Thermocouple sensors consist of two wires composed of distinct metals, joined at two locations. The temperature variation is indicated by the voltage difference between these two wires.
While they may offer slightly less accuracy compared to RTDs, thermocouples cover a broad temperature range from -328°F to 3182°F (-200°C to 1750°C) and are typically more budget-friendly.
Thermocouples come in different types, each designed to handle particular temperature ranges. These various types are tailored to suit specific application requirements.
Uses for Thermocouple Types:
E: suitable for vacuum, inert, mildly oxidizing, or reducing conditions
J: used where there is limited oxygen
K: requires metal or ceramic protection
N: resists oxidation from sulfur
T: used in oxidizing or reducing environments
S, R, and B: must be protected with a form of tubing and used for high temperature applications
C (tungsten/rhenium): very common; requires protective sheathing and used for high temperature applications
A: a variant of type C and has limited use
Type E Thermocouples - Type E thermocouples use a combination of chromel (a nickel-chromium alloy) and constantan. They have a temperature range of -330°F to 1600°F (-200°C to 870°C) and provide excellent EMF (electromotive force) versus temperature characteristics. These thermocouples are suitable for use in sub-zero temperatures and inert environments but need protection when used in sulfurous conditions.
Type J Thermocouples - Type J thermocouples, similar to Type K, are general-purpose sensors made from iron and constantan, with the iron leg being positive and the constantan leg negative. They can be used either exposed or with a protective tube, which is recommended for durability. Type J thermocouples are suitable for use in vacuum, inert, and reducing environments. Like Type K thermocouples, Type J thermocouples require careful calibration and may be sensitive to electrical noise.
Type K Thermocouples - Type K thermocouples are made of Chromel®–Alumel® with small percentages of manganese and silicone. They are a general purpose thermocouple with a temperature range of -328°F up to 2462°F (-200°C up to 1350°C). Type K thermocouples need to be carefully calibrated and have small output signals. They are used in an assortment of environments including water, mild chemicals, gasses, and dry conditions. Common industries that use type K thermocouples are hospitals and food preparation. Regardless of their wide temperature range, type K thermocouples are mostly used for temperatures over 1004°F (540°C).
Type N Thermocouples: These thermocouples are composed of nicrosil (a nickel-chromium alloy) and nisil (an alloy of nickel, silicon, and magnesium). They operate within a temperature range of 32°F to 2300°F (650°C to 1260°C) and are known for their resistance to green rot and hysteresis. Common applications include use in refineries and the petrochemical sector.
Type R Thermocouples: Constructed from a combination of platinum and rhodium, Type R thermocouples can measure temperatures up to 2700°F (1480°C). They require protection through a gas-tight ceramic tube and an additional outer tube. These thermocouples offer enhanced stability and a broader temperature range compared to Type S. They are utilized in fields such as heat treatment, control sensors, semiconductor manufacturing, glass production, and metal processing.
Type S Thermocouples: Known for their precision and stability, Type S thermocouples are ideal for both high-temperature applications in the BioTech and Pharmaceutical industries as well as low-temperature scenarios. They cover a temperature range of -58°F to 2700°F (980°C to 1450°C).
Type T Thermocouples: Featuring copper and constantan, Type T thermocouples are suitable for temperatures ranging from -330°F to 700°F (-200°C to 370°C). They are used in inert atmospheres and are resistant to decomposition even in the presence of moisture. Typical applications include food production and cryogenic environments.
Type B Thermocouples: Type B thermocouples are designed for high-temperature applications and offer the highest temperature range among thermocouples, with remarkable accuracy and stability. Their construction includes platinum with 6% rhodium and platinum with 30% rhodium, operating between 2500°F and 3100°F (1370°C to 1700°C).
Type C Thermocouples: These thermocouples use tungsten and rhenium and are capable of measuring temperatures up to 4200°F (2315°C). They are suitable for use in hydrogen, inert, or vacuum environments to avoid oxidation failure. Type C thermocouples are equipped with protective sheaths made from molybdenum, tantalum, or inconel, and feature insulators such as alumina, hafnia, or magnesium oxide.
