Kamet stocks a wide range of thermocouples, designed and manufactured for different process environments and/or applications. The properties of each thermocouple’s unique alloyed wire composition, determines the sensor’s specifications. When selecting the most suitable (exotic) thermocouple , you need to match these characteristics to the operating/process parameters where it will be used. These include criteria such as: temperature range, response time, chemical and mechanical resistance and installation location.
A thermocouple that is not an optimal match can reduce process efficiency and product quality as well as diminish the accuracy, reliability and long-term performance of the sensor. In order to avoid potential problems in your application, we have compiled a number of questions to consider when choosing a thermocouple:
- Which temperature range will the thermocouple be exposed to?
- What is the desired response time of the measurement?
- What degree of accuracy and sensitivity is needed?
- Do you need to consider chemical compatibility?
- Do mechanical stresses need to be taken into consideration?
- Where in the process will the thermocouple be positioned?
Which temperature range will the thermocouple be exposed to?
Thermocouples can give unreliable measurement data if they are used for temperature measurements that are outside the specified working range. In the table below is an overview of Kamet’s thermocouples and the recommended maximum temperatures for each type
| Thermocouple | Chemical composition | Recommended max. working temperature |
|---|---|---|
| Type E | Cu-CuNi | 850°C |
| Type J | Fe-CuNi | 750°C |
| Type K | NiCr-Ni | 1200°C |
| Type N | NiCrSil-NiSil | 1200°C |
| Type T | Cu-Ni | 350°C |
| Type S | Pt10%Rh-Pt | 1450°C |
| Type R | Pt13%Rh-Pt | 1450°C |
| Type B | Pt30%Rh-Pt6%Rh | 1700°C |
| Type C | W5%Re-W26%Re | 2200°C |
| Type D | W3%Re-W25%Re | 2200°C |
It is important to ensure that the maximum and minimum temperatures of the application are within the range of accuracy and reliability for the chosen thermocouple type, taking error tolerance into account. However, there are more aspects to process temperature dynamics to consider than just the range. For example:
- Are there sustained high temperatures?
- Will the thermocouple be cycling between low and high temperatures repeatedly?
- How quickly will temperatures increase and decrease?
- Will chemical compatibility change at different temperatures?
These factors can influence how a particular thermocouple ages and thus can contribute to measurement error and reduced repeatability of measurements. In the table above you can open a more detailed product page for each thermocouple type to learn more about its (thermal) properties and applications.
What is the desired response time of the measurement?
The response time refers to the interval between the application of a change in temperature to the thermocouple and its corresponding change in output. In general terms, in systems which are characterized by rapid changes in temperature, a rapid response time is desirable as it helps to reduce error.
A thermocouple’s response time is influenced by several parameters, such as the:
- thermocouple dimensions: the thinner the wire the quicker the response.
- environment/medium with which the thermocouple is in contact: depending on the environment, the thermocouple components must be resistant to the presence of acid/alkaline/vacuum/pressure/oxygen/nitrogen/sulfur. Note: for extreme and challenging environments we offer specialized thermocouple solutions.
- type of sheath and junction: this we will explain in more detail below.
Types of Junction
Thermocouple junctions refer to the place where temperature measurement takes place – the point where two wires of different metals are welded together. Thermocouple sensors have three types of junctions: exposed, grounded or ungrounded and all of these influence the response time.
What is a grounded or ungrounded thermocouple?
A grounded thermocouple has the junction welded to the sheath at the sensor tip. The advantage of using a grounded thermocouple is that heat is more easily transferred, causing improved response times to temperature changes. The disadvantage is that the grounding sometimes causes an electrical ground loop that may damage equipment.
An ungrounded thermocouple has insulation, such as magnesium oxide, between the junction and the sheath wall. The insulation material reduces the chance of a ground loop because it is non-conductive. However, the response time is also slower.
What is an exposed junction thermocouple?
An exposed thermocouple has the junction outside the sheath. This provides very fast response times but is also vulnerable to hazardous environments such as those that cause oxidation.
In conclusion, thermocouples with grounded and ungrounded junctions, are both protected by a sheath and are the best choice for corrosive environments. Thermocouples with exposed junctions allow the prioritization of response times in environments with a low risk of damage.
What degree of accuracy and sensitivity is needed?
Accurate temperature measurement can be a critical issue and the various types of thermocouple each have their own specified accuracy and sensitivity levels. It is therefore important to be clear on the required accuracy for your application as part of the process of choosing the most suited thermocouple sensor.
Do you need to consider chemical compatibility?
Are there abrasive or corrosive chemicals in the medium or atmosphere to which the thermocouple will be exposed? Each thermocouple type has specific properties that make it more suited to particular chemical exposure and/or environments (e.g reducing, oxidizing, vacuum).
Or more detailed information in this regard please see our article “Temperature measurement in different atmospheres”.
Do mechanical stresses need to be taken into consideration?
Mechanical stress, pressure and vibrations can all have an impact on thermocouple accuracy. For example, vibration fatigue in thermocouples can cause the sensor to become uncalibrated, however it can also cause the insulation to fail and the thermocouple to short circuit. It is therefore important to consider what physical impact the thermocouple will experience in your process environment and choose a sensor that is designed to be robust and durable.
Where in the process will the thermocouple be positioned?
The exact location of the thermocouple in the process is important. Does the position bring any specific constraints that could be relevant to the choice of thermocouple? Examples of factors to consider include:
- Is there direct heat exposure?
- Does the sheath need to be able to bend?
- Are special extension/lead wires needed for that location?
- Will the thermocouple be a surface mount or immersion?
- Are there size restrictions (e.g. existing holes might determine what the maximum diameter can be.)
Further help with your thermocouple purchase
We hope this information will help you make an informed decision about thermocouple selection. As mentioned before, you can find more detailed information on the properties and specifications of all our thermocouples on our website. We also sell specialized (exotic) models, designed for ultra high temperatures and extremely challenging environments.
Alternatively, if you need advice, please contact the knowledgeable team at Kamet: we are happy to help you find the right thermocouple for your needs.
Considering the many factors to take into consideration, sometimes a particular application will call for a custom-made thermocouple. We have the in-house expertise to design this for you.