In this article we will discuss how temperature is measured with thermocouples today, with enough detail so that you will:

  • See what thermocouples are and how they work
  • Learn the basic types of thermocouples available and how they are used
  • Understand how thermocouples can be interfaced with your DAQ system

Are you ready to get started? Let’s go!

Introduction

Did you know that temperature is the MOST often recorded physical measurement? Knowing the temperature is critical for the correct operation of everything from the human body to an automobile engine, and everything in between. 

We need to know the temperature of objects for an almost infinite number of purposes. Temperature is often an indicator that something is wrong: perhaps you have a fever, or the brake pads on your car are about to fail, or a turbine in an energy plant is running too hot. You get the idea. 

Temperature is measured with one or more kinds of temperature sensors. There are several available on the market today:

  • Thermocouple sensors
  • RTD sensors
  • Thermistor sensors
  • Infrared temperature sensors

What is a Thermocouple?

A thermocouple is a sensor that is used for measuring temperature. The thermocouple is a very popular sensor to its relatively low cost, interchangeability, wide measuring range, and reliability. 

Typical thermocouple sensor
Typical thermocouple sensor
Hartke, Wikimedia Commons, public domain

Thermocouples are widely used across every industry, from factory automation and process control to automotive, aerospace, military, energy production, metals manufacturing, medical sciences, and countless more. 

They have standard connector types, making them interchangeable and easy to source. On the measuring end of the sensor, they can be as simple as the two metals twisted together, or they can be enclosed within a rugged probe for use in rough industrial environments.

Thermocouple probe
Long thermocouple probe connected to a meter
Harke / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Although thermocouples are quite popular, accuracies much better than 1°C are not easily achieved with them. But regardless, due to their many advantages, they remain the most popular type of sensor used for industrial measurements today.

Learn more about Dewesoft and thermocouple temperature measurement:

Dewesoft PRO Training > Temperature Measurement

How Does A Thermocouple Work?

Thermocouples are based on the Seebeck effect, which says that when a pair of dissimilar metals in contact with each other at each end are subjected to changes in temperature, they create a small voltage potential. And they do this passively, i.e., they do not need to be powered by a signal conditioner.

How is this possible? Are we creating free energy from nothing? Not at all - it’s just physics!

Consider that electrons carry both electricity and heat. Take a piece of bare copper wire and close your hand around it at one end. Energized by the heat from your skin, electrons will propagate from the area where you’re touching it to the cooler far end away from you, creating a temperature gradient along the length of the wire. The heat has been transformed into energy.

This phenomenon was originally discovered by Italian scientist Alessandro Volta (for whom we named “the volt”)  in 1794. But German physicist Thomas Johann Seebeck rediscovered it in 1821. He observed that when wires made from two different metals were joined at each end, and there was a temperature difference between these ends, that small voltage potential was created at the junctions.

We call this potential the Seebeck Voltage and the creation of this potential from thermal energy the “Seebeck Effect.” Based on Seebeck’s observations 200 years ago, physicists are able to determine the Seebeck Coefficient, i.e. the magnitude of the thermoelectric voltage that is induced by temperature differences across a given material.

How does thermocouple workThermocouple detects changes in temperature with a pair of dissimilar metals when they come in contact with each other

Decades of research, trial, and error have led to today’s understanding of which metals give us the best results when we pair them to create a thermocouple. Different combinations provide different effective measurement ranges. And of course, each metal has environmental properties, which further determines where and how they can be used.

The science behind thermocouples is quite mature now, and we have industry-standard “types” available on the market today, like Type K, which pairs chromel and alumel metals, providing a very wide measuring range. More about thermocouple types below

It sounds very simple - grab a thermocouple wire pair and connect one end of it to your DAQ system or a voltmeter and start measuring temperature, right? Well, there is a little more to it than that. 

There are two additional steps that must be taken in order to convert a thermocouple sensor’s output to a usable temperature reading: cold-junction compensation and linearization. Let’s look at each of these to see how they work and what they do.

Cold-Junction Compensation

In order to make an absolute measurement, the thermocouple must be “ referenced” to a known temperature on the other end of the sensor’s cables. In the old days, this reference would be an ice bath of nearly frozen distilled water, which has a known temperature of 0°C (32° F). But since this is inconvenient to carry around, another method was created using a tiny thermistor or RTD shielded from the environment to measure the ambient temperature. This is called “cold junction compensation” (CJC). 

CJC inside a Dewesoft IOLITE TH thermocouple module. The white wires connect to a thermistor that is embedded within the white thermal paste.

The “hot junction” is the measuring end of the thermocouple assembly, and the other end is the “cold junction” aka the reference thermocouple junction, where the CJC chip is located. So although the cold junction temperature can vary, it provides a known reference by which the measuring system can derive the temperature at the measuring end of the sensor with very good and repeatable accuracy.

Linearization

The small voltage output of a thermocouple sensor is not linear, i.e., it does not change linearly with changes in temperature. Linearization can be done by the signal conditioner itself or using software running inside the DAQ system.

Linearisation curves for the most popular thermocouple typesLinearization curves for the most popular thermocouple types
Image from Dewesoft online PRO training course

Thermocouple Types

Pairing different kinds of metals give us a variety of measuring ranges. These are called “types.” A very popular one is Thermocouple Type K, which pairs chrome and alumni, resulting in a wide measuring range of −200 °C to +1350 °C (−330 °F to +2460 °F). Other popular types are J, T, E, R, S, B, N, and C.

Thermocouple types J, K, T, and E are also known as Base Metal Thermocouples. Types R, S, and B thermocouples are known as Noble Metal Thermocouples, which are used in high-temperature applications. Here are the most popular thermocouple types in use today:

ANSI IEC Alloys Used Widest Range Magnetic? Comments
J J Iron-Constantan -40° to 750° C
-40° to 1382° F
Yes Better for high than low temperatures
K K Chromel-Alumel −200° to 1350 °C
−330° to 2460 °F
Yes Widest range, most popular. Nickel is magnetic.
T T Copper
(Cu)
-270 to 400° C
-454 to 752° F
No Good for lower temperatures and damp environments.
E E Chromel-Constantan −50° to 740 °C No Good for cryogenic use.
N N Nicrosil
(Ni-Cr-Si)
-270 to 1300° C
-450 to 2372° F
No Wide range of temperatures, more stable than type K
B B Platinum-30% Rhodium
(Pt-30% Rh)
0 to 1820° C
32 to 3308° F
No High temperature, do not insert in metal tubes
R R Platinum-13% Rhodium
(Pt-13% Rh)
-50 to 1768° C
-58 to 3214° F
No High temperature, do not insert in metal tubes
S S Platinum-10% Rhodium
(Pt-10% Rh)
-50 to 1768° C
-58 to 3214° F
No High temperature, do not insert in metal tubes
C C Tungsten-3% Rhenium
(W-3% Re)
0 to 2320° C
32 to 4208° F
No Made for high-temperature applications, but not oxidizing environments

A detailed thermocouple comparison is available on the image below. Click the image to zoom in:

Thermocouple type comparison table

Thermocouple Measuring Challenges and Solutions

Due to the very small microvolt and millivolt output of these sensors, electrical noise and interference can occur when the measuring system is not isolated. Dewesoft DAQ devices address this by means of differential signal conditioning. Nearly all Dewesoft signal conditioning modules are galvanically isolated in addition to being differential. These are the best ways to reject common-mode voltages that get into the signal chain.

Another way to reduce noise is to place the digitizer as close to the sensor as possible. Avoiding long signal lines is a proven strategy for maximizing signal fidelity and reducing costs. Look at our SIRIUS and KRYPTON modular DAQ devices for best-in-breed solutions here.

An inadequate CJC results in wrong readings. This assembly needs to be protected from ambient temperature changes to provide a solid reference. Dewesoft uses a separate CJC chip for each channel in their high-end CJCs, which are milled from a solid block of aluminum, and precisely assembled to achieve the best possible reference.

Thermocouple wires are more expensive than simple copper wires, providing yet another reason that the cold junction should be located as close to the signal source as possible (while still avoiding extreme ambient temperature swings). 

Systems like Dewesoft’s KRYPTON ONE single-channel isolated thermocouple module provide the ultimate in this area, allowing the cold reference to be distributed anywhere the sensors are located and interconnected up to 100 m (328 feet) apart. The signal is converted to the digital right at the measuring point, and transmitted via EtherCAT to the host measuring system, eliminating noise and long runs of expensive thermocouple cables. 

Thermocouple Measurement Applications

A test sample on top of the furnace is being fitted with Type K thermocouplesA test sample on top of the furnace is being fitted with Type K thermocouples (notice the yellow connectors on the side of the furnace)
Achim Hering / CC BY (https://creativecommons.org/licenses/by/3.0)

Temperature is the most measured physical property in the world, and thermocouples are the most popular sensor for temperature measurement. Therefore, there are literally millions and millions of applications for thermocouples, across every industry and sector. Here are just a few of them:

  • Electric power plants (temperature is an indicator of component overheating)
  • Home appliances, where thermistors are not sufficient
  • Industrial process control and factory automation
  • Food and beverage manufacturing
  • Metals and pulp and paper processing mills
  • Environmental monitoring and studies
  • Scientific research and development (R&D)
  • Pharmaceutical and medical supply manufacturing and test
  • Automotive systems and testing applications, hot and cold weather testing, brake tests, ADAS tests, combustion analysis, and more
  • Aircraft and rocket engine systems and testing
  • Satellite and spacecraft manufacturing and test

Advantages and Disadvantages of Thermocouples

Thermocouple advantages:

  • Self-powered (passive)
  • Simple to use
  • Interchangeable, easy connectivity
  • Relatively inexpensive
  • Wide variety of thermocouple probes available
  • Wide temperature ranges in many types
  • Higher temperature capabilities than other sensors
  • Not affected by resistance decreases or increases

Thermocouple disadvantages:

  • Output requires linearization
  • A CJC “cold reference” junction is required
  • Low voltage outputs are susceptible to noise
  • Not as stable as RTDs
  • Not as accurate as RTDs

Temperature Sensors Comparison: Thermocouples, RTDs and Thermistors

Sensor Thermistor Thermocouple RTD (Pt100)
Temperature Range Narrowest
-40°C to 300°C
Widest
Type J is -210 to 1200°C
Type K is 95 to 1260°C
Other types can range as low as -270°C or as high as 3100°C
Narrow
-200- to 600°C
Up to 850°C is possible
Response Fast Medium to Fast
Depends on sensor size, wire diameter, and construction
Slow
Depends on sensor size and construction
Long Term Stability Poor Very Good Best
(±0.5°C to ±0.1°C / year)
Accuracy Fair Good Better
0.2%, 0.1% and 0.05%
Linearity Exponential Non-linear
This is usually done in software
Fairly Good
But linearization is recommended
Construction Fragile Adequate
Sheaths and tubes improve fragility but increase response time
Fragile
Sheaths and tubes improve fragility but increase response time
Size Very small Small Larger
Wiring Very simple Simple Complex 
Excitation/Power Required None None Required
External Requirements None CJC (cold junction compensation) and signal linearization RTD signal conditioner
Cost Lowest 
Low-accuracy types are very inexpensive, but there are some which are more accurate and more expensive. NTC and PTC (negative and positive temperature coefficient) models are available.
Low
R and S types that use platinum are more expensive
Highest

Specifications are typical

Choosing the Right Thermocouple For Your Application

In order to choose the right sensor for your measurement, it’s important to look at a number of different factors:

  • What are the maximum and minimum temperatures you need to measure? 
  • What is the budget?
  • What accuracy range is needed?
  • What atmosphere will it be used in? (oxidizing, inert, etc.)
  • What is the usable sensor lifespan that is needed?
  • What is the required response (how fast must it react to temperature changes)?
  • Will the use of the thermocouple be periodic or continuous?
  • Will the thermocouple be exposed to bending or flexing during its life?
  • Will it be immersed in water, and to what depth?

Based on the answers to these questions, and referencing the Thermocouple Types table above, it should be possible to select the best overall sensor(s) for your application.

Thermocouple Training Video

This video from Dewesoft's measurement conference, explains basic characteristics and working principles of thermocouples and the temperature measurement with Dewesoft DAQ devices and software.

Dewesoft Measurement Devices for Thermocouples

Dewesoft provides several DAQ systems that can effectively measure, store, and display temperature. And they can do so by connecting the most popular temperature sensors in the world for industrial DAQ applications: the thermocouple. Dewesoft systems can measure, store, analyze, and visualize temperature from one to hundreds of channels in real-time.

Note that Dewesoft X data acquisition software allows the temperature output from any sensor to be displayed in your choice of the temperature scale. The default unit of measurement is Celsius, but the software provides easy and simple conversion to the Fahrenheit scale (F) or to the Kelvin scale (K), the base unit of temperature in the International System of Units (SI).

The data file of the Li-ion battery test where the thermocouple sensor was used to measure batteries temperature using Dewesoft X software and DAQ hardware

Dewesoft X is so flexible, in fact, that you can display a given measurement in multiple units of measurement simultaneously if needed.

SIRIUS Thermocouple Measurement

SIRIUS is the flagship of the Dewesoft product range. They represent the highest DAQ system performance, combined with the most powerful DAQ software on the market, DEWESoft X. To connect thermocouples to SIRIUS data acquisition systems, we use our popular Dewesoft Sensor Interface (DSI) adapters to interface with several SIRIUS input modules. 

SIRIUS data acquisition systems are available in a wide array of physical configurations, from modular “slices” that connect to your computer via USB or EtherCAT, R3 rack-mounting systems, and R1, R2, R4, and R8 stand-alone systems that include a built-in computer.

SIRIUS data acquisition device familyThe SIRIUS DAQ devices product line-up

DSI-THx series thermocouple adapters have an industry-standard mini blade type input connector and a short thermocouple cable whose metals are matched to the type. The DSI-THx adapter is compatible with four popular types of thermocouples: J, K, T, and C. 

Dewesoft thermocouple TH-K adapterThe DSI-TH-K adapter from Dewesoft (Types J, T, and C also available)

DSI adapters use a built-in TEDS interface to automatically configure themselves in Dewespft X DAQ software. Simply plug the DSI-TH thermocouple adapter into the selected SIRIUS module’s DB9 input, verify your settings on the hardware setup screen in DEWESoft X software, and you’re ready to start making measurements.

Here is a cross-reference of SIRIUS modules and their compatibility with the DSI-TH8x adapter:

  SIRIUS dual-core modules SIRIUS HD (high density) modules SIRIUS HS (high speed) modules
  STG, STGM, LV HD-STGs, HD-LV HS-STG, HS-LV
DSI-THx 1

1) Note - DSI-TH adapters are available in Types K, J, T, E, and C
2) Note - some SIRIUS DAQ modules have input connector options other than DB9. Please choose DB9 for perfect DSI adapter compatibility.

KRYPTON Thermocouple Measurement

KRYPTON thermocouple DAQ module being tested on vibration shakerKRYPTON thermocouple DAQ module being tested on a vibration shaker

KRYPTON DAQ devices are the most ruggedized range of products available from Dewesoft. Built to withstand extreme temperature and shock and vibration conditions, KRYPTON is rated to IP67, protecting them against water, dust, and more. They connect to any Windows computer (including Dewesoft’s own ruggedized IP67 KRYPTON-CPU model) via EtherCAT and can be separated by up to 100  meters (328 feet), allowing you to locate them near the signal source. Like SIRIUS, they run the most powerful DAQ software on the market, Dewesoft X. 

KRYPTON 8xTH - 8-channel thermocouple data logger and data acquisitionKRYPTONi-8xTH - isolated 8-channel thermocouple data logger and data acquisition

KRYPTONi-16xTH - isolated 16-channel thermocouple data logger and data acquisitionKRYPTONi-16xTH - isolated 16-channel thermocouple data logger and data acquisition

Thermocouples can be connected directly to the KRYPTON-TH multi-channel signal conditioning module and to the HV-TH-1 single-channel high-voltage thermocouple signal conditioning module.

KRYPTON Universal Thermocouple DAQ Device Analog Inputs Setup ScreenDewesoft X software setup screen, showing the 8 universal thermocouple inputs of the KRYPTON thermocouple module

KRYPTON Universal Thermocouple Module Channel Setup ScreenKRYPTON thermocouple module channel setup screen, showing sensor and amplifier settings and live analog signal preview

Here is a cross-reference of KRYPTON DAQ modules and their compatibility with thermocouples, as well as DSI adapters made for temperature measurement:

  KRYPTON multi-channel modules  
  TH STG
Thermocouples Native Thermocouple input (UNIVERSAL - each channel can be set to any type in the software, selectable among these nine types:
J, K, T, E, R, S, B, N, C)
Requires a small DSI-THx 1)

1) Note - DSI-THx adapters are available in Types K, J, T, C and E

KRYPTON 1-channel thermocouple data loggers
 

Left: KRYPTON-1xTH-HV-1 1-channel thermocouple data logger
Right: KRYPTON-1xSTG-1 universal signal data acquisition module

Single-channel KRYPTON ONE provides the ultimate in modularity:

  KRYPTON-1 single-channel modules  
  TH-HV-1 STG-1
Thermocouples Native Type K Thermocouple input, rated to CAT III 600V and CAT II 1000 V isolation. Requires a small DSI-THx 1)

1) Note - DSI-TH adapters are available in Types K, J, T, E, and C 

IOLITE Thermocouple Measurement

IOLITE is a unique product that combines the essential capabilities of a real-time industrial control system with a powerful DAQ system. With IOLITE, hundreds of analog and digital channels can be recorded at full speed while simultaneously sending real-time data to any third party EtherCAT master controller.

IOLITE DAQ systems with thermocouple DAQ modulesLeft: IOLITEr rack-mounting system with 12 input module slots
Right: IOLITEs bench-top system with 8 input module slots

They represent great DAQ system performance plus real-time control via EtherCAT, combined with the most powerful DAQ software on the market, DEWESoft X.

Here is a cross-reference of IOLITE input modules and their compatibility with thermocouples, as well as DSI adapters made for thermocouple measurement:

IOLITE multi-channel modules
  8xTH 6xSTG
Thermocouples Native Thermocouple inputs
(8 channels per module)
Selectable among these types:
K, J, T, R, S, N, E, C, U, B
Via DSI-THx 1)
(up to 6 channels per module)

1) Note - DSI-TH adapters are available in Types K, J, T, E, and C 

The IOLITE-8xTH DAQ module offers both channel-to-ground as well as channel-to-channel isolation up to 1000V. Data are acquired simultaneously from all 8 channels with sample rates up to 100S/s using 24-bit delta-sigma ADC.

The same sample rate and isolation specifications are true of the 6xSTG module, except that it has six channels instead of eight. The 6xSTG is a very versatile module, capable of making strain gage, resistive, and low voltage measurements, in addition to its compatibility with DSI series adapters.

DEWE-43A and MINITAURs Thermocouple Measurement

The DEWE-43A is an extremely portable, handheld DAQ system. Connecting to your computer via a locking USB connector, it features eight universal analog inputs. It’s “big brother” is called the MINITAURs - this is essentially the DEWE-43A combined with a computer and a few other features, in a single highly portable enclosure. The universal inputs of both systems are compatible with Dewesoft’s DSI adapters, allowing you to connect a thermocouple sensor to any or all of their eight input channels. 

DEWE-43 and MINITAURs data acquisition systemsLeft: DEWE-43A handheld DAQ system
Right: MINITAURs model, including built-in computer

The DSI-THx adapters are available for several popular types of thermocouples, including Types J, K, T, and C. DSI adapters use TEDS sensor technology to automatically configure themselves in Dewesoft X DAQ software. Simply plug the DSI-THx adapter into the selected input’s DB9 input, verify your settings on the hardware setup screen in Dewesoft X software, and you’re ready to start making measurements.
 

 

Learn more about Dewesoft and temperature measurement:

Dewesoft PRO Training > Temperature Measurement