Temperature Measurement and Heat Transfer

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.