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What is a thermocouple? What is its importance in the industry?

Termopares Alutal - excelência em termopares

Temperature setting

Temperature has always been very important for humanity, it has been one of the pillars of our evolution, contributing to safety, survival and even well-being. Nowadays it has a profound impact on our daily lives, whether in the measurement of ambient temperature, body temperature, electrical and electronic equipment, temperature of combustion engines and especially in industrial processes.

We can conceptualize temperature as the degree of thermal agitation of the molecules that constitute the substances, and the physical magnitude allows the internal evaluation of a body, the second most used in the world, only wasting for time.

Thermal agitation of molecules - Alutal
Thermal agitation of molecules - Alutal

All modern industrial processes at some point in the production chain benefit from temperature measurement and control,thus providing the control required by current legislation.

There are 3 common ways to refer to temperance measurement:

  1. Thermometry: temperature measurement
  2. Pyrometry: measuring high temperatures
  3. Cryometry: low temperature measurement

Thermocouple, what is it anyway?

The thermocouple (also called thermocouple) is one of the most important sensors used in temperature measurement in the most varied industrial segments.

They can have the most varied shapes and dimensions according to their use
They can have the most varied shapes and dimensions according to their use

They are the simple, robust and low cost temperature sensors used in the most varied processes since their measurement capacity can be applied to a wide temperature range.

Thermocouples are the most suitable sensors for measuring temperatures from a few negative tens to thousands of degrees Celsius. They are the most widely used temperature sensors in the world.

Consisting of two distinct metals, joined at their ends and connected to a thermocouple thermometer or other thermocouple able device, they form a closed circuit that generates an electromotive force when the two joints (T1 and T2) are maintained at different temperatures.

Thermocouple Working Diagram
Thermocouple Working Diagram

Alutal is now recognized as the leading manufacturer of thermocouples in Brazil, producing the sensors according to its design, thus ensuring that they are 100% suitable for measuring your process.

Learn more about the physical principles of thermoelements on our Wiki:

Seebeck effect

The electrical reaction was discovered by chance in 1821 by estonian physicist Thomas Seebeck.

When he noticed that, in a closed circuit, formed by two distinct conductors A and B, a current circulation occurs as long as there is a Temperature Difference DT between its junctions.

We call the measuring joint Tm, and the other, Tr reference joint. The existence of a thermal f.e.m. AB in the circuit is known as the Seebeck effect.

Whenever the temperature of the reference joint is kept constant, it is verified that the thermal f.e.m. is a function of the Tm temperature of the test joint, thus, this fact allows to use a thermoelectric pair as a thermometer.

Seebeck effect
Seebeck effect

Thermocouple Types

They are available in various types or calibrations, it is important to carefully select the appropriate thermocouple for your application.

The table below lists the main types of thermocouples:

Calibration Positive Element Negative Element Usual Temperature Range Standard Error Line (Choose the Largest) Special Error Line (Choose the Largest)
Type T Copper
Constantan -200°C~0°C +/-1°C ou +/-1,5% --/--
Type T Copper Constantan 0°C~370°C +/-1°C ou +/-0,75% +/-0,5°C ou +/-0,4%
Type J Iron Constantan 0°C~760°C +/-2,2°C ou +/-0,75% +/-1,1°C ou +/-0,4%
Type E Cromel Constantan 0°C~870°C +/-1,7°C ou +/- 0,5% +/-1,0°C ou +/-0,4%
Type K Cromel Alumel 0°C~1260°C +/-2,2°C ou +/- 0,75% +/-1,1°C ou +/-0,4%
Type N Nicrosil Nisil 0°C~1260°C +/-2,2°C ou +/- 0,75% +/-1,1°C ou +/-0,4%
Type S 90% Platinum / 10% Rhódio Pt 100% 0°C~1480°C +-1,5°C ou +/- 0,25% +/-0,6°C ou +/-0,1%
Type R 87% Platinum / 13% Rhódio Pt 100% 0°C~1480°C +/- 1,5°C ou +/- 0,25% +/-0,6°C ou +/-0,1%
Type B 70% Platinum / 30% Rhódio 94% Platinum / 06% Rhódio 870°C~1700°C +/- 0,5% +/-0,25%

Thermocouple Type B

It has characteristics very similar to those of models R and S. They are more stable, however, due to their reduced sensitivity, they are usually used only to measure temperatures above 300°C, up to 1800°C.

It can also be suitable for vacuum use for short periods. However, it should not be used in reducing or vapor-containing atmospheres, both metallic and non-metallic. This equipment must not be inserted directly into a primary metal protective tube, and requires the use of high alumina ceramic insulators and protective tubes.

Type E Thermocouple

It can be used in oxidizing, inert or vacuum atmospheres. However, it is not indicated for alternating oxidizing and reducing atmospheres.

Compared to other thermocouples usually used, it has higher thermoelectric power – something very advantageous for those who want to detect small temperature variations.

Thermocouple Type J

It can be used in reducing, neutral or oxidizing atmospheres. However, it is not recommended in atmospheres with high moisture content and also at low temperatures, as the thermoelement JP can become brittle.

Thermocouple Type K

It stands out for being of generic use. It has a low cost, and for its popularity is available in the most diverse probes. The temperatures covered by this product range from -200°C to 1200°C.

Thermocouple Type N

Its great stability and resistance to oxidation at high temperatures makes it more suitable for measurements at high temperatures, without resorting to thermocouples that incorporate platinum in their constitution (types B, R and S). It was designed to be a K-type 'evolution'.

Thermocouple Type S

It can be used in inert or oxidizing atmospheres. It presents a good stability index when exposed to high temperatures over time, standing out for being higher than that of non-platinum thermocouples.

However, the thermoelements of this appliance must not be exposed to reducing atmospheres or metal vapours. The indicated is that they are never inserted directly into metal protection tubes, but first in a ceramic protection tube with the following specifications: manufactured with alumina (Al2O3) of high purity (99.7%), commercially called type 799 (former 710).

However, it is worth mentioning that ceramic tubes with a lumina content of 67% are available on the market, called type 610, but their use for platinum thermocouples is not recommended.

Type T Thermocouple

It is indicated for inert, oxidizing or reducing atmospheres. It has good precision because of a great homogeneity with which copper can be processed.

At temperatures above 300°C, copper oxidation becomes very intense, thus reducing its useful life and causing deviations in its original response curve.

Thermocouple Type R

They have the same characteristics of Type S Thermocouples, being suitable for measuring temperatures up to 1600 °C, but because their higher cost is not so common their use in the industry in general.

What is the difference between thermocouple and thermoresistance?

There are two very usual thermoelements for temperature measurement: thermocouples and thermoresistors. Despite the similar names, each of them has its own characteristics and functionings.

Thermocouples transform thermal energy into kinetic energy and are suitable for high temperatures up to 1700°C, have low cost and are used in the most varied processes and in wide temperature ranges.

While thermoresistances are high precision sensors and good reading repeatability, based on the principle of variation of the electrical resistivity of a metal as a function of temperature.

Generally the thermoresistance is made of platinum, but other materials can also be used, such as nickel and its range of use range from -200ºC to 650ºC.

The main differences between them are:

Thermocouple Thermoresistance
- Based on voltage
- Higher resistance to temperatures
- More economical
- Based on resistance
- Higher accuracy
- Resistance curve as a function of the most linear temperature

Learn about the top 5 advantages of thermocouple on Thermoresistence (RTD)

  1. Lower cost when compared to RTD in the same range
  2. Temperature range greater than thermoresistors
  3. Better response time
  4. Mechanically more robust
  5. Can be used in places with a lot of vibration

How to choose the correct temperature sensor?

It is essential that it is correctly specified respecting your application.

Using sensors with inappropriate characteristics can lead to measurement errors, process failures, and low service life.

Our engineers highlight 5 points to be observed:

  1. Determine in detail the application where it will be installed
  2. Carefully analyze the temperature variation at the location that the thermocouple will be exposed to
  3. Carefully evaluate any chemical resistance that the thermocouple will be exposed during its operation
  4. Consider the need for abrasion and/or vibration resistance
  5. Create a list with all installation requirements (observe compatibility with existing equipment; existing holes can determine probe diameter, etc.)

Importance of temperature sensor calibration

As important as choosing the appropriate sensor is its proper calibration.

It is through it that we can validate whether the instrument corresponds between a known or standardized physical magnitude and its readings.

Calibration should follow market standards such as CGCRE (General Accreditation Coordination) RBC Accreditation, ABNT NBR ISO/IEC 17025,among others.

Find out how much 1°C of variation in fuel costs in your process

Have you ever stopped to think how much 1°C costs inside your process?

How much fuel, energy do you spend financially to heat or cool your process over the course of 1 year?

Alutal's calculating engineers developed a powerful algorithm that based on the variables of your process (such as material type, fuel, calorific value, etc.) calculates and delivers an exclusive report showing the cost of 1ºC in your company in 1 year.

Thermocouples manufactured by Alutal

Frequently asked questions

Below we list the most frantic questions about Thermocouples and temperature measurement. Click on the title to access the content.

What is the difference between Heat and Energy?

Heat is thermal energy in transit, which occurs because of the temperature differences between the bodies or systems involved. To deepen, it is worth studying about the first law of thermodynamics, which is the principle of energy conservation applied to thermodynamic systems.

Energy is the ability that a body has to perform work. To deepen, research on the second law of thermodynamics, which shows us the limitations imposed by nature when it turns heat into work.

What are the forms of heat transfer?

Heat is a type of energy that can be transferred from one body to another when there is temperature difference between them. Heat transfer can occur in three ways: radiation, conduction and convection.

Thermal radiation, also known as irradiation,is a form of heat transfer that occurs through electromagnetic waves. As these waves can propagate in the vacuum, there is no need for contact between the bodies for heat transfer.

Driving is the process of transferring energy through a material medium, without transporting matter. thermal energy propagates from particle to particle of the medium. It occurs mainly in solid materials, this process is more efficient in materials such as metals, which are good heat conductors.

Convection is the common form of heat transfer to gases and liquids.

Thermocouples act by transferring heat by conduction, however they can be used in all types of environments (radiation, conduction or convection).

What is a temperature sensor and what are its types?

A temperature sensor is a device that responds to a physical/chemical stimulus in a specific and measurable manner analogously.

Temperature sensors can be:

  • Glass liquid thermometers
  • Thermocouples
  • Temperature-sensitive resistors (thermistors)
  • Resistance Temperature Detector (RTD)
  • Bimetallic thermometers
What are special thermocouples?

They are thermocouples that use noble materials in positive and negative elements, for example, Thermocouple Gold/Platinum (Au/Pt), Tungsten–Rhênium (Type W, D, C), Iridium 40%-Rhodio/Iridium.

What types of assemblies can a Thermocouple have?

Thermocouples can have 3 types of assembly:

Conventional: They are assembled in a simple way, using insulators and ceramic connection blocks.

Mineral Insulation: Also known as TIM (Mineral Insulation Thermocouple), It consists of a metallic protection sheath in which the conductors are highly compacted with magnesium oxide providing an excellent electrical insulation, conductors completely isolated from environmental conditions.

Flexible: Ideal for use in the plastics processing industry or in applications where they are needed: ease of installation, easy removal and quick response time.

Learn more about Thermocouples and Industrial Thermometry

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