Thermocouple Sensor

Thermocouples are widely used temperature sensors that convert temperature variations into electrical voltage. They operate based on the Seebeck Effect, where two dissimilar metals, forming a closed circuit, generate an electromotive force (e.m.f.) when maintained at different temperatures.

Working Principle:

• Seebeck Effect:
  • When two dissimilar metals form a closed circuit at different temperatures, an e.m.f. is induced.
• Transducer Function:
  • Thermocouples act as transducers by converting temperature (non-electrical) into voltage (electrical), making them active transducers.
• No External Power:
  • They don’t require an external power source for operation.

Working of Thermocouple:

• Circuit Setup:
  • Two dissimilar metals (A and B) are joined at two junctions (P and Q).
  • P is the measuring junction (hot), and Q is the reference junction (cold).
• Temperature Difference:
  • When P and Q are at different temperatures, an e.m.f. is generated.
• Measurement:
  • The e.m.f. is measured using a PMMC instrument, calibrated directly in terms of temperature.
• No Temperature Difference:
  • When P and Q are at the same temperature, no e.m.f. is generated, and no current flows.

Reference Junction:

• Cold Junction:
  • Normally connected to the measuring instrument and held at 0°C.
  • Automatic reference compensation is often used for accuracy.

Specifications:

• Temperature Range:
  • 0 to 1800 °C.
• Display:
  • 3½ digit / 4 digit 7 Segment Red LED’s.
• Power Supply:
  • 230/110 VAC or 24 VDC.
• Accuracy:
  • ± 0.2% of the specific range.
• Resolution:
  • 0.1/1 °C, subject to specific input and range.
• Input Sensors:
  • Thermocouple types J, K, RTD Pt-100, 2 wire, 3 wire.
• Number of Set Points:
  • Two (Configurable for High & Low).
• Control Output:
  • 2 relay contacts NO/C/NC, rating 5 A @ 230 Vac.

Types of Thermocouples:

1. T – Type:
   • Positive wire: Cu, Negative wire: Constantan.
   • Up to 350°C, stable and inexpensive.
2. E – Type:
   • Positive wire: Chromel, Negative wire: Constantan.
   • Up to 850°C, highly sensitive and generates high output voltage.
3. J – Type:
   • Positive wire: Iron, Negative wire: Constantan.
   • Up to 1000°C, common type with high stability.
4. K — Type:
   • Positive wire: Chromel, Negative wire: Alumel.
   • Up to 1200°C, widely used and cost-effective.
5. S – Type:
   • Positive wire: Pt 10% Rhodium, Negative wire: Pt.
   • Up to 1400°C, high precision for accurate requirements.

Advantages:

• Follow temperature changes with a small time-lag.
• Convenient for measuring temperature at specific points.

Disadvantages:

• Low accuracy, not suitable for high precision.
• Prone to contamination, requiring protection.
• Placed at a large distance from the measuring device, introducing errors.