What is a Thermocouple?
A thermocouple is defined as a thermal junction that functions based on the phenomenon of the thermoelectric effect, i.e. the direct conversion of temperature differences to an electric voltage. It is an electrical device or sensor used to measure temperature.
A thermocouple can measure a wide range of temperatures. It is a simple, robust, and cost-effective temperature sensor used in various industrial applications, home, office, and commercial applications.
In 1821, German physicist Thomas Johann Seebeck found that a magnetic field is produced when two different metals are connected at one end and create a temperature difference between two ends.
He observed that due to the magnetic field the voltage is induced by the thermoelectric effect. However, this voltage is very small (in terms of mV) and depends on the type of metal used in the thermocouple.
According to the applications, different designed configurations of thermocouples include thermocouple probes, probes with connectors, infrared thermocouples, base wire thermocouples, and just thermocouple wire.
How does a Thermocouple Work?
A thermocouple consists of two plates of different metals. Both plates are connected at one end and make a junction.
The junction is placed on the element or surface where we want to measure the temperature. This junction is known as a hot junction. And the second end of the plate is kept at a lower temperature (room temperature). This junction is known as a cold junction or reference junction.
According to the Seebeck effect, the temperature difference between the two different metals induces the potential differences between two points of the thermocouple plates.
If the circuit is closed, a very small amount of current will flow through the circuit. A voltmeter is connected to the circuit. The voltage measured by the voltmeter is a function of a temperature difference between two junctions.
Hence, by measuring the voltage, we can calculate the temperature of the hot junction.
How measurement of temperature is achieved using a thermocouple is expanded upon in the video below:
Types of Thermocouples
According to different types of combinations of alloys, the thermocouples are available in different types. The type of thermocouple is chosen according to the application, cost, availability, stability, chemical properties, output, and temperature ranges.
Here we will discuss different types of thermocouples with their characteristics.
Type K Thermocouple
The K-type thermocouple is the most common type of thermocouple, and it has the widest temperature measuring range.
The positive lead of Type K thermocouple is composed of approximately 90% nickel and 10% chromium. The negative lead is composed of approximately 95% nickel, 2% aluminum, 2% manganese, and 1% silicon.
The positive lead is colored yellow and it is a non-magnetic material. The negative lead is colored red and it is a magnetic material. And the overall jacket is colored yellow.
The temperature range of type K thermocouple is -200˚C to +1260˚C (-328 F to +2300 F). It is inexpensive and widely used in general-purpose applications where temperature sensitivity requires approximately 41μV/˚C.
The accuracy of type K thermocouple is ±2.2 C% (0.75%). The accuracy of thermocouples also depends on the deviation in alloys.
Type K thermocouple performs best in a clean oxidizing atmosphere. It is not recommended to use for partially oxidizing conditions in a vacuum.
Type T Thermocouple
This type of thermocouple is suitable to measure low temperature. The positive lead is composed of copper and the negative lead is composed of constantan (45% nickel and 55% copper).
The sensitivity of the type T thermocouple is 43 µV/°C. This thermocouple is suitable to works in an oxidizing atmosphere. The temperature range of this thermocouple is between -200°C to 350°C.
Type J Thermocouple
This type of thermocouple is a low-cost and most used thermocouple. The positive lead is made of iron and a negative lead is made of constantan (45% nickel and 55% copper).
The positive lead is colored white and the negative terminal is colored red. And the overall jacket is colored black.
The temperature range of type J thermocouple is between -210˚C to 750˚C (-346F to 1400F). This type of thermocouple has a smaller temperature range and short life span compared to type K thermocouple. But this type of thermocouple is well suited for oxidizing atmospheres.
The accuracy of this type of thermocouple is ±2.2˚C (0.75%). This type of thermocouple is not recommended for lower-temperature applications. And the sensitivity of this type of thermocouple is approximately 50μV/˚C.
Type E Thermocouple
This type of thermocouple has higher accuracy and stronger signal than type K and J thermocouple at moderate temperature ranges. And it is a more stable type thermocouple than type K. The type E thermocouple produces the highest EMF per degree than other types of thermocouple.
The positive lead is composed of nickel-chromium (90% nickel and 10% chromium) and a negative terminal is composed of constantan (95% nickel, 2% aluminum, 2% manganese, and 1% silicon). The positive lead is colored purple and the negative lead-colored red.
The temperature range of this type of thermocouple is between -270˚C to 870˚C (454 to 1600F). And standard accuracy is ±1.7˚C%.
This type of thermocouple is used where high accuracy and fast response are required. It is not used for vacuum or low oxygen applications and Sulphuric environments. The cost of type E thermocouples is more than the type J and K thermocouples.
Type N Thermocouple
Type N thermocouple is designed by the Defense Science and Technology Organization (DSTO) of Australia, by Noel A. Burley. The accuracy and temperature limits of type N thermocouple is the same as the type K thermocouple.
The temperature range of type N thermocouples is between -270 °C and 1300 °C. Sensitivity is slightly lower than type K thermocouple and it is 39μV/˚C.
Type N thermocouple is composed of Nicrosil and Nisil. Where Nicrosil is a combination of Nickel, Chromium, and Silicon. And it made positive wire. The Nisil is a combination of Nickel and Silicon. And it made negative wire.
This thermocouple is the best alternative to type K thermocouples for low oxygen conditions. This type of thermocouple is suitable to use in vacuum, oxidizing atmosphere, inert atmosphere, or dry atmosphere.
Type S Thermocouple
Type S thermocouples are used for higher temperature applications. Because of high accuracy and stability, sometimes it is used for low temperature also.
The temperature range of type S thermocouples is between 630 °C to 1064 °C. The positive lead is composed of 90% platinum, 10% Rhodium, and the negative led is composed of Platinum.
Generally, this type of thermocouple is used in the application like Pharmaceutical and Biotech industries where the high temperature should be measure with high accuracy.
Type R Thermocouple
Type R thermocouple is also composed of Platinum and Rhodium. But the output range and stability of the type R thermocouple is slightly more than the type S thermocouple.
The positive lead pf type R thermocouple is composed of 87% Platinum and 13% Rhodium. And the negative lead is composed of Platinum. The temperature range of this thermocouple is between 0 °C to 1600 °C.
Type B Thermocouple
Type B thermocouples are also made up of a combination of Platinum and Rhodium. The positive lead of a thermocouple is composed of 70% Platinum and 30% Rhodium. And the negative lead is composed of 94% Platinum and 6% Rhodium.
Type B thermocouple is used to measure the temperature up to 1800°C. But the output of this thermocouple is lower compared to type R and S thermocouples.
How Do You Know if You Have a Bad Thermocouple?
To understand when we have a bad thermocouple, we first have to understand the working principle of a good thermocouple (one that is working)
A thermocouple works through the thermoelectric effect i.e. the direct conversion of temperature differences to an electric voltage. When the probes of a thermocouple are placed on a surface whose temperature we want to measure, the probes are at slightly different temperatures.
Due to this temperature difference, an EMF is produced. And this EMF is proportional to the temperature.
You can measure the generated EMF with the help of a millivoltmeter. The millivoltmeter is attached with both probes of a thermocouple.
Now if you increase the temperature, the generated EMF should also increase.
So if the EMF reading is not varying with respect to the temperature, then the thermocouple is bad / not working properly.
Before using a thermocouple, you must have the reference datasheet of the thermocouple you are using. From the datasheet, you can find the table of temperature and corresponding EMF.
RTD vs Thermocouple
RTD (Resistance Thermometer Detectors) and Thermocouple both devices are used to measure the temperature. And it is difficult to conclude which is better for overall performance as a temperature sensor.
But if you specify some performance parameters like cost, range of temperature, ruggedness, and speed of measurement, the thermocouple has a better performance compared to RTD.
The cost of a thermocouple is much less (almost 2.5 to 3 times) compared to RTD. And also, the cost of installation is cheaper. The RTD is designed to measure a limited range of temperatures.
The advantage of an RTD is that it is more accurate compared to the thermocouple. And the repeatability of measurement is more compared to the thermocouple. Hence, RTD is preferred in the application where the most accurate temperature is required.
So, both devices have their advantages and disadvantages. The thermocouple has a wide range of temperature measurements, is cheaper, and is durable. On the other hand, RTD has better accuracy and reliable measurement.
Thermocouple Color Codes
The color code of thermocouples is different according to different country standards. In the below table, we have discussed color code with different standards.
How Long Should a Thermocouple Last?
The life span of a thermocouple depends on the application where it is used. Hence, we cannot exactly define the life span of the thermocouple.
If you maintain it properly, it will last up to years. But, after some years of continuous use, maybe there will be an aging effect. And due to this, it will generate a weak output signal.
The cost of the thermocouple is not much higher. Hence, it is recommended to change the thermocouple after 2 to 3 years.
The applications of thermocouples are listed below:
- It is used to monitor the temperature in the steel and iron industries. For, this type of application, type B, S, R, and K thermocouples are used in the electric arc furnace.
- The principle of a thermocouple is used to measure the intensity of incident radiation (especially visible and infrared light). This instrument is known as a thermopile radiation sensor.
- It is used in the temperature sensors in thermostats to measure the temperature of the office, showrooms, and homes.
- The thermocouple is used to detect the pilot flame in the appliances that are used to generate heat from gas like a water heater.
- To test the current capacity, it is installed to monitor the temperature while testing the thermal stability of switchgear equipment.
- The number of thermocouples is installed in the chemical production plant and petroleum refineries to measure and monitor temperature at different stages of the plant.