Temperature measurement principles

Why is temperature measurement important?

How to measure temperature in industrial applications?

Bimetallic thermometers

Thermocouples

Resistance thermometer detectors

Thermowells

Temperature transmitters

How to select an industrial temperature measurement device?

1) What is the application?

2) Which temperature range is rquired?

3) What is the required accuracy and response time?

4) What are the installation conditions?

More information on temperature sensors needed?

Why is temperature measurement important?

Temperature measurement is so important, because it plays a vital role in the final products’ quality and the plant’s safety. This is explained by the fact that temperature can, for example, have a significant impact on several aspects such as taste, viscosity, density, solubility, and the aggregate state of a product.

In this context, the question arises, what is the definition of temperature in the first place? In a few words, temperature can be defined as the physical quantity of thermal energy, measuring the level of kinetic energy in the particles. The higher the kinetic energy of the particle, the higher the temperature and vice versa.

How to measure temperature in industrial applications?

There are different temperature measurement devices to measure the temperature in industrial areas. The most used sensors are the bimetallic thermometers, industrial thermometers with thermocouples or resistance temperature detectors, also known as RTDs, and the temperature transmitters.

Bimetallic thermometers

Bimetallic thermometers, also known as bimetal thermometers or temperature gauges, comprise two different metals with different expansion coefficients. As they are heated, one metal expands more than the other, causing a deformation. The deformation is mechanically transferred to move a pointer in a dial, where the temperature can be measured and read on the scale. Bimetal thermometers are an excellent and cost-effective solution for local measurement and indication of temperature.

Figure 1 - Bimetal thermometer construction

To the bimetal thermometers

Bimetallic thermometers advantages:

Low cost and reliable
Do not require a power supply
Good option for local indication

Bimetallic thermometers limitations:

Internal moving parts may need maintenance over time
Re-calibration and correction might be necessary over time
No output signal

Thermocouples

Figure 2 - Thermocouple functioning

Thermocouples works based on the Seeback effect, where a pair of wires of two electrical conductors of different materials are joined together in one point, called the hot junction. A voltage can be measured between the two wires, called the cold junction, in relation to the sensed temperature. Thermocouples can be made of different materials, different names, and designed for different temperature ranges.

Type K (Chromel-alumel) is the most common thermocouple, an inexpensive yet reliable sensor on a temperature range of -200 to 1300 °C. For more demanding applications with higher temperatures, thermocouples made of noble alloys with platinum and rhodium, like the thermocouples type B, R, and S can measure temperatures above 1700 °C.

Thermocouples require special cables. If the connection is made with a cable of a different material from the thermocouple, the connection between the thermocouple and the cable creates another thermocouple, which will also produce a voltage, causing errors in the measured value. For this reason, thermocouples extension and compensation cables need to be specifically for this purpose. For long distances, the special cabling can get expensive, and the system is susceptible to interferences. For this reason, it is recommended to use a temperature transmitter close to the sensor that will transmit the measured temperature using a standardized and robust telemetry signal, like 4-20 mA, 0-10 V, or digital communication protocols.

Thermocouples advantages:

Low cost and reliable
Different versions for different temperature ranges
Support higher temperatures compared to RTDs
Faster response time compared to RTDs
Can be connected to a temperature transmitter

Thermocouples’ limitations:

Non-linear signal
Low voltage signal susceptible to electrical noise
Requires special cabling
Accuracy may drift over time, requiring re-calibration

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Resistance thermometer detectors

Resistance Thermometer Detectors, also known as resistance thermometers or simply as RTDs, are temperature sensors made of a metallic material where the electrical resistance changes based on the temperature. With the relation between temperature and electrical resistance, it is then possible to measure the temperature. The most common material for RTDs is the platinum, found on Pt100 and Pt1000 RTDs, which provide a well-defined, stable, and linear resistance value. However, more inexpensive materials can also be used on RTDs like copper and nickel.

RTDs can vary in construction depending on the manufacturer and the model. The two most common designs are the wire-wound RTD and the thin-film RTD. Wire-wound RTDs are more accurate and can work under a wider temperature range. However, they are more expensive and less resistant to mechanical stress than thin-film RTDs. Thin-film RTDs are also more compact and have a better response in the sensor’s tip.

Figure 3 - Wire-wound RTD (left); Thin-film RTD (Right)

RTDs can be found in a 2, 3, or 4-wire configuration. In RTDs, what is being measured is the sensor resistance. However, the cable resistance will sum with the sensor resistance and can have a significant impact on the accuracy, depending on the cable distance. 3 and 4-wire RTDs were designed to be wired into a compensating electrical circuit that can compensate the cable resistance, diminishing its impact in the measurement, where a 4-wire version would be the most accurate of them all. In any case, it is recommended to use a temperature transmitter close to the sensor for connecting sensors over long distances. This way, the cabling electrical resistance won’t affect the measurement value.

Get to know the resistance thermometers

RTDs advantages:

More accurate compared to thermocouples
More linear and stable compared to thermocouples
Can be connected to a temperature transmitter
Do not require special cabling

RTDs limitations:

More expensive compared to thermocouples
Usually designed for temperatures below 600 °C
Cable resistance can affect the measurement value

Thermowells

Thermowells are used to protect the temperature sensor from abrasion, corrosion, vibrations, and mechanical impact. The thermowell is basically a tube, with one closed end, serving as a jacket for the temperature probe. Besides protecting the temperature sensor, it also allows the removal or substitution of a sensor without having to stop the process.

When selecting a thermowell, it is important to consider a few aspects like internal and external diameter, length, material, process connection, and design, which should be compatible with the sensor that will be inserted on it.

Explore the thermowells

Figure 4 - Temperature probe and thermowell

Thermowell advantages:

Offers mechanical resistance to the sensor
Sensors can be removed or replaced without having to stop the process

Thermowell limitations:

Increases the sensor response time
Increases the pressure loss due to the bigger footprint

Temperature transmitters

Figure 5 - Head transmitter (lef), Din-rail transmitter (right)

Temperature transmitters are used for transmitting the measurement signal over long distances using a standardized telemetry signal like 4-20 mA or digital communication protocols, which are very robust and resistant to electrical noise. Compact thermometers can have an integrated temperature transmitter, but a standalone sensor can be connected to din-rail and head transmitters, which are compatible with most of the thermocouple and RTD types.

Some temperature transmitter models will also enable a possibility of parametrization, calibration, integrated diagnostics, and other functionalities.

Discover the din-rail and head transmitters

Temperature transmitters advantages:

Robust industrial telemetry signal for long distances
Offers parametrization, calibration, and diagnostics, depending on the model
Compatible with a wide range of temperature sensors

Temperature transmitters limitations:

Additional cost

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How to select an industrial temperature measurement device?

Many factors can influence the selection of a temperature sensor, and due to all the technical aspects to be considered, the correct selection can be challenging and time-consuming. However, by answering a few guiding questions, it is possible to a better idea for selection.

1) What is the application?	It is important to understand what the application is, e.g. if it’s to measure temperatures of liquids and gases enclosed in a pipe or tank, the ambient temperature in a room, or even the temperature of a surface. Each sensor is designed for a specific task and may not perform well if not used for its purpose. Therefore it is important to check if the sensor is designed for the application.
2) Which temperature range is required?	Each temperature sensor is designed to work within a temperature range. If applied outside of the range limits, inaccuracies, malfunction, and even damage could be caused to the sensing element.
3) What is the required accuracy and response time?	Different classes of accuracy and response time can be found on temperature sensors. Therefore, it is important to check if the selected sensor meets the process requirements.
4) What are the installation conditions?	Temperature probes can have different insertion lengths, probe diameter, and process connections. For this reason, it is worth checking if the selected sensor has dimensions and connections compatible with the place of installation.

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Back to overview

Temperature measurement principles

Why is temperature measurement important?

How to measure temperature in industrial applications?

Bimetallic thermometers

Bimetallic thermometers advantages:

Bimetallic thermometers limitations:

Thermocouples

Thermocouples advantages:

Thermocouples’ limitations:

Resistance thermometer detectors

RTDs advantages:

RTDs limitations:

Thermowells

Thermowell advantages:

Thermowell limitations:

Temperature transmitters

Temperature transmitters advantages:

Temperature transmitters limitations:

How to select an industrial temperature measurement device?

1) What is the application?

2) Which temperature range is required?

3) What is the required accuracy and response time?

4) What are the installation conditions?

More information on temperature meters needed?