The physical principle: Heat becomes tension
The basis of every thermocouple is the Seebeck effect. Two conductors made of different materials are connected at the measuring point. If there is a temperature gradient between this hot measuring point and the colder connection or reference point, a thermal voltage arises.
What is physically important is not simply “two metals touching”. The different Seebeck coefficients of the materials along the temperature gradient are crucial. That's why each pair of thermocouples has its own characteristic curve and requires the appropriate evaluation.
The thermocouple does not measure absolute temperature directly. It delivers a small voltage in the microvolt to millivolt range. The measuring device calculates the temperature from this and must properly compensate for the reference junction.
Schematic structure: The thermal voltage arises from the temperature gradient between the measuring point and the reference point. The measuring device must take into account the thermocouple type and the cold junction temperature.
The entire measuring chain is crucial: thermocouple, measuring point, protective tube, cable, connection and evaluation must fit together.
Structure and components of an industrial thermocouple
An industrially used thermocouple consists of several levels. Each level influences measurement quality, service life and response behaviour. With sheathed thermocouples, the thermal wires are in mineral insulation and protected by a metal jacket.
Longitudinal section of a sheathed thermocouple: Depending on the design, the measuring point can be grounded, insulated or free-standing.
Four components determine the measuring point
| component | function | Typical version |
|---|---|---|
| thermocouple | Generates the thermal voltage through the Seebeck effect. | NiCr/Ni, Fe/CuNi, PtRh/Pt |
| insulation | Electrically isolates the conductors and conducts heat to the measuring point. | MgO, Al₂O₃ ceramic, glass fibre |
| protective tube | Protects against mechanics, medium and atmosphere. | Stainless steel, Inconel, SiC, Al₂O₃ |
| Connection | Connects thermocouple and measuring line. | Connection head, plug, compensation cable |
Measuring point: grounded, isolated or free-standing?
Grounded measuring point
The measuring point has thermal contact with the jacket. This reacts quickly, but can become problematic in the event of electrical interference or potential differences.
Isolated measuring point
The measuring point is electrically separated from the jacket. This reduces interference, but usually reacts more slowly than a grounded version.
Standardized thermocouple types according to IEC 60584
IEC 60584 or DIN EN 60584 describes standardized thermocouple types with standardized characteristics. The type designation alone is not sufficient for selection. The decisive factors are the temperature window, atmosphere, protective tube, wire diameter, installation position and the required measurement quality.
| type | material pair | Typical direction of use | What to pay attention to? | Practical benefits |
|---|---|---|---|---|
| K | NiCr/Ni | widely used base metal type | Check drift, atmosphere and green rot in critical areas | good introduction to many oven, tool and machine processes |
| J | Fe/CuNi | low to medium temperature range | Pay attention to iron legs and oxidizing atmosphere | suitable if the process, controller and environment match type J |
| N | NiCrSi/NiSi | Base metal type for more demanding high temperature areas | check as an alternative if type K becomes critical due to drift or atmosphere | More stable measuring point in a suitable protective tube design |
| E | NiCr/CuNi | Applications with high thermal voltage | Coordinate temperature window and evaluation | strong when signal resolution and dynamics are important |
| T | Cu/CuNi | low and moderate temperature ranges | Take process humidity and copper legs into account | useful with suitable media and moderate process conditions |
| R | Pt13Rh / Pt | Precious metal type for high temperatures | Pay attention to the cleanliness of ceramics and surroundings | for processes in which stability and material purity are crucial |
| S | Pt10Rh / Pt | Precious metal type for high temperatures | sensitive to contamination; Select protective tube carefully | for high temperature and testing tasks with a clean measuring chain |
| b | Pt30Rh / Pt6Rh | very high temperature ranges | Consider lower thermal voltage and evaluation | for high process temperatures with a cleanly designed protective tube concept |
The table serves as a pre-selection. The specific design must be checked according to temperature range, atmosphere, design, protective tube, cable and evaluation.
Qualitative representation: The curves run differently depending on the material pair. Therefore, the thermocouple type, compensation cable and evaluation device must match.
Protective tubes and materials: The casing plays a decisive role
The protective tube is not a secondary component. It determines whether the measuring point survives the process, how quickly it reacts and whether the thermal wires remain chemically clean. A fast measuring point requires a low thermal mass. A robust measuring point needs material reserves and the right material.
Metallic protective tubes
For many industrial processes, hot gases, mechanical engineering and oven areas. Materials such as stainless steel, 1.4841 or Inconel are selected based on temperature, atmosphere and mechanical stress.
- mechanically resilient
- good heat conduction
- suitable for many standard processes
Ceramic protective tubes
For very high temperatures, aggressive media or electrically insulating requirements. Al₂O₃ and SiC are used depending on the process environment.
- high temperature resistance
- chemical resistance
- relevant for precious metal thermocouples
| Material group | Examples | Typical use | Crucial point |
|---|---|---|---|
| Stainless steel / steel | 1.0305, 1.4571, 1.4841 | Mechanical engineering, furnace construction, neutral to oxidizing atmosphere | Check temperature, corrosion and scale formation |
| Nickel base | Inconel 600 | High temperature, hot gases, salts, carburizing areas | Balance service life against chemical exposure |
| Ceramics | C530, C610, C799 | high temperatures, electrical insulation, precious metal types | Pay attention to gas tightness, purity and thermal shock |
| Silicon carbide | SiC, RSiC | Melting, strong temperature changes, furnace processes | Check resistance to temperature changes and media attack |
For Type R and Type S, purity is critical. Contamination from unsuitable ceramics can measurably distort the precious metal thermocouples.
Interference, aging and typical sources of error
Thermocouples are robust, but not independent of the environment and installation. The most common errors are not caused by the measuring principle, but by drift, incorrect cables, unsuitable protective tubes, cold junction errors, contamination or mechanical tension in the thermocouple.
Green rot in type K
In the range of around 800 to 1050 °C, chromium in the NiCr leg can become depleted, especially if there is a lack of oxygen. The result is drift due to falling thermal voltage. Type N or a tight conduit may be a better choice.
K-effect
With type K, a short-range order in the crystal lattice between around 400 and 600 °C can lead to reversible measurement deviations. Rapid cooling through this temperature range is critical.
Poisoning and diffusion
Sulfur, silicon, phosphorus or hydrogen can change thermal wires. With precious metal types, small amounts of impurities are enough to shift the characteristic curve.
Management and comparison point
Incorrect compensation lines or missing cold junction compensation create systematic errors. This is not a sensor error, but an error in the measurement chain.
| Source of error | Caused | risk | Practical measure |
|---|---|---|---|
| Wrong management | Compensation line does not match the type | systematic measurement error | Select cable by thermocouple type |
| Comparison point | missing or incorrect compensation | shifted temperature display | Check cold junction compensation |
| Aging/drift | Structural change, oxidation, diffusion | creeping deviation | Coordinate the type, protective tube and test interval |
| Green rot | Chromium depletion in type K | decreasing thermal voltage and drift | Check type N or protective tube concept |
| K-effect | Short-range order in the NiCr leg | reversible deviation in the middle range | Check cooling conditions and type selection |
| Construction too sluggish | massive protective tube, unfavorable immersion depth | Control reacts too late | Balance response time against service life |
| Mechanical tension | tight bending radii, tension, vibration | Breakage or unstable measurement | Lay out the installation and strain relief neatly |
Thermocouple or Pt100?
A Pt100 is not a thermocouple. Both types of sensors measure temperature, but according to completely different physical principles. The thermocouple uses the Seebeck effect and generates a small voltage itself. The Pt100 is a resistance temperature sensor: Platinum has a nominal resistance of 100 ohms at 0 °C, which changes with temperature. For this, the measurement requires a measuring current.
It is precisely this separation that is important for users. Those who only ask about “temperature sensors” often overlook the type of signal, controller connection, cable, required measurement quality and operating temperature.
| feature | Thermocouple | Pt100/Pt1000 |
|---|---|---|
| principle | Seebeck effect, thermal voltage from temperature difference | Change in resistance of a platinum measuring element |
| signal | µV to mV, type-dependent characteristic | Ohm signal, Pt100 = 100 Ω at 0 °C |
| energy | self-generating, no supply voltage for the sensor element | Measuring current required, note self-heating |
| Strength | high temperatures, small size, fast response, vibration | Reproducibility, stable evaluation, control processes in the low and medium temperature range |
| Typical limit | Evaluation, drift, compensation line, protective tube | Temperature range, mechanical robustness, response time |
Thermocouple
- high temperatures possible
- small designs can be achieved
- fast response times
- robust in the face of vibration and tight installation space
Pt100/Pt1000
- suitable for high measurement quality requirements
- very good reproducibility
- stable characteristic curve
- clean for the laboratory, test stand and control processes
Rule of thumb: Pt100 is strong when measurement quality and reproducibility are more important than temperature peaks. Thermocouple is strong when heat, dynamics, size or mechanics determine the process.
Selection aid: find the right measuring point in six steps
The selection does not start with the sensor type, but with the measurement task. This sequence reduces wrong decisions in design, purchasing and maintenance. As a manufacturer with 100% product depth, Keller, Ihne & Tesch can view the measuring point from the thermocouple to the connection as a complete version.
Define measuring range
Consider normal range, minimum temperature, maximum temperature and short-term peaks separately. The highest number alone is not decisive.
Clarify medium
Oxidizing, reducing, neutral, moist, aggressive, melt or gas flow influence the type, protective tube and service life.
Determine measurement quality
Check limit deviation, drift, calibration requirement, controller and reference junction together.
Check mechanics
Define installation depth, diameter, pressure, flow, vibration, bend radius and strain relief.
Evaluate response time
Thin protective tubes react more quickly and robust structures can withstand more mechanically. This is often where the conflict of goals lies.
Determine connection
Match the connection head, plug, cable, compensating cable, built-in parts and controller connection to the measuring chain.
Industries and typical applications
Thermocouples are used wherever temperature must be reliably recorded under industrial conditions. The design is determined more by the process and installation than by the sensor type alone.
| Industry | Typical application | Common types | Special feature |
|---|---|---|---|
| Plastic processing | Cylinder, nozzle, tool, hot runner | J, K | tight installation spaces and quick response |
| Furnace construction / heat treatment | Oven room, batch, hot air | K, N, S | Temperature, atmosphere and protective tube are crucial |
| Metal / Foundry | Melt, crucible, pouring trough | K, N, S | Protective tube against attack by the medium |
| Chemistry / process engineering | Reactor, pipeline, distillation | K, J, N | Check medium, pressure and connection type |
| Test bench / automotive | Exhaust gas, engine, component testing | K, N, J | Vibration and short response times |
| Electronics/Semiconductors | Process furnaces, heat processes | S, B, K | Pay attention to cleanliness and process environment |
Technically clarify thermocouple Send temperature range, medium, installation situation, desired response time and connection type. We test the thermocouple, protective tube, cable and connection as a complete measuring point.
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