Thermocouple Types For Measuring Different Temperature Ranges
Thermocouples are fast temperature sensors with a wide temperature range. You can get all different thermocouple types, and you have to choose the type of thermocouple suitable for your project. The thermocouple types have different temperature ranges and tolerances.
If you are already familiar with thermocouples you can skip straight to the overview of all the thermocouple types:
What Is A Thermocouple?
The thermocouple is a thermometer used for high temperature ranges. It is called a thermocouple because it works by coupling two different materials with different thermoelectric properties. Thermocouples can be used to measure temperatures and with a transmitter in between, the thermocouple can be used with a PLC. It it not rare to see temperature control loops in ladder logic for example.
What Does The Thermocouple Types Look Like?
You might have seen a thermocouple before. Thermocouple thermometers can look very different, depending on where it is used. You will typically see the thermocouple in a housing to protect the wires and the junctions. Some thermocouples are merely just wires with a thin layer of insulation. The insulation of the thermocouple wires are resistant to higher temperatures. To electrically isolate the the wires you will often find the thermocouples as a thermocouple probe. The thermocouple probe can be used to measure temperatures without anything interfering with the electricity in the wires. In many digital multimeters or VOM’s, you can even connect a thermocouple prope directly with a connector and see the temperature.
If you havn’t seen a thermocouple before, here are some examples of how thermocouples can look like:
Thermocouples can look very different. Some of them are barely just wires, while others have metal houses to protect them from heat, EMI and other types of 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 the higher temperatures. There are good reasons for that. The main reason that the temperature range is so high is, that RTD’s or resistance temperature detectors become inaccurate at higher temperatures.
Here are some of the most common thermocouple applications, where the thermocouple is the best option for measuring the temperature. Feel free to write a comment below this article, if you know about other common thermocouple applications.
- Heating – Gas burners for ovens
- Cooling – Freezers
- Engine protection – Temperatures and surface temperatures
- High temperature control – Iron casting
As you an see the thermocouple is often used for industrial purposes. This is due to the fact that they work best in higher temperatures, and industrial automation often requires temperature measurement for high temperatures.
But you will also find people using thermocouples for hobby projects. Some times you need to measure higher temperatures even in small projects. In those cases it is common to use cheap thermocouples to use with for example the Arduino or the Raspberry Pi platform. These thermocouples costs as low as $10.
Thermocouple theory – How does a thermocouple work?
So, how does the thermocouple really works? As mentioned before, the thermocouple works by taking advantage of the different thermoelectric properties in different conductors. To really understand how this works, you first need to know a little more about the thermoelectric properties of these conductors.
The thermoelectric properties is better known as the thermoelectric effect. This is the connection between a change in temperature over a conductor and the change in voltage. When you apply different temperatures to each end of a conductor a temperature gradient along the conductor is created. Where a temperature gradient is there will also be a voltage gradient.
The Seebeck Effect: Thermal Gradients And Voltage Gradients In Conductors
Take a look at this simple example of an electrical conductor. To illustrate how the thermoelectric effect affects the voltage over the conductor:
When you apply this temperature difference of 100 degrees, we say that there is a thermal gradient (change of temperature) over the conductor. According to the Seebeck effect, a thermal gradient in a conductor will generate a voltage gradient. This generated voltage is very small and often measured in mikrovolts – μV. This is one of the reasons thermocouple temperature ranges are for higher temperatures.
You can even calculate the voltage gradient, by using the Seebeck formula:
ΔV = -S * ΔT
Where ΔV is the voltage gradient; the change in voltage over the conductor. And ΔT is the temperature gradient. At last S is the Seebeck coefficient of the conductor. The Seebeck coefficients are measured in volts per kelvin.
Junction Of Different Thermoelectric Conductors
The next principle you need to understand is the junction of two conductors. This is the principle of how the thermocouples work. When you connect two conductors with different thermoelectric properties, we call the connection a junction.
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 has 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 appoximately 1.5 μV/kelvin. And for aluminium the Seebeck coefficient is -1.5 μV/kelvin.
The thermocouple takes advantage of exactly this difference in material properties. You can take a look at this principle drawing of a thermocouple to understand this better:
As you can see the thermocouple has three junction points. A junction between the two conductors. And two junctions between the conductors and the wires to the voltmeter.
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
Where the two conductors with different thermoelectric effects are connected is called the hot junction point. This is where the temperature is measured. So, if you need to measure the temperature in an oven, the hot junction point has to be in the oven. As to say, that the temperature of the hot junction point has to be the temperature you need to measure. We refer to the hot junction point as Tsense or the measured temperature.
Thermocouple Voltage To Temperature Principle
Two different temperatures will now be generated over the two conductors, between the hot junction and the cold junction. At the two reference junctions you will now measure two different voltages. Each of them depending on the Seebeck coefficients of the used conductors. Many thermocouple types are made with two conductors with a big difference in their Seebeck coefficients, to generate the largest voltage difference. Each thermocouple type is made with two different conductors. More on the different types of thermocouples 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.
But do we really measure the temperature at the hot junction point here?
The short answer to that is no. Because what we really are measuring here is the temperature difference between the hot junction (measured temperature) and the cold junction (reference temperature). We need to compensate for the reference temperature to get the correct measured temperature at the hot junction point.
Cold Junction Compensation Of The Thermocouple
To get to the real measured temperature at the hot junction, you need to compensate for the reference temperature. This is called cold junction compensation and can be done in a couple of ways.
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.
When the temperature at the cold junction is measured, the temperature has to me measured at exactly the junction (not the surrounding temperature). Many transmitters for thermocouples do have an integrated pt100 RTD right at the cold junction. For example, the EM231 analog input thermocouple module from the Siemens S7-200 PLC series has an internal thermometer to measure and compensate for the cold junction.
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 Accuracy And Limits Of Error
Is the thermocouple an accurate temperature sensor?
Each thermocouple type has a tolerance or limits of error. This is the initial thermocouple accuracy. The accuracy of the thermocouple when it leaves the factory. For example a type k thermocouple 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
These tolerances or thermocouple accuracies are written as standards in ASTM E230. As mentioned above, the thermocouples are divided into two types, when it comes to the accuracy. Standard and special. The standard thermocouples all has a so-called standard limits of error. While the special thermocouples are made with wires that are more pure in the alloy. This improves the accuracy of the thermocouple. Therefore the special limits of error are lower then the standard limits of error.
Thermcouple Resonse Time: How Fast Is The Thermocouple?
Thermocouple response time is the time is takes for the thermocouple to change according to the change in temperature. How fast will you be able to measure a change in voltage when the temperature changes?
One of the big advantages of thermocouples is their fast reponse time. Thermocouples has a very fast response time, because they work by temperature gradients along the conductors. Only a one end of the conductor has to change in temperature to change the voltage gradient. This makes the thermocouple much faster than for example RTD thermometers, that 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
Thermocouples comes in a series of standardized types. Each of these thermocouple types has its own name – a letter – and its own specific properties. The types of thermocouples are made with two specific conductors with different thermal effects.
Luckily for us they use the same names for the different types of thermocouples. This is because all the standard thermocouple types and the letters for their names are made by ASTM. ASTM E230 is the standard for thermocouples. All the specifications for thermocouples are written in that standard.
All these thermocouple types are also standards in both IEC, BS and ANSI, just with different wire colors etc. More about the thermocouple wires and connectors later in this article.
Before listing all the thermocouple types, let me introduce you to the most common types of thermocouples. These are the types i have seen the most out there. Most of them used for industrial purposes.
K Type Thermocouple: The Most Common Thermocouple Type
The first widely used thermocouple type is the K type thermocouple. The therocouple K type is used in many applications. This is because the thermocouple type k is inexpensive and has a big temperature range.
The conductors used in the K thermocouple are chromel and alumel 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 or 0.039 mV.
ITS-90 Table For Type K Thermocouple
The sensitivity of the K type thermocouple can be explored further in the ITS-90 tables. The ITS tables are made by National Institute of Standards and Technology or NIST. They are also called reference tables, and are made for each thermocouple type. In the tables you can see the thermoelectric voltages as a function of the temperature and 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 them 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 has nickel as one of the constituent metals in the alloys. Hence nickel is magnetic, it has 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
The next thermocouple type I want to highlight is the T type thermocouple. Until now, I have only talked about thermocouples used to measure high temperatures. But the thermocouple type T has a lower temperature range. The two conductors in the T type thermocouple are made of cobber and constantan. That makes the thermocouple very stable at low temperatures. To be more specific, the T thermocouple are stable at temperatures between -200°C and 200°C.
Thermocouple Types Chart
Each of the thermocouple types all has a standard color code for the wires and the cable. The tricky part here is that each type of thermocouple has different cable- and wire colors depending on what standard you use. Generally speaking there are 3 sets of standards for this.
- 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
The IEC standards mainly for use in Europe, but the standards are international. The BS standards are some other international standards. It is called BS because they are published by the British Standards Institution. And at last the ANSI for use in America.
If you want to identify a thermocouple type, this can be a bit confusing. Some of the standard wire color codes for thermocouples overlap. So, in order to identify which thermocouple type you have, the first thing you need to know is what standard the thermocouple is made under.
Keep in mind that you don’t have to remember all these color codes for thermocouple wires. You can always look at a thermocouple type chart like the one below.
Thermocouple Types Chart: Wire And Cable Colors
|Thermocouple type||ANSI MC96.1||IEC 60584-3||BS 1843|
Just like the thermocouple cable and wires, each thermocouple has its own standard connector. Those connectors has color codes. One smart thing about these connectors is that they are polarized. They can only be connected in one way, so to keep the thermocouple polarity.
The thermocouple connectors all has standard colors according to both IEC and ANSI.
ANSI Thermocouple Connector Color Codes
For the ANSI standard the standard colors for the thermocouple connector are the same as the colors of the extention cables. As an example, the connectors for the K type thermocouples are yellow. Because the cable color of the extension cable is yellow according to the ANSI standard.
IEC Connector Color Codes for Thermocouples
And for the IEC thermocouple connectors, the colors are the same as the cable color. For example the K type thermocouple connector is green, just like its cable color.
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. Beside that, they are relatively linear sensors. This makes them 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.
Do you have experience with thermocouples? Maybe even something you want to add to this article. Feel free to join the discussion below.