Thermocouples 101 - An Introduction to Thermocouples

Introduction to thermocouples

Part of the Industrial Temperature Measurement and Control Blog

What is a Thermocouple?

The thermocouple is one of the most versatile industrial temperature measurement sensors.  It can measure temperatures as high as 3,000°C and as low as –270°C.  It can measure the temperature of liquids, solids and gases and can be used in both industrial and laboratory applications.  The cost of a thermocouple can be as wide ranging as their application suitability, costing only  few dollars in simple forms to several thousand dollars for more advanced configurations. Although it has an inherently simple construction, understanding the nuisances of choosing the right thermocouple for an application can be challenging.  In this post we will  provide a basic understanding of thermocouples including theory of operation, the different types and styles of thermocouples and which thermocouple configurations are best suited for different application environments.

How Do Thermocouples Work?

A thermocouple is a simple device created by joining two metals of non-similar materials.  When the thermocouple is exposed to a temperature gradient across its length, a voltage is produced.  

Basic Thermocouple

The voltage produced by a thermocouple is a result of the Seebeck Effect.  As the temperature gradient across a thermocouple increases the voltage produced by the thermocouple also increases.   The relationship of temperature to the output of the thermocouple is not linear but it is repeatable making the thermocouple useful as a temperature sensor.   The output of the thermocouple not only varies with temperature but is also dependent on its materials of construction.  The electrical output is a very low level voltage where hundreds of degrees of temperature change may only represent a few millivolts change in the output. Not only is the output small but it is non-linear.  The output also requires a technique known as cold junction compensation to convert it to a temperature value.  Because of the complexity in converting the electrical signal  to temperature, specialized instrumentation is required for thermocouples.  Fortunately these instruments are commonly available on the market.

IOThrifty's DL-IR-EXCELOG-6 temperature data logger can read and log data from up to 4 thermocouples plus 2 RTD temperature sensors

 

Thermocouple Types

Almost any two metals can be used to make a thermocouple but there are certain metals that have been adopted as industry standards.  These industry standard thermocouples are made of both metals as well as metal alloys.  For simplicity sake they are identified by one letter identifiers, often referred to as the thermocouple “type” or “calibration”.  The table below shows the four of the most common thermocouples by letter designator, metal of construction and temperature range.   These four thermocouples are known as Base Metal Thermocouples.

Base Metal Thermocouples

Thermocouple Type

Positive Lead

Negative Lead

Temp. Range(°C)

J

Iron

Copper-nickel alloy

(Constantan) 

-210 to 1200

K

Nickel-chromium alloy

(Chromel*)

Nickel-aluminum alloy

(Alumel*)

-270 to 1372

T

Copper

 

Copper-nickel alloy

(Constantan)

-270 to 400

E

Nickel-chromium alloy

(Chromel*)

Copper-nickel alloy

(Constantan)

-270 to 1000

*The terms Chromel and Alumel where originally trademarked by Hoskins Alloys.

 

Although base metal thermocouples are the most common there are other thermocouple types used for more demanding applications requiring higher temperature ranges.  They are often made of rare and exotic materials.

Rare Metal Thermocouples

Thermocouple Type

Material (Alloy trade name and alloy shown)

Lead Polarity

Temperature Range(°C)

B

Platinum-30% Rhodium Alloy

Platinum-6%Rhodium Alloy

0 to 1820

R

Platinum-13% Rhodium Alloy

Platinum

 

-50  to 1768

S

Platinum-10% Rhodium Alloy

Platinum

 

-50  to 1768

 

Thermocouple Color Codes

Most thermocouples have a color code designator.  The charts above show the thermocouple letter designator highlighted in the color code for that thermocouple.  For example, J thermocouples use a black color code, K thermocouples use a yellow color code etc., etc.. The color codes above are from the American National Standards Institute (ANSI) which are those used in the United States.  There are other standards which some other countries follow.

The color code is used to help identify thermocouples since without the color identification it is difficult to visually distinguish one thermocouple from another.  The color is often incorporated into the insulation of the positive lead of the thermocouple.  Unlike most other electrical conventions, the negative lead of a thermocouple commonly has a red insulation.

Thermocouple wire is bundled as positive and negative pairs where the outer insulation is usually colored with the color code.  Thermocouple connectors also follow the color codes convention.

 

Thermocouple Accuracy

Many factors may have an impact on measurement accuracy of a thermocouple.  These include:

  • Sensor accuracy as specified by the thermocouple type or calibration
  • Electrical noise in the environment including ground loops
  • Sensor size (eg. response time related errors and/or inadequate penetration depth)
  • Sensor shape (eg. Surface measurement errors)
  • Instrumentation errors

Future editions of this blog will explore these error sources in more detail but the fundamental starting point for determining measurement accuracy is the sensor accuracy.  Each thermocouple type has a characteristic accuracy.  Normally there are two accuracy classes provided.  They are referred to as Standard Limits of Error and Special Limits of Error.  The table below shows both accuracy classes for the most common thermocouples.

 

Thermocouple Type

Standard Limits of Error

(degrees or % of reading whichever is greater)

Standard Limits of Error

(degrees or % of reading whichever is greater)

K

±2.2°C or ±0.75%

±1.1°C or ±0.4%

T

±1.0°C or ±0.75%

±0.5°C or ±0.4%

J

±2.2°C or ±0.75%

±1.1°C or ±0.4%

N

±2.2°C or ±0.75%

±1.1°C or± 0.4%

E

±1.7°C or ±0.5%

±1.0°C or ±0.4%

S

±1.5°C or ±0.25%

±0.6°C or ±0.1%

R

±1.5°C or ±0.25%

±0.6°C or ±0.1%

B

±0.5%

±0.25%

 

 

Thermocouple Styles and Configurations

Because of the simple nature of thermocouple construction, it can be made into a variety of shapes and styles which can be optimally suited for different types of temperature measurement applications.  Below we list some of the most common styles of thermocouple.

Beaded Wire Thermocouples

One of the simplest forms of  thermocouples.  It consists of the two thermocouple wires joined at a welded end.   It has the advantages of being fast responding, and relatively low cost.  Its use is generally limited to gases.

 IOThrifty GT-K-36 Beaded Thermocouple

IOThrifty's GT-K-36 Beaded Wire Thermocouple

Surface Measurement Thermocouples

A specialty style thermocouple designed to measurement the temperature of a solid surface.  In order to provide optimal contact with the measurement surface, the welded joint is machined flat.

 TC-SA-K Type K Surface Thermocouple

IOThrifty's TC-SA-K Type K Surface Thermocouple

Thermocouple Probes

The workhorse of thermocouples; it consists of a thermocouple inside a metal tube. They are general purpose sensors and can be used to measure the temperature of liquids or gases.   Probes are generally slower responding than a beaded wire sensor but they can be made in small sizes which provide  fast response times.

 Thermocouple Probe

Thermocouple Probe

Head and Well Assemblies

Design for rugged industrial environments.  The thermocouple is housed inside a steel enclosure called a thermowell.  A metallic "head" is mounted on top of the thermowell.  The head typically contains a connection block as well as electronics used to convert the thermocouple signal into a more robust format (such as 4-20mA) which is better suited for industrial environments.

 

Industrial Head and Well Assembly

Industrial Head and Well Assembly

 Thermocouple Styles Comparison

Beaded Thermocople

Surface Measurement Thermocople

Thermocople Probe

Head &Well Assembly

Price

$

$$

$$

$$$

Application

non-corrosive Gases

Solid surfaces

liquids, gases

liquids, gases

Environment Suitability

Laboratory, Light Commercial

Laboratory, Light Commercial

laboratory, commercial, light industrial

heavy industrial

Response Time

Fast

Fast

Dependent on probe size

Slow

 

Buy Thermocouples or Make Your Own

 The thermocouple market is huge with many different suppliers.  Some specialize in large installations while others cater to small purchases. Selecting a supplier depends on the style of thermocouple you need, your budget and your delivery requirements.  IOThrifty has a section of thermocouples which can be seen here,

For users of large quantities of thermocouples, making your own thermocouple is also an option.  Making a beaded wire thermocouple is a simple process requiring thermocouple wire and a means of connecting the wires together.  While any mechanical connection will work there are commercially available thermocouple welders that are easy to use and create a professional welded junction with minimal experience.  IOThrifty offers a variety of welders that can be used.  Our thermocouple welders can be seen here.

 TC-Welder Thermocouple Welder

 TC-WELD Thermocouple Welder