Based on different temperature range there are many types of thermocouple which are listed and explain below.
Thermocouples are divided into three functional classes:
1. Base metal thermocouple (E, J, K, N, T)
2. Noble metal thermocouple (R, S, B)
3. Refractory metal thermocouple (C)
BASE METAL THERMOCOUPLES :
Base metal thermocouple types are composed of common, inexpensive metals such as nickel, iron and copper. The thermocouple types E, J, K, N and T are among this group and are the most commonly used type of thermocouple. Each leg of these different thermocouples is composed of a special alloy, which is usually referred to by their common names.
Type E – The type E thermocouple is made up of a positive leg of chromel (nickel/10% chromium) and a negative leg of constantan (nickel/45% copper). The temperature range for this thermocouple is –330 to 1600°F (-200 to 900°C). The type E thermocouple has the highest millivolt (EMF) output of all established thermocouple types. Type E sensors can be used in sub-zero, oxidizing or inert applications but should not be used in sulfurous, vacuum or low oxygen atmospheres. The color code for type E is purple for positive and red for negative.
Type J – Type J thermocouples have an iron positive leg and a constantan negative leg. Type J thermocouples have a useful temperature range of 32 to 1400°F (0 to 750°C) and can be used in vacuum, oxidizing, reducing and inert atmospheres. Due to the oxidation (rusting) problems associated with the iron leg, care must be used when choosing this type for use in oxidizing environments above 1000°F. The color code for type J is white for positive and red for negative.
Type K – The type K thermocouple has a Chromel positive leg and an Alumel (nickel/ 5% aluminum and silicon) negative leg. The temperature range for type K alloys is –328 to 2282°F (-200 to 1250°C). Type K sensors are recommended for use in oxidizing or completely inert environments. Type K and type E should not be used in sulfurous environments. Because type K has better oxidation resistance then types E, J and T, its main area of usage is at temperatures above 1000°F but vacuum and low oxygen conditions should be avoided.
Type N – Type N thermocouples are made with a Nicrosil (nickel – 14% chromium – 1.5 % silicon) positive leg and a Nisil (nickel – 4.5% silicon - .1% magnesium) negative leg. The temperature range for Type N is –450 to 2372°F (-270 to 1300°C) and the color code is orange for positive and red for negative. Type N is very similar to Type K except that it is less susceptible to selective oxidation. Type N should not be used in vacuum and or reducing environments in an unsheathed design.
Type T – Type T thermocouples are made with a copper positive leg and a constantan negative leg. The temperature range for type T is –328 – 662°F (-200 to 350°C) and the color code is blue for positive and red for negative. Type T sensors can be used in oxidizing (below 700°F), reducing or inert applications.
NOBLE METAL THERMOCOUPLES :
Noble metal thermocouples are manufactured with wire that is made with precious or “noble” metals like Platinum and Rhodium. Noble metal thermocouples are for use in oxidizing or inert applications and must be used with a ceramic protection tube surrounding the thermocouple element. These sensors are usually fragile and must not be used in applications that are reducing or in applications that contain metallic vapors.
Type R – Type R thermocouples are made with a platinum/13% rhodium positive leg and a pure platinum negative leg. The temperature range for type R is 32 – 2642°F (0 to 1450°C) and the color code is black for positive and red for negative.
Type S – Type S thermocouples are made with a platinum/10% rhodium positive leg and a pure platinum negative leg. The temperature range for type S is 32 – 2642°F (0 to 1450°C) and the color code is black for positive and red for negative.
Type B – Type B thermocouples are made with a platinum/30% rhodium positive leg and a platinum/6% Rhodium negative leg. The temperature range for type r is 32 – 3092°F (0 to 1700°C) and the color code is gray for positive and red for negative.
REFRACTORY METAL THERMOCOUPLES :
Refractory metal thermocouples are manufactured with wire that is made from the exotic metals tungsten and Rhenium. These metals are expensive, difficult to manufacture and wire made with these metals are very brittle. These thermocouples are intended to be used in vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 500°F.
Type C – The type C thermocouple is made with a tungsten/5% rhenium positive leg and tungsten 26% rhenium negative leg and has a temperature range of 32 – 4208°F (0 – 2320°C). The color code for this type is white with red tracer for positive leg and red for the negative leg.
POLARITY OF THERMOCOUPLE :
Standard practice is to use colour red for negative lead. Other standards are the negative lead of bare wire thermocouple is approximately ¼”shorter than the positive lead and the large pin on a thermocouple connector is always the negative conductor. Two types of thermocouples (types J and K) have one leg, which is magnetic. With these 2 types, you can use a magnet to determine polarity.
Easy order to remember types of thermocouple (S, R, K, T, B, C, E, N).