ANS:B - iron - constantan

The iron-constantan thermocouple is a type of temperature sensor commonly used for measuring temperatures in various industrial and scientific applications. Here's how it works and why it's used:

1. Composition: The iron-constantan thermocouple consists of two wires made of different materials: iron and constantan. Iron is used as one of the conductors, and constantan (a copper-nickel alloy) is used as the other conductor.
2. Principle: The operation of a thermocouple is based on the Seebeck effect, which states that when two dissimilar metals are joined together at two different temperatures, a voltage is generated across the junction proportional to the temperature difference. In the case of the iron-constantan thermocouple, when one end of each wire is connected together to form a junction, and the other ends are subjected to different temperatures, a voltage is generated.
3. Measurement Range: Iron-constantan thermocouples are suitable for measuring temperatures over a wide range, typically from -200°C to around 900°C. Beyond this range, the accuracy and reliability of the thermocouple may decrease.
4. Advantages:
   • Wide temperature range: Iron-constantan thermocouples can measure both high and low temperatures effectively.
   • Low cost: Compared to some other types of thermocouples, iron-constantan thermocouples are relatively inexpensive.
   • Durable: They are robust and can withstand harsh environments.
5. Applications:
   • Iron-constantan thermocouples are commonly used in industries such as manufacturing, food processing, and HVAC (heating, ventilation, and air conditioning) for temperature monitoring and control.
   • They are also used in laboratory settings for various scientific experiments and research activities.
6. Limitations:
   • Limited temperature range at high temperatures: While iron-constantan thermocouples can measure temperatures up to around 900°C, they may not be suitable for applications requiring higher temperature measurements.
   • Less accuracy at lower temperatures: At temperatures close to or below freezing, the accuracy of iron-constantan thermocouples may decrease.