A Positive Temperature Coefficient (PTC) thermistor's resistance increases with temperature.
A Negative Temperature Coefficient (NTC) thermistor's resistance decreases with temperature.

Resistive Temperature Devices

The sensing element in resistive temperature devices is an electrical resistor.
Resistance changes with temperature.

Selecting Temperature Sensors

Thermistors are highly sensitive, have a wide range of applications, are inexpensive, but are nonlinear and have a limited temperature range.
Consider these factors when selecting a temperature sensor:
- How tightly must temperature be controlled?
- How important is the total system cost?

Electro-Mechanical Bi-Metal Thermostats

Advantages:
- Direct interface with applications for fast response.
- No additional circuitry or components required.
- Available in hermetic and non-hermetically sealed designs.
- High current carrying capabilities.
- Wide operating temperature range.
- Market-based pricing.
- NASA qualified, high reliability and military versions are available.
Disadvantages:
- Less accurate than most electronic-based systems.
- Larger size than electronic-based systems.
- Cannot interface with electronic components.
- May experience "closed" failure at the end of life.

Electro-Mechanical Bulb and Capillary Thermostats

Advantages:
- Control can be located a significant distance from the sensing application.
- Built-in overtemperature systems available.
- Broad operating temperature range.
- High current carrying capability.
Disadvantages:
- Large size.
- Relatively expensive.
- Limited number of potential applications.

Electronic Silicon Sensors

Advantages:
- Less expensive than RTDs.
- More linear than thermistors.
- Easier to use than RTDs or thermocouples due to higher output.
- IC types offer on-chip signal conditioning.
- Many IC types have communication protocols with bus-type data acquisition systems.
Disadvantages:
- Not as linear as RTDs.
- Less accurate than other electronic-based systems.
- More expensive than thermistors or thermocouples.
- Limited temperature range.
- Slower thermal response than other electronic-based systems.
- Typically larger than RTDs and thermistors.
- Require larger package sizes for immersion.
- Additional components/circuitry required to control application loads.

Electronic Infrared (IR) Pyrometry

Advantages:
- Allows for non-contact measurement of moving objects or hazardous materials.
- Can be used with fiber optics for remote sensing.
- Typical temperature range of -18°C to +538°C (0°F to 1000°F).
- Accuracy to ±1%.
Disadvantages:
- Accuracy can be affected by surface finish.
- Field of view must be matched to the target size.
- Ambient temperature can affect readings.
- Wavelength filter must be matched to the application.
- Higher cost can be even higher if control circuitry is required.
- Calibration can be difficult and costly.
- Additional components/circuitry required to control application loads.
- Dust, gas, or other vapors in the environment can affect accuracy.

Electronic Thermocouples

Advantages:
- Small size for rapid temperature response.
- Relatively inexpensive.
- Wide temperature range.
- More durable than RTDs in high-vibration and high-shock applications.
- ANSI established calibration types.
Disadvantages:
- Must be protected from corrosive environments.
- Smaller gage wires are less stable and have a shorter operating life.
- Plated-copper instrumentation wire results in errors when ambient temperatures change.
- Special extension wires are required.
- Reference junction compensation is required.
- Less stable than RTDs in moderate or high temperatures.
- Should be tested to verify performance under controlled conditions for critical applications.
- Additional components/circuitry required to control application loads.

Resistive Thermistors

Advantages:
- Low component cost.
- Fast thermal response.
- Large change in resistance vs. temperature for more resolution.
- Extremely small size, providing faster reaction to change in temperature and flexibility in assembly.
- Linearized resistance types are available.
- High resistance values eliminate the need for lead wire compensation.
Disadvantages:
- Limited temperature range.
- Lower temperature exposures than RTDs or thermocouples.
- No established resistance standards.
- Self-heating can affect accuracy.
- Nonlinear resistance change requires additional components for accurate interpretation.
- Increased component count decreases system reliability.
- Additional components/circuitry required to control application loads.

Resistive Resistance Temperature Detectors (RTDs)

Advantages:
- Very accurate and repeatable.
- Wide temperature range of -200°C to +650°C (-328°F to +1202°F) depending on type.
- Extremely stable over time: >0.1°C/year drift.
- Larger voltage output than thermocouples.
- Excellent resistance linearity.
- Resistance can be determined in the lab and remains stable over time.
- Area or point sensing capabilities available.
- Low variation for better interchangeability.
- Standard instrumentation cable can be used to connect to control equipment.
- Established industry-accepted resistance curves.
Disadvantages:
- Higher cost than thermistors or thermocouples.
- Self-heating of the RTD affects overall system accuracy.
- Larger size than thermistors or thermocouples.
- Not as durable as thermocouples in high-vibration and high-shock environments.

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Description

Explore the fundamental concepts of temperature scales, heat transfer mechanisms, and temperature sensing devices. This quiz will cover Celsius, Fahrenheit, Kelvin, and their applications in real-world scenarios, as well as the functionality of thermistors and resistive temperature devices. Test your understanding of temperature measurement and sensor selection.