Temperature measurement: thermocouple, Pt100 or infrared?
Temperature can be measured with contact sensors or non-contact methods. Thermocouples cover a very wide range and are robust, Pt100 resistance sensors are more accurate, and infrared thermometers read temperatures fast and without contact. The right choice depends on range, accuracy, access to the object and the required response time.
View measurement toolsThermocouple or Pt100 - which contact sensor fits?
Contact sensors touch the object and take on its temperature. Two principles dominate: the thermocouple generates a temperature-dependent voltage from two dissimilar metals and covers a very wide range robustly. The Pt100 resistance sensor (RTD) uses the resistance change of platinum and measures more accurately, but over a narrower range.
Thermocouples come in standardised types. Type K (nickel-chromium/nickel-aluminium) is the universal standard with a wide range and good robustness. Type J (iron-constantan) is common in older plants, while Type T (copper-constantan) suits low temperatures and is accurate there. All share fast response times and high mechanical durability.
| Sensor type | Measuring range | Use |
|---|---|---|
| Thermocouple Type K | -200 to +1300 °C | universal standard, robust |
| Thermocouple Type J | -40 to +750 °C | older plants, reducing atmospheres |
| Thermocouple Type T | -200 to +350 °C | low temperatures, accurate |
| Pt100 (RTD) | -200 to +600 °C | high accuracy, lab and process |
| Infrared / pyrometer | -50 to +1000 °C and above | non-contact, fast, moving objects |
As a rule of thumb: where robustness, high temperatures and speed matter, a thermocouple is the first choice. Where accuracy and repeatability in the mid range count, the Pt100 shows its strength. Both, however, require direct contact with the object.
When is an infrared thermometer the better choice?
Infrared thermometers and pyrometers measure without contact: they capture the thermal radiation emitted by the object and calculate its surface temperature from it. This is fast, wear-free and ideal for moving, very hot, live or hard-to-reach objects that cannot or must not be touched.
The reading takes a fraction of a second without contact - good for moving belts and processes.
Emissivity must be set correctly; bare metals radiate little and otherwise distort the reading.
It reads a spot whose size grows with distance (distance-to-spot ratio) - mind the distance.
It captures surface temperature, not the core; reflections and windows can interfere.
The key setting is emissivity: it describes how strongly a surface radiates compared with an ideal emitter. Matte, dark surfaces are close to 0.95, while bare or polished metals are much lower. Without a correct setting, the device reads too low on shiny surfaces. A stuck-on matte-black measuring dot or tape with a known emissivity helps in such cases.
How do I choose by range, accuracy and response time?
The choice follows four questions: what temperature range must be covered, what accuracy is required, is contact with the object possible, and how quickly must the reading settle? The answers almost always make it clear whether a thermocouple, a Pt100 or infrared is the right solution.
On range, thermocouples reach the widest and highest, the Pt100 covers the mid range well with high accuracy, and infrared has the edge on very hot or inaccessible objects. On accuracy the Pt100 leads, the thermocouple is coarser, and infrared depends heavily on the correct emissivity.
Response time is decisive in fast processes: thin thermocouples and non-contact infrared react very quickly, while more massive Pt100 sensors are a little slower but measure more stably and with less drift. If contact is not possible - for example on rotating or live parts - only infrared remains. For documented measurements, all methods should be calibrated regularly.
Frequently asked questions
Thermocouple or Pt100 - which is more accurate?
The Pt100 resistance sensor is more accurate and repeatable in the mid range, typically around ±0.15 °C. The thermocouple is coarser but covers a much wider and higher range and responds faster. Choose Pt100 for high accuracy and the thermocouple for wide ranges and robustness.
Why must I set the emissivity on an infrared thermometer?
Emissivity describes how strongly a surface radiates heat. If it is set wrongly, the device reads too low, especially on bare metals. Matte, dark surfaces are close to 0.95; for shiny surfaces a matte-black measuring dot or tape with a known emissivity helps.
When do I need a non-contact measurement?
Whenever contact is impossible or undesirable: on moving or rotating parts, very hot surfaces, live components or hard-to-reach spots. Infrared then measures fast and wear-free, but only the surface temperature.
Find the right temperature meter
From handheld thermocouple meters and Pt100 probes to infrared thermometers - we help you choose by range, accuracy and response time.
Neutral advice
Recommendation by application, range and accuracy, independent of brand.
Contact and non-contact
Thermocouple, Pt100 and infrared combined from one source.
Practical focus
Selection by the real process, not by the datasheet maximum.
Related reading
More on climate monitoring at /en/ratgeber/humidity-measurement.