**3.3 Base metal types**

Type E, Chromel (nickel-10% chromium) (+) vs. Constantan (nickel-45% copper (-)). Type E is recommended for use to 900°C (1600°F) in oxidizing or inert atmospheres. Type E has been recommended as the most suitable of all standardized types for general lowtemperature use, about -230°C (-380°F), since it offers the best overall combination of desirable properties i.e. high thermopower, low thermal conductivity, and reasonably good thermoelectric homogeneity typical values for the thermopower of type E at 4, 20, and 50K are 2.0, 8.5 and 18.7VK-1 respectively (Spark *et al*., 1972).

Type **J,** Iron (+) vs. Constantan (nickel-45% copper (-)) is one of the most commonly used thermocouples in industrial pyrometry due to its relatively high thermopower and low cost. These thermocouples are suitable for use in vacuum, air, reducing, or oxidizing atmospheres to 760°C (1400°F) in the heavier gage sizes. Rapid oxidation of the iron wire at temperatures above 540°C (1000°F) limits the expected service life of the finer sized wires.

Types K (Chromel (nickel-10% chromium) (+) vs. Alumel (nickel-5% aluminum and silicon (-)) and T (Copper (+) vs. Constantan (nickel-45% copper) (-) thermocouples are often used below 0oC, but type J is not suitable for general low-temperature use because the positive thermo element (noted as JP) is composed of iron and thus is subject to rusting and embrittlement in moist atmospheres. Type K is more resistant to oxidation at elevated temperatures than types E, J and T and consequently it finds wide application at temperatures above 500oC. Type E has the highest thermopower above 0oC of any of the standardized types.

Type N, Nicrosil (nickel-14% chromium, silicon) (+) vs. Nisil (nickel-4% silicon, magnesium) (-). This type differs from type K by having silicon in both legs and containing magnesium in the negative leg. It was developed to be more stable (exhibit less calibration drift) than type K when used at temperatures above about 1000°C (1800°F). Both type N wires are similar in color and both are non-magnetic, so identification is usually made by gently heating the junction and observing the polarity of the resultant emf.
