Thermocouple basic principal

Two different materials were joined at both ends and one end was at a different temperature than the other, a current was created. This phenomenon is known as the Seebeck effect and is the basis for all thermocouples.

A thermocouple is a type of temperature sensor, which is made by joining two dissimilar metals at one end. The joined end is referred to as the HOT JUNCTION. The other end of these dissimilar metals is referred to as the COLD END or COLD JUNCTION. The cold junction is actually formed at the last point of thermocouple material Positive leg Hot junction Cold Junction (Joined End) Negative leg.

BASE METAL THERMOCOUPLES

Base metal thermocouples are known as Types E, J, K, T and N and comprise the most commonly used category of Thermocouple. The conductor materials in base metal thermocouples are made of common and inexpensive metals such as Nickel, Copper and Iron.

NOBLE METAL THERMOCOUPLE TYPES

Noble Metal Thermocouples are another category of thermocouples and are made of the expensive precious metals Platinum and Rhodium. There are three types of noble metal thermocouples:

• Type B (Platinum/Platinum-30% Rhodium)
• Type R (Platinum/Platinum-13% Rhodium)
• Type S (Platinum/Platinum-10% Rhodium)

Types R and S have temperature ranges of 1000 to 2700 F°have Type B thermocouples  have a temperature range of 32 to 3100°F.

Refractory Metal Thermocouples
These thermocouples are made of the exotic metals Tungsten and Rhenium, which are expensive, difficult to manufacture, brittle, and must be handled carefully. There are three types of refractory metal thermocouples: 

• Type G (Tungsten/Tungsten 26% Rhenium)
• Type D (Tungsten 3% Rhenium/Tungsten 26% Rhenium)
• Type C (Tungsten 5% Rhenium/Tungsten 26% Rhenium

All of these types have a temperature range of 32-4200 deg F. Limits of error should be as per ASTM E230.