Five questions that decide the sensor
Most temperature measurements come down to three sensor types: thermocouples, platinum resistance thermometers (RTDs, usually PT100 or PT1000) and NTC thermistors. Work through the questions below in order. The first question that rules out a type usually settles the choice, and the later ones refine it: class, construction and connection.
1. What is the temperature range?
The range rules out more options than anything else.
- Above about 600 °C: a thermocouple. The tolerance classes of IEC 60751 end at 600 °C for wire-wound platinum elements. Type K or N works up to 1200 °C, and types R, S and B go higher, to about 1600 °C and 1700 °C in continuous use.
- About −200 to 600 °C: a PT100 or PT1000 gives the best accuracy and stability. A thermocouple also works and may be preferred for the reasons in step 3.
- About −40 to 150 °C in equipment and electronics: an NTC thermistor is often enough and costs least.
- Below about −200 °C: outside the scope of this guide. Cryogenic sensors such as silicon diodes or special RTDs are used there.
Take the extremes of the process, not just the normal operating point. A sensor that sees 700 °C during a cleaning cycle must be rated for 700 °C.
2. How accurate must the whole measurement be?
Write down the error you can accept at the operating temperature. Then compare it with the sensor’s tolerance class, using the tables on the sensor accuracy page. At 100 °C:
- PT100 class AA: ±0.27 °C
- PT100 class A: ±0.35 °C
- PT100 class B: ±0.8 °C
- Type K class 1: ±1.5 °C
- Type K class 2: ±2.5 °C
- NTC thermistor: depends on its R25 and B tolerances. A ±1 % R25 part is roughly ±0.2 °C near 25 °C, with more error away from it.
The sensor is only one term. Add the connection (lead resistance for an RTD, the reference junction for a thermocouple), the transmitter or input card, and the effect of ambient temperature on the electronics. Combine independent terms as the root sum of squares. The temperature measurement error calculator does this and shows which term dominates. Often it is not the sensor.
If the budget is tighter than the best class at your temperature, calibrate the sensor and transmitter together, or choose a sensor class with a narrower tolerance.
3. What does the installation demand?
- Fast response or a small probe: a thermocouple. Its junction can be made in sheaths well under a millimetre in diameter.
- Heavy vibration or shock: a thermocouple or a thin-film RTD. A wire-wound RTD element is the most delicate.
- Long service without recalibration: an RTD, which drifts least. Among thermocouples, type N is more stable than type K at high temperature.
- Low-oxygen or reducing atmospheres at high temperature: avoid type K (green rot). Use type N, or a platinum type in a suitable protection tube.
- Pressurised or flowing process: use a thermowell, so the sensor can be replaced without opening the process. The well slows the response, so allow for it.
- Hazardous area: check the Ex certification of the probe and transmitter as an assembly (see the Ex marking decoder).
4. How far is the instrument, and what does it accept?
- Thermocouple: extension or compensating cable of the same type must run all the way to the instrument. Over long runs that cable is expensive and picks up noise.
- RTD: use three or four wires so lead resistance does not appear as temperature. Ten metres of 0.5 mm² cable in a two-wire connection adds about 1.8 °C on a PT100. A PT1000 reduces this tenfold.
- Head-mounted transmitter: for long distances, it converts either sensor to a 4–20 mA signal close to the probe. The loop power calculator checks that the transmitter still has enough voltage at the end of the cable.
- NTC thermistor: usually read by a microcontroller with a voltage divider. Lead resistance is negligible against kilohms. Industrial PLC cards rarely accept NTCs, but they do accept PT100 and PT1000.
5. What does it cost over its life?
Compare the cost of the whole point: sensor, protection tube or thermowell, cable, transmitter or input channel, installation and recalibration.
- Base-metal thermocouple (type K, J or T): usually the cheapest probe, but needs matching cable and drifts with time at high temperature.
- PT100 or PT1000: costs more as a probe, uses ordinary copper cable, and keeps its calibration longer.
- NTC thermistor: the cheapest element by far for electronics, if its range and interchangeability are acceptable.
- Platinum thermocouple (type R, S or B): expensive, but the only option at the highest temperatures.
Quick decision table
| Situation | Usual choice |
|---|---|
| Furnace, kiln, exhaust gas above 600 °C | Thermocouple type K or N; R, S or B above about 1200 °C |
| Process liquid 0 to 300 °C, accuracy within ±0.5 °C | PT100 class A, four-wire or with a head transmitter |
| HVAC, building automation, energy metering | PT1000 or PT100; matched pairs for heat metering |
| Food and pharmaceutical processes | PT100 class A or AA, hygienic connection |
| Fast surface or small-gap measurement | Thin mineral-insulated thermocouple |
| Battery packs, chargers, electronics | NTC thermistor |
| Below zero down to about −200 °C | PT100, or thermocouple type T |
Common mistakes
Choosing by the sensor tolerance alone. The transmitter and wiring often add more error than the sensor class.
Specifying class A above its range. For wire-wound elements, class A is defined only up to 450 °C.
Running thermocouple signals on copper. Every connection between the probe and the instrument must use cable of the same thermocouple type.
Forgetting the thermowell. A well protects the sensor but adds response time and some heat-conduction error. Immerse the sensor deep enough.
Frequently asked questions
Which temperature sensor is the most accurate?
Within its range, a platinum RTD (PT100 or PT1000). Class AA allows ±0.27 °C at 100 °C under IEC 60751, against ±1.5 °C for a class 1 type K thermocouple. An NTC can resolve smaller steps near one temperature, but its accuracy depends on the part’s tolerances rather than a common standard.
Which sensor should I use above 600 °C?
A thermocouple. Type K or N to about 1200 °C, and types R, S or B up to about 1600 °C and 1700 °C in continuous use. The tolerance classes for wire-wound PT100 elements end at 600 °C.
Is a thermocouple or an RTD cheaper?
A base-metal thermocouple probe is usually cheaper than a PT100 probe of the same size. Over the whole measurement point the gap narrows: thermocouples need extension cable of the same type and more frequent checks at high temperature, while RTDs use ordinary copper cable.
When is an NTC thermistor the right choice?
In equipment and electronics working between about −40 and 125 °C, where low cost, small size and a large signal for a simple ADC matter more than interchangeability with a standard curve. Battery packs, chargers, motors and appliances are typical examples.
Do I need a 4–20 mA transmitter?
Not for short runs to a controller or PLC card that accepts the sensor directly. Over longer distances, or with electrical noise, a head-mounted transmitter converts the small sensor signal to 4–20 mA close to the probe, which is robust over hundreds of metres.
How do I compare sensors for my accuracy budget?
Add the sensor tolerance, the connection, the transmitter input and output errors and the ambient temperature effect, combining independent terms as the root sum of squares. The temperature measurement error calculator does this for an RTD or a thermocouple and shows which term limits the result.