Thermocouple Types For Measuring Different Temperature Ranges

Thermocouples measure temperature quickly across a wide range. There are several types to choose from, each with its own temperature range and tolerance. Choose the thermocouple type that suits your project.

If you have used a particular thermocouple type in a project, we would love to share images of your work. Leave a comment below or contact me.

If you are already familiar with thermocouples you can skip straight to the overview of all the thermocouple types:

Thermocouple Types Chart: Wire And Cable Colors ⇓.

What Is A Thermocouple?

A thermocouple is a thermometer for high-temperature ranges. Its name comes from coupling two materials with different thermoelectric properties. A transmitter between the thermocouple and a PLC lets the PLC use its temperature measurements. Temperature control loops in ladder logic are a common example.

What Does The Thermocouple Types Look Like?

You may already have seen a thermocouple, though thermocouple thermometers look quite different depending on where they are used. You will usually see a housing that protects the wires and their junctions. Some thermocouples consist of little more than wires covered by a thin layer of insulation that resists high temperatures. To electrically isolate those wires, thermocouples often come in the form of a probe. The probe lets you measure temperatures without interference with the electricity in the wires. Many digital multimeters or VOM’s also accept a thermocouple probe directly through a connector, allowing you to read the temperature on the meter.

These examples show some of the forms a thermocouple can take:

Thermocouple type K with green and white wires.

Thermocouple probe thermometer

Some thermocouples consist of little more than wires. Others have metal housings for protection against heat and EMI or other noise.

Thermocouple applications – Where do I use a thermocouple?

Thermocouples are used in a variety of different applications. It is mainly used for higher temperatures since the thermocouple temperature range is at higher temperatures. There are good reasons for that. The main reason that the temperature range is so high is, that RTDs or resistance temperature detectors become inaccurate at higher temperatures.

The list below shows common applications where thermocouples are the best choice for temperature measurement. If you know of another common application, leave a comment below.

• Heating – Gas burners for ovens
• Cooling – Freezers
• Engine protection – Temperatures and surface temperatures
• High-temperature control – Iron casting

Industrial automation often needs measurements at high temperatures. Thermocouples work best in that range, which explains their frequent use in industry.

Thermocouples can even be used for rocket science. NASA uses thin-film thermocouple (PDF) to measure temperatures at the engine surfaces.

Thermocouples also appear in hobby projects. Even a small project may need high-temperature measurements, often using an inexpensive thermocouple with an Arduino or the Raspberry Pi platform. Prices for these thermocouples start as low as $10.

Thermocouple theory – How does a thermocouple work?

A thermocouple measures temperature through the different thermoelectric properties of its materials. Understanding those properties helps explain how the measurement works.

The thermoelectric effect describes these properties through the relationship between a temperature change along a conductor and a resulting voltage change. Applying different temperatures to the ends of the conductor creates a temperature gradient along it. Wherever that temperature gradient exists, a voltage gradient accompanies it.

The Seebeck Effect: Thermal Gradients And Voltage Gradients In Conductors

This simple conductor illustrates how the thermoelectric effect changes the voltage along it:

Seebeck effect: When the thermal gradient is applied to a conductor a voltage gradient will be generated.

Applying the temperature difference of 100 degrees shown here gives us a thermal gradient, meaning a change in temperature along the conductor. According to the Seebeck effect, that thermal gradient generates a voltage gradient. The generated voltage is small enough that it is often measured in microvolts, μV. This small voltage is one of the reasons thermocouple temperature ranges cover higher temperatures.

You can calculate the voltage gradient with the Seebeck formula:

ΔV = -S * ΔT

In this formula, ΔV is the voltage gradient, or change in voltage along the conductor, and ΔT is the temperature gradient. S is the conductor’s Seebeck coefficient, measured in volts per kelvin.

Junction Of Different Thermoelectric Conductors

The connection between two conductors is called a junction. Connecting conductors with different thermoelectric properties is the next principle behind thermocouple operation.

Each conductor has its own thermoelectric property or Seebeck constant. This is how much voltage that is generated through the conductor. For example, cobber and chrome have different Seebeck coefficients. This means, that when we apply the same difference in temperature over the two materials, the generated voltage will be different. The voltage generated in the cobber conductor will be different from the voltage generated in the chrome conductor. For cobber, the Seebeck coefficient is approximately 1.5 μV/kelvin. And for aluminum, the Seebeck coefficient is -1.5 μV/kelvin.

A thermocouple uses this difference between material properties. The following drawing shows the principle:

Thermocouple principle – How the thermocouple works.

The thermocouple in the drawing has three junction points: one joining the two conductors and two joining the conductors to the voltmeter wires.

The Cold Junction Point: Reference Temperature

The junction between the two different conductors are two different junctions, but with the same temperature. These two junctions are referred to as Tref or temperature reference. The temperature at these junctions is the reference temperature, also known as the cold junction point. In thermocouples, this point is used as a reference to measure the temperature at the other junction.

The Hot Junction Point: Measured Temperature

Joining the two conductors with different thermoelectric effects creates the hot junction point. This is where you measure the temperature. To measure an oven, for example, place the hot junction inside the oven so that its temperature matches the temperature you want to measure. This point is called Tsense, or the measured temperature.

Thermocouple Voltage To Temperature Principle

Between the hot junction and the cold junction, the two conductors now have different temperatures along them. At the two reference junctions, you measure two different voltages, each depending on the Seebeck coefficient of its conductor. Many thermocouple types use conductors with a large difference between their coefficients so that they generate the largest voltage difference. Every thermocouple type uses two different conductors. We will return to those different types later.

We call this the voltage to temperature principle because we measure different voltages at the two cold junction points. This can be translated directly into temperatures since the voltage gradient is proportional to the gradient in temperature.

Does this give us the temperature at the hot junction itself?

We are measuring the temperature difference between the hot junction (the measured temperature) and the cold junction (the reference temperature). To find the correct hot junction temperature, we need to compensate for the temperature at the reference point.

Cold Junction Compensation Of The Thermocouple

Compensating for the reference temperature gives us the measured temperature at the hot junction. This is called cold junction compensation, and there are a couple of ways to do it.

Insert a cold junction compensation thermometer at the cold junction.
The thermometer is usually an RTD like a pt100. It is used to measure the temperature at the cold junction terminals. The cold junction thermometer will give you the temperature at the reference point. Add the temperature to the temperature difference measured by the thermocouple. Now you have the temperature measured at the hot junction point.

Measure the temperature at the cold junction itself, rather than the surrounding temperature. Many thermocouple transmitters have an integrated pt100 RTD at that junction. For example, the EM231 analog input thermocouple module in the Siemens S7-200 PLC series has an internal thermometer for measuring and compensating for the cold junction.

Cold junction compensation of thermocouple with pt100 temperature sensor.

The known temperature at the cold junction.
You can also place the cold junction point in a place where the temperature is known. A common way of doing this is using a reference box where the temperature is controlled by a thermostat. In this way, you can easily make a cold junction compensation of your thermocouple. You just have to add the known temperature to the difference in temperature measured by the thermocouple.

Another practical way of doing this is to make a bath of semi-frozen water or ice water. Semi-frozen distilled water is approximately 0 degrees Celsius. By placing the cold junction at 0 degrees Celsius you do not have to compensate for the reference temperature. The reference temperature is now 0. You can use the voltage difference from the thermocouple to translate directly into the measured temperature.

Thermocouple cold junction compensation with 0°C ice water.

Thermocouple Accuracy And Limits Of Error

How accurate is a thermocouple temperature sensor?

Each type has a tolerance, also called its limits of error, which describes its initial accuracy when it leaves the factory. A type k thermocouple, for example, has a tolerance of ± 2.2°C or ± .75% for a standard type and ± 1.1°C or ± .4% for a special type.

Standard Limits Of Error Or Special Limits Of Error

ASTM E230 sets out these accuracy tolerances. Thermocouples have either standard or special limits of error. The standard types meet the standard limits, while special types use wires with purer alloys. That improves their accuracy, giving them lower limits of error than standard types.

Thermocouple Response Time: How Fast Is The Thermocouple?

Response time describes how quickly the thermocouple reacts and gives you a temperature readout. Thermocouples react very quickly because they work through temperature gradients along their conductors. Only one end of a conductor needs to change temperature for the voltage gradient to change. That is what makes a thermocouple much faster than an RTD thermometer, which works by measuring resistance.

The response times of thermocouples depend on how the thermocouples are wired. Or to be more precise, how the hot junction point is wired. You can, as an example, use grounded junctions to protect the hot junction point from electromagnetic interference or EMI. How the hot junction point of the thermocouple is wired affects the response time. But thermocouples as thermometers are generally fast in their response time.

Thermocouple Types: Thermocouples With Different Properties

Thermocouple types chart

Standardized thermocouple types each have a letter name and their own properties. Each type pairs two specific conductors with different thermal effects.

ASTM defines the standard thermocouple types and their letter names, so those names remain consistent. The thermocouple standard is ASTM E230, which contains all the specifications.

IEC, BS and ANSI also standardize these thermocouple types, with differences such as wire colors. We will look at wires and connectors later.

Before the full list, I will describe the thermocouple types I encounter most often. Most of these are used in industry.

K Type Thermocouple: The Most Common Thermocouple Type

Type K is widely used across many applications because it is inexpensive and covers a large temperature range.

The conductors used in the K thermocouple are chrome and aluminum which are both metal alloys. By combining these two materials this thermocouple type has a sensitivity of 39 µV/°C. For each one degree Celsius increase in the temperature gradient, the voltage will increase by 39 µV or 0.039 mV.

ITS-90 Table For Type K Thermocouple

For more on K type thermocouple sensitivity, see the ITS-90 tables. The National Institute of Standards and Technology, or NIST, produces these reference tables for each type. They show thermoelectric voltages as a function of temperature, with the reference junctions at 0°C.

Take a look at the ITS-90 table for the k type thermocouple. As you can see the voltage generated in the thermocouple conductors are 0 mV if the temperature is 0°C. But if you look at the voltage level, when the temperature is 10°C, you will see that the voltage is now 0.397 mV. The change of 0.039 mV/°C is continuous. The thermocouple output is almost linear, and that makes it easy to work with. If you want to use the thermocouple with a PLC, you would still have to convert the signal though. The PLC hardware, or to be specific the PLC inputs are not capable of detecting the small changes in voltage generated by the thermocouple alone.

The Curie Temperature In The K Type Thermocouple

Also called a nickel alloy thermocouple is the k type thermocouple. Be aware that the nickel alloy thermocouples have nickel as one of the constituent metals in the alloys. Hence nickel is magnetic, it has a Curie point. The Curie point or the Curie temperature is the point of where the permanent magnetism of the material changes to induced magnetism. At around 185°C this point will be reached for K type thermocouples. And at this point, the output will deviate.

Type T Thermocouple For Extreme Low Temperatures

Type T measures lower temperatures than the high-temperature types discussed so far. Its conductors are cobber and constantan, which make it very stable at low temperatures. The T thermocouple is stable between -200°C and 200°C.

Thermocouple Types Chart

Wire and cable colors are standardized for each thermocouple type. However, the colors for the same type differ between standards. There are generally 3 sets of standards to consider.

• International Electrotechnical Commission (IEC)
  IEC 60584-1 – Thermoelectric voltages of thermocouples
  IEC 60584-2 – Tolerances of thermocouple voltages
  IEC 60584-3 – Cables and wire colors
• BS – British Standard Institution
  BS EN 60584 – Merges some of the former IEC 60584 standards
  BS 1843 – Wire and color codes for thermocouples
• ANSI – American National Standards Institute
  ANSI MC96.1 – Thermocouple color codes

IEC standards are international, though mainly used in Europe. The BS standards are also international and take their name from their publisher, the British Standards Institution. ANSI standards are used in America.

The overlapping wire colors in some standards can make thermocouple identification confusing. A color used for one type under one standard may appear elsewhere too. To identify the type you have, you first need to know which standard your thermocouple was made under.

You do not need to memorize every thermocouple wire color. A chart like the one below lets you look them up.

Thermocouple Types Chart: Wire And Cable Colors

Thermocouple type	ANSI MC96.1	IEC 60584-3	BS 1843
K
T
J
E
N
R	–
S	–
U	–		–
B	–		–

Thermocouple Connectors

Standard thermocouple connectors for thermocouple wires.

Each thermocouple type has a standard, color-coded connector, just as it has standard wire and cable colors. The connectors are polarized to fit in only one direction, preserving thermocouple polarity.

IEC and ANSI both specify standard thermocouple connector colors.

ANSI Thermocouple Connector Color Codes

Under ANSI, connector colors match the extension cable colors. A K type thermocouple therefore has a yellow connector because ANSI specifies a yellow extension cable for that type.

IEC Connector Color Codes for Thermocouples

IEC connector colors also match the cable colors. For a K type thermocouple, both the connector and cable are green.

According to IEC K type thermocouples has green connectors.

Thermocouple Signals and PLC Inputs

Thermocouples are a great choice for measuring temperature. Especially in very cold and very hot environments. They have a wide temperature range and they are quite robust. Since they are merely just two wires, sometimes with protection, they can withstand a lot of noise and vibrations. Besides that, they are relatively linear sensors. This makes it easy to work with. The signal can with ease be transformed into a 4-20 mA signal. This is helpful when you want to connect to an analog PLC input.

If you have experience with thermocouples or something to add, join the discussion below.