Thermocouples are used very frequently in furnace construction, process heat, plastics processing, mechanical engineering, laboratories, heat treatment and service. They are robust, fast and suitable for high temperatures. At the same time, thermocouple measurement chains are prone to errors because the sensor itself is not the only factor that matters. Thermocouple type, compensating cable, terminal point, cold junction, temperature controller, transmitter, PLC input and parameterization must all match exactly.
When a temperature display appears incorrect, the thermocouple sensor is often replaced immediately. However, this only solves the problem if the sensor is actually the cause. Frequently, the fault lies in the controller, the compensating cable, cold junction compensation or the incorrect setting of the input type. This is exactly why thermocouple simulation is a very effective tool for troubleshooting.
When simulating a thermocouple, a calibrator generates a defined thermocouple or mV signal. This makes it possible to check whether a temperature controller, transmitter, data logger or PLC input displays the correct temperature value for a specified Type K, J or N signal. The measurement chain can therefore be checked step by step without immediately heating up the process or removing a sensor.
Table of contents
- Basics: what does thermocouple simulation mean?
- Understanding mV signal, thermocouple type and characteristic curve
- Type K, J and N: why the correct thermocouple type is decisive
- Correctly evaluating cold junction and cold junction compensation
- Testing the measurement chain: sensor, cable, controller, transmitter and PLC input
- Simulation of test points: zero point, working range and full-scale value
- Detecting compensating cable, polarity and connection errors
- Testing thermocouple transmitters and 4–20 mA output
- Practical example: furnace controller shows incorrect temperature despite new sensor
- Which measuring instruments / products are suitable?
- Conclusion: thermocouple simulation separates sensor faults from evaluation errors
- FAQ: frequently asked questions about thermocouple simulation
Basics: what does thermocouple simulation mean?
A thermocouple generates a very small electrical voltage depending on the temperature difference between the measuring point and the reference junction. This voltage is in the millivolt range. A temperature controller, transmitter, data logger or PLC input evaluates this voltage using the characteristic curve of the configured thermocouple type and displays a temperature from it.
When simulating a thermocouple, a calibrator takes over the role of the sensor. It outputs a defined signal that corresponds to a specific thermocouple type and a specific temperature. For example, the user sets “Type K, 600 °C”. The evaluation device should then also display approximately 600 °C, provided that input, parameterization, terminals and cold junction compensation are working correctly.
The major advantage is the clear separation of fault causes. If the controller displays correctly at a simulated temperature, the input is basically set correctly. If the real sensor still measures incorrectly afterwards, the cause is more likely to be in the sensor, compensating cable, installation situation or process. If the controller already displays incorrectly during simulation, the cause must be sought in parameterization, input, reference junction or wiring.
Simulation does not replace every temperature calibration. It mainly checks the electrical evaluation of the measurement chain. If the sensor itself is to be tested at a real temperature, a temperature calibrator, dry block calibrator or comparison bath is additionally required. For quick troubleshooting on controllers, transmitters and PLC inputs, however, thermocouple simulation is particularly efficient.
| Test approach | What is tested? | Typical benefit |
|---|---|---|
| Thermocouple simulation | Controller, transmitter, data logger or PLC input with defined TC signal | Fast test of electrical evaluation without a real temperature process |
| mV simulation | Pure input voltage independent of TC type selection | Testing whether an input processes electrical signals correctly |
| Comparison measurement with reference sensor | Real temperature at the process or component | Plausibility check between installed sensor and reference measurement |
| Dry block calibrator / temperature calibrator | Thermocouple sensor at a defined real temperature | Testing the sensor itself and, if required, the entire measurement chain |
Understanding mV signal, thermocouple type and characteristic curve
Thermocouples do not work like resistance thermometers. A Pt100 changes its electrical resistance with temperature. A thermocouple, on the other hand, generates a thermoelectric voltage. This voltage is very small and depends on the metal pair used as well as on the temperature difference between measuring point and connection point.
For this reason, the evaluation device must know which thermocouple type is connected. Type K, Type J and Type N have different characteristic curves. At the same temperature, they do not generate the same voltage. If a Type K sensor is operated on an input set to Type J, the display can deviate significantly even though sensor and cable are technically intact.
A calibrator internally converts these characteristic curves. The user selects the thermocouple type and the desired temperature value. The device then generates the appropriate mV voltage for this type. Alternatively, many calibrators can output a direct mV value. This is helpful when not the temperature characteristic curve, but the electrical input itself is to be tested.
During troubleshooting, a clear distinction should be made: am I simulating a thermocouple type with a temperature value, or am I simulating a pure mV voltage? Thermocouple simulation checks the input including type characteristic curve and cold junction reference. mV simulation focuses more strongly on electrical signal processing without automatic interpretation as temperature.
Type K, J and N: why the correct thermocouple type is decisive
Type K is one of the most widely used thermocouple types in industrial applications. It is frequently used in furnaces, machines, exhaust ducts, plastics processing, heat treatment and service tasks. However, its widespread use also means that in practice it is sometimes assumed too quickly that an existing sensor is automatically Type K.
Type J is often found in older systems and in industrial temperature measurements with more limited temperature ranges. If an old Type J sensor is replaced by a mechanically compatible Type K sensor without changing the controller settings, a measurement error occurs. The reverse also applies. Mechanically compatible sensors are not automatically electrically interchangeable.
Type N is often used where higher temperatures and better long-term stability are required. It is not automatically available in every evaluation unit. Therefore, before simulation and before replacing the sensor, it must be checked whether the controller, transmitter or PLC input actually supports Type N.
Simulating Type K, J or N makes it very quick to check whether the evaluation unit is set to the correct type. If a calibrator specifies Type K at 500 °C and the controller only responds plausibly when set to Type J, the parameterization or documentation must be checked. Such errors are particularly critical because the displayed values often appear plausible but are still incorrect.
| Thermocouple type | Typical application | Typical practical error | Test with calibrator |
|---|---|---|---|
| Type K | Furnace construction, mechanical engineering, exhaust gas, plastics processing, service | Sensor is assumed to be Type K although another type is installed | Simulate Type K and compare display on controller or PLC |
| Type J | Older industrial systems, heat processes, machines | Replacement sensor is mechanically suitable but wrong thermocouple type | Simulate Type J and compare parameterization with documentation |
| Type N | Higher temperatures, applications with good long-term stability | Evaluation device does not support Type N or is set incorrectly | Simulate Type N and specifically test input function |
Correctly evaluating cold junction and cold junction compensation
A thermocouple does not only measure at its hot measuring point. The decisive factor is always the temperature difference between the measuring point and the reference junction. The connection point at the controller, transmitter or calibrator is often referred to as the cold junction or reference junction. In order to convert the thermoelectric voltage into a correct temperature, this reference junction temperature must be taken into account.
Modern evaluation devices have cold junction compensation for this purpose. It measures or takes into account the temperature at the connection terminals and corrects the displayed value. If this compensation is faulty or influenced by unfavorable ambient conditions, temperature deviations occur.
During simulation, it is therefore important how the calibrator is connected and which cold junction compensation is active. Depending on the test setup, the calibrator can internally account for the reference junction or output an mV signal that tests the cold junction logic of the connected device. The user must know whether the complete thermocouple input including cold junction compensation or only pure mV processing is to be tested.
In practice, errors often occur in control cabinets. Terminals there can be heated differently by power electronics, ambient temperature, fans, sunlight or neighboring modules. If the thermocouple cable transitions to copper at unfavorable points or if the terminal temperature is not stable, the display can drift even though the sensor itself is in good condition.
Testing the measurement chain: sensor, cable, controller, transmitter and PLC input
A thermocouple measurement chain consists of several parts. At the beginning is the sensor with its measuring junction. This is followed by thermocouple cable or compensating cable, connectors, terminals, possibly a temperature transmitter and finally a controller, display, data logger or PLC input. Each of these parts can cause an error.
Simulation is especially helpful for testing the evaluation unit. If the calibrator is connected directly to the controller, it can be determined whether the input is parameterized correctly. If the calibrator is connected at an intermediate terminal, part of the cable route is also included. If simulation is carried out at the beginning of the measurement chain, compensating cable and terminal points can also be evaluated.
For troubleshooting, it is useful to proceed step by step. First, the input is tested directly. If it responds correctly, the simulation is moved further toward the field. This makes it possible to narrow down whether the fault is in the evaluation device, control cabinet wiring, compensating cable or sensor area.
Proper documentation is important here. Which thermocouple type was simulated? At which point was the signal injected? Which temperature points were tested? What values did the controller or PLC display? Without this information, troubleshooting can hardly be traced later.
Simulation of test points: zero point, working range and full-scale value
A single test value is usually not sufficient to reliably assess a measurement chain. If only 100 °C is simulated, gross errors can be detected, but not necessarily incorrect scaling, non-linearity, shifted limit values or an incorrect display unit. It is more useful to test several points across the relevant working range.
In a furnace process, for example, 100 °C, 500 °C and 900 °C can be simulated, provided that these values match the thermocouple type used and the system. In a plastics process, other points may be more appropriate, such as start temperature, typical production temperature and upper process limit. The decisive factor is not a rigid scheme, but the real working range of the measuring point.
Testing several points shows whether the input responds plausibly and linearly across the range. If all points are shifted by approximately the same amount, this points more toward cold junction, offset or connection problems. If the deviation increases with rising temperature, wrong thermocouple type, characteristic curve or parameterization become more likely.
| Test point | Why useful? | Typical conclusion |
|---|---|---|
| Low point in the working range | Checks basic function, connection and display near start or ambient temperature | Helps identify gross connection or type errors |
| Typical process point | Checks the measurement chain where it most often operates during production | Particularly relevant for control and process release |
| Upper working range | Checks display, characteristic curve and limit value behavior at high temperature | Shows scaling, type or linearization problems more clearly |
| Limit value / switching point | Checks alarm, shutdown, relay or PLC logic | Important for process safety and documented functional testing |
Detecting compensating cable, polarity and connection errors
With thermocouples, the cable is not just an electrical wire, but part of the measurement chain. Arbitrary copper cables must not be used between sensor and evaluation device if this creates additional thermoelectric voltages. Suitable thermocouple cables, extension cables or compensating cables that match the thermocouple type must be used.
An incorrect compensating cable can generate stable but incorrect values. This is particularly dangerous because the display does not necessarily jump or issue an error message. The temperature value may appear plausible even though it is systematically shifted. Additional errors occur especially at terminal points with temperature differences.
Polarity is also a common source of error. Thermocouple cables have positive and negative conductors. If these are reversed, the display may react implausibly or move in the wrong direction when the temperature changes. When connecting calibrator, controller and compensating cable, thermocouple type, polarity and terminal assignment must therefore always be observed.
Simulation can make such errors visible. If the calibrator displays correctly when connected directly to the controller, but incorrect values occur at a more distant terminal, the fault is probably in the cable, terminal, polarity or material transition between them. This significantly narrows down troubleshooting.
Testing thermocouple transmitters and 4–20 mA output
In many systems, the thermocouple signal is not routed directly to the PLC. Instead, a temperature transmitter converts the thermocouple signal into a robust standard signal, frequently 4–20 mA. This is particularly useful when longer cable routes, interference, galvanic isolation, simple PLC connection or a uniform signal structure are required.
When testing a thermocouple transmitter, there are two sides. On the input side, a Type K, Type J or Type N signal is simulated. The transmitter should process the corresponding temperature value from it. On the output side, it is checked whether the 4–20 mA signal correctly matches the configured temperature span. For example, if the transmitter is scaled to 0 to 1000 °C, 4 mA should correspond to the lower range and 20 mA to the upper range.
A thermocouple calibrator is suitable for the input side. For the output side, the UPS4E loop calibrator is a suitable tool. It can be used to measure and simulate 4–20 mA signals, check loop supply and verify PLC scaling. This makes it possible to separate whether the fault lies at the thermocouple input, the transmitter or the PLC evaluation.
Scaling errors are common, especially with temperature transmitters. The transmitter may be correctly set to Type K, but scaled to 0 to 800 °C, while the PLC expects 0 to 1000 °C. In that case, neither the sensor nor the calibrator is at fault. The measurement chain is simply not parameterized consistently.
Practical example: furnace controller shows incorrect temperature despite new sensor
A company operates an industrial furnace with a Type K thermocouple. After maintenance, the furnace controller displays about 35 °C less than a comparison measurement during operation. The sensor has already been replaced, but the error remains. Initially, it is assumed that the new sensor is also faulty.
A service technician connects a thermocouple calibrator directly to the controller input and simulates Type K at several test points. At 200 °C, 500 °C and 800 °C, the controller displays plausible values in each case. This makes it clear that the controller input is basically working correctly and that the parameterization for Type K is probably correct.
The simulation is then repeated at an intermediate terminal in the control cabinet. There, a clear deviation suddenly occurs. When checking the wiring, it becomes apparent that a section of unsuitable copper cable has been inserted between the compensating cable and controller. In addition, this terminal point is located in a warm area of the control cabinet near a power contactor.
After replacing it with a continuous, type-compatible compensating cable and clean terminal routing, simulated values and real comparison measurement match again. The fault was not in the thermocouple sensor, but in the measurement chain between sensor and controller.
Which measuring instruments / products are suitable?
For testing thermocouple measurement chains, the category simulators is relevant. Simulators help to specify defined sensor signals and thus test controllers, transmitters, displays, data loggers or PLC inputs. This is particularly helpful with thermocouples because wrong type, compensating cable and cold junction compensation are frequent sources of error.
The C.A 1621 calibrator for thermocouple sensors J, K, T, E, R, S, B and N is particularly suitable for simulating and measuring thermocouple types as well as mV signals. It allows temperature controllers, transmitters and inputs to be tested specifically with defined Type K, Type J or Type N values.
The category process calibrators / electrical calibrators is useful when additional process and electrical signals need to be tested alongside thermocouple signals. In maintenance and service, measurement chains rarely occur in isolation. Temperature, mV, mA, voltage, switching points or transmitters often need to be considered together.
For measurement chains with temperature transmitter and 4–20 mA output, the UPS4E loop calibrator should also be considered. While the thermocouple calibrator tests the input side of the transmitter, the UPS4E checks the output side to the PLC, display or control system. This combination makes troubleshooting significantly safer and faster.
| Product / area | Typical use | Particularly relevant for |
|---|---|---|
| Simulators | Simulation of defined sensor signals | Testing controllers, displays, transmitters and PLC inputs |
| C.A 1621 thermocouple calibrator | Measurement and simulation of thermocouple types and mV signals | Type K, J, T, E, R, S, B and N, furnace construction, process heat, service and troubleshooting |
| Process calibrators / electrical calibrators | Testing process, temperature and electrical signals | Maintenance, calibration, measurement chain testing and plant service |
| UPS4E loop calibrator | Testing and simulation of 4–20 mA signals | Temperature transmitters, PLC scaling, current loops and output signal testing |
Conclusion: thermocouple simulation separates sensor faults from evaluation errors
Thermocouple simulation is a very practical method for systematically checking temperature measurement chains. It quickly shows whether temperature controllers, transmitters, data loggers or PLC inputs respond correctly to Type K, J or N. This makes it clear whether the fault is actually in the sensor or whether evaluation, parameterization, compensating cable or cold junction compensation is the cause.
Testing several temperature points in the real working range is particularly important. A single test value is often not sufficient for a reliable assessment. Low, medium and high test points make it much easier to detect wrong thermocouple type, scaling errors, offset, cold junction problems and limit value errors.
The most important recommendation is: always consider thermocouple measurement chains as a complete system. Sensor, compensating cable, terminals, reference junction, controller, transmitter and PLC must match. A thermocouple calibrator checks the input side, while a loop calibrator such as the UPS4E checks the 4–20 mA output side for transmitters. This allows faults to be narrowed down faster and unnecessary sensor replacements to be avoided.
FAQ: frequently asked questions about thermocouple simulation
What does thermocouple simulation mean?
Thermocouple simulation means using a calibrator to generate a defined thermocouple or mV signal. The connected evaluation device, for example a temperature controller, transmitter or PLC input, should display the corresponding temperature value from it.
Why is thermocouple simulation useful?
It helps separate sensor faults from evaluation errors. If a controller displays correctly with simulated thermocouple values, the input is probably parameterized correctly. If the real sensor still measures incorrectly, the cause is more likely to be in the sensor, cable, installation or process.
Can Type K be simulated with a thermocouple calibrator?
Yes, a suitable thermocouple calibrator can simulate Type K. A defined temperature value is set, and the calibrator generates the corresponding thermocouple signal. The connected controller or input should display the same value.
Can Type J and Type N also be simulated?
Yes, provided that the calibrator supports these thermocouple types. Type J is common in many older industrial systems. Type N is often used at higher temperatures and in applications with good long-term stability. It is important that the evaluation device also supports the respective type.
What is the difference between thermocouple simulation and mV simulation?
In thermocouple simulation, a thermocouple type and a temperature value are selected. The calibrator generates the corresponding signal from this. In mV simulation, a voltage is specified directly. This is helpful when the electrical input is to be tested independently of the temperature characteristic curve.
Why is the correct thermocouple type so important?
Each thermocouple type has its own characteristic curve. Type K, J and N generate different voltages at the same temperature. If the input is set to the wrong type, the display can deviate significantly even though the sensor is technically functioning.
What is the cold junction?
The cold junction is the connection point where the thermocouple signal enters the evaluation device. Since a thermocouple measures a temperature difference, the temperature of this reference junction must be taken into account. This is done by cold junction compensation.
Why can cold junction compensation cause errors?
If the connection point is thermally influenced or the cold junction compensation does not work correctly, an incorrect temperature value results. Typical causes include warm control cabinet areas, power electronics, fans, sunlight or unfavorable terminal points.
How many test points should be simulated?
In practice, at least three test points are useful: a low point, a typical process point and an upper working range. In addition, switching points or alarm limits can be checked. The test points should match the real application.
How do you test a temperature controller with thermocouple input?
The calibrator is connected to the controller input instead of the thermocouple. Defined temperature values for the correct thermocouple type are then simulated. The controller should display these values within the permissible deviation and trigger switching points correctly.
How can an incorrect compensating cable be detected?
An incorrect compensating cable can generate stable but incorrect temperature values. If simulation directly at the controller is correct, but deviates when injected at a more distant terminal, the cause is often in the cable, terminal point, polarity or material transition.
What happens if polarity is reversed?
If polarity is reversed, the temperature display may react implausibly or move in the wrong direction when heated. For this reason, plus and minus of the thermocouple as well as terminal assignment of the calibrator and controller must be checked carefully.
Can a thermocouple calibrator test the sensor itself?
A thermocouple calibrator mainly tests the electrical evaluation or can measure thermocouple signals. To test the sensor at a real temperature, an additional temperature calibrator, dry block calibrator or comparison measurement with a defined temperature is required.
How do you test a thermocouple transmitter?
On the input side, a defined thermocouple value is simulated. On the output side, it is checked whether the transmitter delivers the appropriate output signal, for example 4–20 mA. This makes it possible to identify whether input, scaling and output are working correctly.
Why is the UPS4E helpful with temperature transmitters?
If a temperature transmitter sends a 4–20 mA signal to the PLC, this current loop must also be checked. The UPS4E can measure and simulate mA signals. This makes it possible to determine whether PLC scaling and signal processing are correct.
When should a thermocouple measurement chain be calibrated?
Calibration is useful when measured values are quality-relevant, deviations have been detected, sensors or transmitters have been replaced, or process releases depend on the temperature measurement. Depending on the requirement, the complete measurement chain should be considered.