The thermocouples that could be used for this purpose are as versatile as your applications. In measurement and control technology and in the control of processes and plants, correctly recorded temperatures are crucial for the trouble-free operation of the plants. Thermocouples are always used when the temperature has to be measured particularly quickly or when a large measuring range with sometimes very high temperatures is required.
In this guide, we have compiled information on what you should pay attention to when selecting, purchasing, installing and operating your thermocouple.
What is a thermocouple?
Thermocouples are thermoelectric sensors with two wires made of different materials. These are connected to each other at one point (measuring point) and one can measure a thermoelectric voltage at their ends (reference junction), which also changes when the temperature changes. This thermoelectric effect is also called Seebeck effect.
Thermocouples measure the temperature difference between the measuring point and the reference junction, i.e. the temperature at the reference junction must be known.
Like platinum sensors, thermocouples are also standardized and can be immersed regardless of the manufacturer. They also have a very fast response time and can be used at very high or very low temperatures. They are inexpensive and come in many different types. Thermocouples also have disadvantages: they are more sensitive than resistance-based temperature probes. Their characteristic curve is not linear. The measuring principle should be known to the user to avoid measuring errors.
How does a thermocouple work?
A thermocouple consists of two wires that are made of different materials. This is very important because the different materials also behave differently, creating a measurable thermoelectric voltage when you connect the two wires at one end. This thermoelectric voltage changes when the temperature changes. This effect is named after its discoverer - Seebeck effect.
Unlike resistance-based temperature probes, the temperature difference between the measuring point and the reference junction is determined. The temperature at the reference junction must therefore be known. Therefore, a resistance-based temperature sensor is often installed there. The advantage of thermocouples is that the measuring point and the reference junction do not have to be located next to each other and that the temperature is not "falsified" by line resistances. This means that the measuring tip can be inserted into the process, where it may be very hot, for example - but the temperature is determined at the reference junction.
What types of thermocouples are there?
Thermocouples differ in the different alloys from which the thermocouple wires are made. Based on these materials, different characteristic curves and measuring ranges can be mapped.
At Testo Sensor, we offer the following thermocouples depending on the type:
- Thermocouple type J (Fe/Cu-Ni)
- Thermocouple type K (Cu/Cu-Ni)
- Thermocouple type L (Fe-CuN/Cu-Ni)
We mark the thermocouples according to the IEC 60584-1 standard or, in the case of type L, according to Din 43710.
The exact measuring ranges of our thermocouples as well as the characteristic curves can be found in the operating instructions for each probe or in our download area.
What are the different types of thermocouples?
Thermocouples are used in many different applications. Therefore, just as with resistance-based temperature probes, there are numerous types. On the one hand, the type differs according to the type of measurement: thermocouples with a handle are used for insertion or spot measurements. If, on the other hand, the probe is to be permanently inserted into the process, then screw-in thermocouples or immersion thermocouples with cable, connector or measuring head are often used. The materials used, such as the compensating or thermocouple cables, may determine the temperature ranges in which the thermocouple can be installed. At Testo Sensor we offer the following thermocouples:
In each type we have different versions, so that you can find a thermocouple for many measuring tasks. However, if there are any open requests or you cannot find the type you need, please contact us.
What should I pay attention to when selecting the right thermocouple?
The right choice of thermocouple depends on your measuring task and the associated requirements for the temperature probe. We often hear questions like:
- Is the probe also compatible with?
- Which thermocouple should I choose?
- Can I change my thermocouple?
- Which cable do I need or even can I replace the cable?
Answering the following questions can help you with your selection:
Which type do you need for your thermocouple?
Select the required type for your thermocouple using the categories. You will find an overview of all types here.
What is the right thermocouple for me?
The choice of the thermocouple depends on the one hand on which measuring device you want to use the thermocouple. Please check which thermocouple is to be used according to the technical data or the operating instructions of the measuring device. The different thermocouple types are not compatible with each other and also have different characteristic curves. The characteristic curves of our thermocouples can be found here at a glance.
Does the thermocouple fit your measuring range?
In addition to the measuring device, please also check in which measuring range and with which accuracy / tolerance you want to use the thermocouple. Information on the measuring range and tolerance can also be found in the characteristic curves and in the operating instructions for each probe. With thermocouples, it is particularly important to adhere to the measuring range, because if the limit is exceeded, the metal alloy can be irreversibly damaged and then the thermocouple no longer measures within the specified tolerance range.
- How do I connect the thermocouples? Should I use a thermocouple cable or a compensating cable? Thermocouples are connected to the reference junction. By using thermocouple cables or compensating cables, the reference junction may be physically separated from the measuring junction, because often the transducer cannot be operated in close proximity to the measuring junction, as it is located in machinery or similar hot, dirty environments. Thus, it is necessary to transmit the measurement signal over a certain distance.
When do I use a compensating cable?
Compensating leads are made of most cost-efficient substitute materials that are not identical to the metal pairing of the thermocouple. Therefore, they behave exactly the same as the thermocouple only within a certain measuring range. Because of their limited measuring range, compensating cables are used primarily for cost reasons, as they are less expensive than thermocouple cables.
When do I use a thermocouple cable?
A thermocouple cable is made of the same material as a thermocouple, therefore the thermocouple cable also has the same physical properties and the same area of application as the thermocouple itself. Connecting the two wires at one end creates a thermocouple.
What do I have to pay attention to when insulating the compensating cable or the thermo cable?
In addition to the type of cable (i.e. whether a thermoelectric cable or a compensating cable is used), the area of application of the cable also depends on the insulation material used for the cable and its properties. PVC, PFA, silicone or glass fiber are used. In addition to the area of application, the cables differ in other properties such as resistance to dust and moisture (IP class) and resistance to aggressive media. You will find the following cables in our range:
Do I need a connector and what size should the connector be?
Thermocouples are either connected directly to the meter. However, it is much more common for thermocouples to be connected to the measuring device via a plug connector. At Testo Sensor you will find the following connectors on offer:
- Mini TC connector
- Standard TC connector
- Mini TC connector
- Standard TC coupling
How do you ensure the correct mechanical connection of the thermocouple to the process?
Correct mechanical connection of the probe to the process ensures efficient, error-free and trouble-free measurement.
Which screw-in thread / process connection does my thermocouple need?
Please make sure to choose the correct thread size for screw-in thermocouples.
Which neck tube length / sleeve length do I need for my thermocouple?
For thermocouples with variable sleeve length or neck tube length, please measure in advance which length you need. Each probe requires a minimum immersion depth to accept and correctly measure the temperature of the medium being measured. As a rule of thumb, the following advice can serve you: optimally, the immersion depth should correspond to 10 times the thermowell diameter, but at least a 5-fold value should be used.
Which sheath diameter should my thermocouple have?
Please consider what diameter you need: When measuring temperature in solids or when the probe is mounted in an immersion sleeve, the air surrounding the probe slows the conduction of heat from the medium to the probe. This also slows down the temperature measurement and, in extreme cases, leads to incorrect readings. Therefore, please make sure that the thermocouple is coupled to your measuring medium in the best possible way in terms of thermal technology, i.e. select the sleeve diameter so that the air gap is as small as possible.
We have provided more detailed information on mounting and immersion depths in the operating instructions, which you can download for each probe.
What environmental influences is the thermocouple subject to?
Please also pay attention to whether you have certain requirements at the measuring point or in your process regarding temperature, pressure, IP protection, steam or similar and compare your specification with our data sheets. If you have any questions or are unsure, please feel free to contact us.
What do I have to consider when mounting the thermocouple?
If you are wondering: Where do I have to mount the probe? Or how can I mount the thermocouple? Then take a look at the instruction manual. Because mounting is as different as the probes themselves and also differs depending on the probe design, we have stored in the operating instructions of the respective probe exactly where and how the probe is to be mounted and what you have to pay attention to during mounting. You will find the operating instructions with each probe or in our download area.
Which accessories do I need for my thermocouple?
When ordering, please consider whether you need any accessories. Please check the delivery details for each probe to see what is included.
We offer the following accessories:
Replacement of a defective thermocouple
When replacing a defective temperature probe, please pay close attention to which thermocouple you need, because the thermocouple types are not compatible with each other. Therefore, please refer to the technical data of the respective system to find out which type has been installed. If the required information is not apparent in the operating instructions of the measuring device / equipment, the manufacturer of the machine or system should be contacted in case of doubt.
When the exact thermocouple type has been determined, the mechanical adaptation must be tested. The temperature can only be measured correctly if the probe can be mounted correctly at the installation site. Otherwise, machines and equipment can easily malfunction.
Also make sure that you order thermal paste and mounting accessories such as compression fittings, cutting ring screw fittings or bayonet nipples if these also need to be replaced.
Frequently asked questions about temperature sensors
How do I test a thermocouple for function?
The measurement curve of thermocouples is not linear. Therefore you need a special multimeter with an appropriate measuring input, which linearizes the signal and at the same time corrects the terminal point compensation, so that only the temperature measured at the measuring point is displayed.
Alternatively, you can check whether you can detect mechanical damage such as cable breakage or replace the thermocouple and see whether the fahler still occurs.
How can I tell that my thermocouple is defective?
The temperature probe shows strange values? The temperature probe does not show any values? This behavior may be a sign that your temperature probe is defective or that it has not been connected correctly.
To find out if there is a defect, you can simply measure the resistance value of resistance-based temperature probes and compare it with the specification on the probe itself or in the user manual. You can use a standard multimeter to do this.
If you are using a thermocouple instead of a resistance-based temperature probe, the situation is somewhat different. Here you need a multimeter with an appropriate connection for thermocouples, because thermocouples do not have a resistance signal, but a voltage signal.
Alternatively, you can test on a trial basis whether a replacement will remedy the situation.
How do I connect the temperature probe correctly?
When connecting your temperature probe, please make sure that you follow our wiring diagrams. You will find the wiring diagrams in the operating instructions for your temperature probe.
How can I protect a temperature probe from certain influences?
Select a temperature probe that is well suited to your measuring task. Depending on the medium in which you want to measure, you will need a different probe design. We have specified exactly which material we use in our operating instructions so that you can check for yourself whether the probe is suitable. Please pay attention to the measuring range and other environmental conditions such as dust, water, steam, solar or UV radiation.
We offer immersion sleeves in our accessories, which can protect the probe. In addition, especially for outdoor probes, you will also find a sun shield and a ball throw protection to shield your outdoor probes from environmental influences and to obtain the correct measurement result.
For easier orientation, we have always indicated suitable accessories in the respective operating instructions of the temperature probe.
To the thermocouples