Thermocouple can be defined as a sensor or device which is used for sensing or measuring temperature. The thermocouples are consisting of two metals or wires and the metals are known as legs. Both legs are welded together so as to create two junctions. Temperature is measured in these junctions with the help of a reference of known temperature. Whenever the junction is experiencing a change or alteration in temperature, a difference in voltage is created which is then interpreted through the reference tables of thermocouple for the calculation of temperature. This article describes about Thermocouple Types, Temperature ranges and color codes. Following is the figure of a practical thermocouple.
Generic Circuit Diagram
The following is generic circuit diagram of a thermocouple.
Thermocouple Types & Ranges
There are numerous types of thermocouples and each has its un-matched characteristics and temperature ranges. Each thermocouple is different from other on the basis of durability, temperature ranges, resistance, and applications etc. The following are major types of thermocouples along with its subtypes explained in detail along with comparison of thermocouple types.
Thermocouples of Base Metals
The thermocouples of base metals are types E, T, N, J, and K etc. known as the most common categories. The material from which these thermocouples are made are inexpensive and commonly available metals such as iron, copper, and nickel etc. Below is details of each type of base metal thermocouples.
Type E :
The type E thermocouple is having chromel as a positive leg and constantan as negative leg. Chromel is having a percentage of 10 percent nickel while constantan has a 45 percent nickel. These thermocouples are having a temperature range of over -330 till 1600 degrees Fahrenheit. These are having greatest EMF versus temperature values and are also usable in the sub-zero temperatures. These can also be utilized in inert or oxidizing atmospheres, however these must not be used in the sulfurous atmospheres, lower oxygen or vacuums for avoiding dangers. The color of such thermocouples is either red or purple.
Type J :
The type J thermocouple is having iron as their positive legs which constantan as negative legs. These types of thermocouples are very usable in oxidizing, vacuum, inert, and reducing atmospheres. These thermocouples must be taken care of due to iron leg because it may get oxidized or rusted and hence special care is required for its operation. The temperature range of these types of thermocouples is from 32 till 1400 degrees Fahrenheit. The color for these thermocouples is either red or white.
Type K :
The type K thermocouple is having chromel as their positive leg and an alumel as their negative leg. Alumel is having 5 percent nickel and rest is silicon and aluminum. These types of thermocouples are suggested to be used in the environment of inert or oxidation. These thermocouples are having temperature ranges from -300 till 2300 degrees Fahrenheit. The color coding for these thermocouples are red or yellow.
Type N :
The type N thermocouple is having nicrosil as its positive leg and nisil as its negative leg. Nicrosil is having 14 percent of nickel and 1.5 percent of chromium and rest is silicon, while nisil is having 4.5 percent nickel, 0.1 percent of silicon, and rest is magnesium. These types of thermocouples are having temperature ranges from 32 till 2300 degrees Fahrenheit. The color coding for such thermocouples is either red or orange.
Type T :
The Type T thermocouple is having copper as its positive leg and constantan as its negative leg. Such thermocouples are having temperature ranges from -330 till 700 degrees Fahrenheit and color coding is either red or blue.
Thermocouples of Refractory Metals
The thermocouples of refractory metals are the considered as the last category of thermocouples which can be manufactured. These thermocouples are created from certain exotic metals which are rare and costly as well such as Rhenium and Tungsten etc. These thermocouples are very difficult to manufacture and must be handled with great care as these are brittle and have chance to break. There are a total of 3 types of such thermocouples which are explained below.
Type G :
These thermocouples are having Tungsten and 26 percent of Rhenium. The color code for such thermocouples are either red or white with blue tracer.
Type D :
These thermocouples are having 3 percent of tungsten and 26 percent of Rhenium. The color code for such thermocouples are either red or white with yellow tracer.
Type C :
These thermocouples are having 5 percent tungsten and 26 percent of Rhenium. The color code for such thermocouples are either red or white with red tracer.
All of the refractory types of thermocouples are having temperature ranges from 32 till 4200 degrees Fahrenheit. The main usages of such thermocouples are mainly in the vacuum furnaces which are having temperature ranges beyond the temperature capacity of platinum. These types of thermocouples are rarely used in applications where temperature range is below 2500 degrees Fahrenheit. These types of thermocouples and their metal protection tubes must not be utilized in the presence of oxygen when the temperature range is above 500 degrees Fahrenheit.
The types of thermocouples which are mentioned above are only for the refractory thermocouples and are used in industry designations, but these are not for the standardize thermocouples.
Rules for Thermocouples
There are some important rules for the use of thermocouples which must be taken in to consideration. The following are major rules.
- Red is always negative terminal or leg of thermocouples regardless of the type of thermocouples.
- Each and every type of thermocouples are having different color coding which are given in the handouts or manuals. The handouts will make you sure the proper use of thermocouple and calibrate it in accordance to proper technique.
Applications of Thermocouples.
There are hundreds of applications of thermocouples mainly in scientific, industrial, and OEM aspects. Thermocouples can be seen in almost every industry starting from Oil and Gas, Power Generation, Cement, Pharmaceutical, Pulp and Paper industry etc. There are numerous other applications of thermocouples such as toasters, furnaces, and stoves etc.
Selection Criterion of Thermocouples
Thermocouples are selected because of many reasons, some of which are mentioned below.
- Thermocouples have high temperature sensitivity ranges.
- These have lower costs.
- These have wide temperature ranges.
- These are durable and long lasting.