Optoelectronic Devices: Thermocouples

Optoelectronic and Miscellaneous Devices

• Temperature Measurement Circuits (Thermocouples):

  • A thermocouple is formed by joining two dissimilar metals.
  • A Seebeck voltage is produced across the junction when heated.
  • The voltage is proportional to the temperature of the junction.
  • The voltage is typically less than 100 mV.
  • Thermocouple voltage varies with temperature (curves are given, showing various types).
  • Output is dependent on the reference temperature; commonly 0°C is used.
  • Signal-conditioning circuits are used to avoid unwanted thermocouple effects.

• Eliminating Unwanted Thermocouple Effects:

  • A "cold junction" can be present where the thermocouple wires connect, creating an unwanted thermocouple.
  • Adding a reference thermocouple at a known constant temperature (often 0°C) is one method of elimination.
  • Another approach uses a compensation circuit with a resistor and an integrated circuit sensor (a cold junction compensation).

• Resistance Temperature Detector (RTD):

  • An RTD is a resistive device whose resistance changes directly with temperature.
  • Most common RTDs are made of platinum, nickel, or nickel alloys.
  • RTDs offer a more linear temperature response than thermocouples.
  • RTDs can be used in a constant current circuit or a three-wire bridge circuit.

• Thermistors:

  • A thermistor is a semiconducting material (e.g., nickel oxide, cobalt oxide).
  • Its resistance varies inversely with temperature (negative temperature coefficient).
  • Thermistors are more non-linear than RTDs or thermocouples.
  • Thermistors have a higher sensitivity than RTDs or thermocouples. But, typically have a more limited temperature range than thermocouples.

• Strain Measurement:

  • Strain is the deformation (expansion or compression) of a material due to a force.
  • Strain gauges are used to measure strain. Strain gauges are thin resistive strips attached to a surface to measure strain.
  • Strain gauges have a gage factor (a ratio of resistance change to length change), and thus change their resistance when deformed.
  • Strain gauges are usually incorporated in bridge circuits or constant-current circuits.

• Pressure Transducers:

  • Pressure transducers are devices whose resistance changes proportionally to pressure.
  • They often use a strain gauge bonded to a diaphragm to measure pressure.

• Flow Rate Measurement:

  • Differential pressure is used to measure fluid flow rate through a restriction (Venturi section).
  • Velocity and pressure increase as fluid passes through a restriction.
  • The volume of fluid remains constant, but the velocity changes.
  • Flow meter outputs are measured consistently.

This quiz explores the fundamental principles of thermocouples and temperature measurement circuits. You'll learn about Seebeck voltage, unwanted thermocouple effects, and methods to eliminate them. Test your knowledge on signal-conditioning circuits and their application in thermocouple measurements.