Thermocouple:

A thermocouple is a device that converts temperature differences into an electric voltage, based on
the principle of the thermoelectric effect. It is a type of sensor that can measure temperature at a
specific point or location. Thermocouples are widely used in various fields, such as industrial,
domestic, commercial, and scientific applications, because of their simplicity, durability, low cost,
and wide temperature range.
The thermoelectric effect is the phenomenon of generating an electric voltage due to a temperature
difference between two different metals or metal alloys. This effect was discovered by German
physicist Thomas Seebeck in 1821, who observed that a magnetic field was created around a closed
loop of two dissimilar metals when one junction was heated, and the other was cooled.
The thermoelectric effect can be explained by the movement of free electrons in the metals. When
one junction is heated, the electrons gain kinetic energy and move faster toward the colder junction.
This creates a potential difference between the two junctions, which can be measured by a
voltmeter or an ammeter. The magnitude of the voltage depends on the type of metals used and the
temperature difference between the junctions.
A thermocouple consists of two wires made of different metals or metal alloys, joined together at
both ends to form two junctions. One junction called the hot or measuring junction, is placed at the
location where the temperature is to be measured. The other junction called the cold or reference
junction, is kept at a constant and known temperature, usually at room temperature or in an ice
bath.
When there is a temperature difference between the two junctions, an electric voltage is generated
across the thermocouple circuit due to the thermoelectric effect. This voltage can be measured by a
voltmeter or an ammeter connected to the circuit. By using a calibration table or a formula that
relates the voltage to the temperature for a given type of thermocouple, the temperature of the hot
junction can be calculated.

Types of Thermocouple:
There are many types of thermocouples available, each with different characteristics and
applications. The type of thermocouple is determined by the combination of metals or metal alloys
used for the wires. The most common types of thermocouples are designated by letters (such as K,
J, T, E, etc.) according to international standards.
The following table summarizes some of the main types of thermocouples and their properties:
Temperature
Type Positive Wire Negative Wire Color Code Sensitivity Accuracy Applications
Range

Nickel- Yellow (+),

Nickel-aluminum -200°C to General purpose,
chromium Red (-), ±2.2°C
K (95% Ni, 2% Al, +1260°C (-328°F 41 µV/°C wide range, low
(90% Ni, 10% Yellow (0.75%)
2% Mn, 1% Si) to +2300°F) cost
Cr) (overall)

White (+),
-210°C to +750°C Oxidizing
Iron (99.5% Constantan (55% Red (-), ±2.2°C
J (-346°F to 50 µV/°C atmospheres,
Fe) Cu, 45% Ni) Black (0.75%)
+1400°F) limited range
(overall)

Blue (+), Red -200°C to +350°C Low temperatures,

Copper (99.9% Constantan (55% ±1°C
T (-), Brown (-328°F to 43 µV/°C oxidizing
Cu) Cu, 45% Ni) (0.75%)
(overall) +662°F) atmospheres

Nickel-
Purple (+),
chromium Constantan (55%
E Red (-),
(90% Ni, 10% Cu, 45% Ni)
Purple
Cr)

Thermocouples have many advantages and disadvantages compared to other

temperature sensors, such as RTDs (Resistance Temperature Detectors), thermistors, or
infrared sensors.
Some of the advantages of thermocouples are:
 They can measure a wide range of temperatures, from cryogenic to very high
temperatures.
 They are simple, robust, and reliable devices that can withstand harsh environments
and vibrations.
 They are inexpensive and easy to install and replace.
 They have a fast response time and can measure dynamic temperature changes.
 They do not require external power or amplification for their operation.
Some of the disadvantages of thermocouples are:
 They have low accuracy and stability compared to other sensors.
 They are susceptible to errors due to corrosion, oxidation, contamination, or aging
of the wires.
 They require a reference junction at a known temperature for accurate
measurement.
 They have a non-linear output that requires complex calibration or compensation.
 They may generate unwanted thermoelectric voltages due to parasitic junctions in
the circuit.
There are many factors to consider when selecting the right thermocouple for your
application, such as:
 The temperature range and accuracy required for your measurement.
 The chemical compatibility and durability of the thermocouple wires with the
environment where they will be exposed.
 The physical size and shape of the thermocouple probe or wire that will fit your
installation.
 The electrical characteristics and noise immunity of the thermocouple circuit and
the measuring instrument.
 The availability and cost of the thermocouple type and accessories.
To select the right thermocouple for your application, you can follow these steps:
1. Determine the temperature range and accuracy that you need for your
measurement. Choose a thermocouple type that covers your temperature range and
has an acceptable accuracy level for your application. Refer to the table above for a
comparison of different types of thermocouples.
2. Check the chemical compatibility and durability of the thermocouple wires with the
environment where they will be exposed. Choose a thermocouple type that has
good resistance to corrosion, oxidation, contamination, or aging in your
environment. For example, type K thermocouples are suitable for oxidizing
atmospheres, while type T thermocouples are suitable for low temperatures and
oxidizing atmospheres.
3. Select the physical size and shape of the thermocouple probe or wire that will fit
your installation. Choose a thermocouple probe that has the appropriate shape,
length, diameter, and tip for your application. For example, you can choose from
needle probes, surface probes, air probes, penetration probes, or immersion probes.
You can also choose from exposed, grounded, or ungrounded junctions, depending
on the response time and insulation required.
4. Choose the electrical characteristics and noise immunity of the thermocouple circuit
and the measuring instrument. Choose a thermocouple type that has a suitable
output voltage and sensitivity for your measuring instrument. You may also need to
consider the resistance, capacitance, and inductance of the thermocouple wires and
connectors. To reduce noise and interference in the thermocouple circuit, you can
use shielded wires, twisted pairs, or differential inputs. You can also use filters,
amplifiers, or signal conditioners to improve the signal quality.
5. Compare the availability and cost of the thermocouple type and accessories. Choose
a thermocouple type that is readily available and affordable for your application.
You may also need to consider the cost of accessories, such as connectors, extension
wires, terminals, adapters, or mounting hardware.
To install and maintain a thermocouple properly, you should follow these steps:
1. Select a suitable location for the thermocouple probe or wire that will ensure good
contact with the object or medium whose temperature is to be measured. Avoid
locations that are exposed to excessive heat, moisture, corrosion, vibration, or
mechanical stress.
2. Connect the thermocouple wires to the measuring instrument according to the
polarity and color code of the thermocouple type. Use appropriate connectors,
terminals, or adapters that match the thermocouple type and size. Avoid loose or
broken connections that may cause errors or noise in the thermocouple circuit.
3. Calibrate the thermocouple and the measuring instrument before use to ensure
accuracy and consistency of measurement. You can use a reference thermometer or
a calibration source to compare the thermocouple output with a known temperature
value. You can also use a calibration table or a formula to correct for any deviation
or error in the thermocouple output.
4. Check the thermocouple regularly for any signs of damage, corrosion,
contamination, or aging that may affect its performance or reliability. Replace the
thermocouple if it shows any signs of deterioration or failure.
Application:
Thermocouples are used in a wide range of applications across various industries and
domains. Some of the common applications of thermocouples are:
 Steel and iron industries: Thermocouples are used to monitor the temperature and
chemistry of molten metal during various stages of the steel-making process. Types
B, S, R, and K thermocouples are commonly used in electric arc furnaces, ladles,
tundishes, molds, and rollers.
 Gas appliances: Thermocouples are used to detect the presence of a pilot flame in
gas heaters, boilers, ovens, stoves, and fireplaces. If the pilot flame goes out, the
thermocouple shuts off the gas supply to prevent gas leakage or explosion.
 Thermopile radiation sensors: Thermopiles are arrays of thermocouples connected
in series that measure the intensity of incident radiation (especially visible and
infrared light). They are used in devices such as pyrometers, radiometers,
spectrometers, thermal cameras, and solar panels.
 Manufacturing: Thermocouples are used to measure and control the temperature of
various processes and products in manufacturing industries such as food
processing, chemical processing, pharmaceutical, aerospace, automotive, and
biomedical industries. Types K, J, T, E, and N thermocouples are commonly used to
measure and control the temperature of various processes and products in these
industries.
 Power production: Thermocouples are used to measure and monitor the
temperature of various components and systems in power plants, such as boilers,
turbines, generators, transformers, reactors, and fuel cells. Types R, S, B, K, and N
thermocouples are commonly used in power production applications.
 Process plants: Thermocouples are used to measure and control the temperature of
various fluids and gases in process plants, such as oil refineries, petrochemical
plants, gas pipelines, and water treatment plants. Types K, J, T, E, and N
thermocouples are commonly used in process plant applications.
 Thermocouples as vacuum gauge: Thermocouples can be used to measure the
pressure of a vacuum by measuring the temperature difference between a heated
wire and an unheated wire in a thermocouple circuit. The pressure of the vacuum is
inversely proportional to the temperature difference. This type of vacuum gauge is
known as a thermocouple gauge or a Pirani gauge.