The short answer
Use an NTC thermistor when the temperature stays within about −40 to 150 °C, cost and size matter, and you can calibrate or accept a tolerance of a few tenths of a degree to a degree or so. Use a PT100 (or PT1000) when sensors must be interchangeable against a published standard, the range is wider, or the measurement has to be traceable over years.
The table above shows the main difference. Near room temperature a 10 kΩ NTC changes its resistance by about 4.4 % per kelvin, while a PT100 changes by about 0.35 % per kelvin. The NTC gives roughly twelve times more signal per degree, but that signal is far from linear: the same NTC is 33.6 kΩ at 0 °C, 10 kΩ at 25 °C and about 0.7 kΩ at 100 °C.
How each sensor works
A PT100 is a platinum resistor of 100 Ω at 0 °C whose resistance rises almost linearly with temperature: 138.51 Ω at 100 °C. IEC 60751 defines the curve (Callendar–Van Dusen equation) and the tolerance classes, so a PT100 from any manufacturer follows the same table within its class. A PT1000 is the same curve at 1000 Ω, which makes lead resistance and self-heating less important.
An NTC thermistor is a ceramic semiconductor whose resistance falls steeply as temperature rises. It is described by its resistance at 25 °C (R25, for example 10 kΩ) and by a B value, which describes how steep the curve is (for example 3950 K). The Beta equation R = R25 · exp(B · (1/T − 1/T25)) is a good approximation over a limited range. The three-term Steinhart–Hart equation, fitted to three points from the maker’s table, is closer over a wide range. The NTC thermistor calculator does both.
Sensitivity and resolution
The NTC’s large change in resistance is its main advantage. With a simple voltage divider and a 12-bit ADC, an NTC resolves a small fraction of a degree around its centre temperature. A PT100 needs an excitation current, a precision amplifier or a dedicated RTD-to-digital converter to resolve the same step, because 0.1 K is only about 0.04 Ω.
The sensitivity of an NTC is not constant, though. It is highest at low temperature and falls as the resistance drops. At 125 °C the 10 kΩ, B = 3950 K part changes by about 2.5 %/K, and a divider sized for 25 °C loses much of its resolution there. The NTC voltage divider calculator shows the resolution across the whole range for a chosen series resistor.
Accuracy and interchangeability
PT100 accuracy is defined by the standard: class A is ±0.15 °C at 0 °C and ±0.35 °C at 100 °C, class B ±0.3 °C and ±0.8 °C. Any class A PT100 can replace another without recalibration.
NTC accuracy is defined by the datasheet, usually as a tolerance on R25 (for example ±1 % or ±5 %) and a tolerance on B. Near 25 °C a ±1 % R25 tolerance is worth roughly ±0.2 °C, because the part changes by about 4.4 % per kelvin there. Away from 25 °C the B tolerance adds to it, and the error grows. There is no common NTC curve across manufacturers: two “10 kΩ, B = 3950 K” parts from different makers can differ by a degree or more at the ends of their range. Replacing an NTC with another type needs its own table in the firmware.
Range
IEC 60751 covers platinum RTDs from −200 to 850 °C, with tolerance classes up to 600 °C for wire-wound elements. Standard epoxy-coated or glass-encapsulated NTC thermistors are typically specified from about −50 to 150 °C. Glass parts reach higher, but the resistance at the hot end becomes so low that resolution suffers. For anything outside about −50 to 150 °C, an RTD or a thermocouple is the safer choice.
Self-heating
Both sensors are heated by the current that measures them.
For a PT100 at 1 mA, the power is I²R = 0.1 mW at 100 Ω. The resulting error depends on the probe’s self-heating coefficient and on how well the medium carries the heat away.
For an NTC, datasheets give a dissipation constant in mW/K: the power that raises the part by one kelvin. In a voltage divider, the power is highest when the NTC resistance equals the series resistor. With 3.3 V across a 10 kΩ divider at that point, the NTC dissipates about 0.27 mW. At a dissipation constant of 2 mW/K, that is about 0.14 K of self-heating. Keep the divider current low, or switch it on only for the measurement.
Lead resistance and wiring
A PT100 is sensitive to its leads: 0.69 Ω of two-wire lead resistance reads as about +1.8 °C. Three- or four-wire connections, or a PT1000, are standard practice.
A 10 kΩ NTC hardly notices a few ohms of cable, since a 1 Ω lead is about 0.002 °C near 25 °C. That makes NTCs easy to wire on a circuit board or at the end of a long thin cable.
Linearisation and electronics
A PT100 is nearly linear. Over 0 to 100 °C a straight line through the end points is within about 0.4 °C of the IEC 60751 curve, and a simple polynomial or the exact equation removes the rest.
An NTC needs linearisation in the firmware or a lookup table: the Beta equation for modest accuracy, Steinhart–Hart or the maker’s table for better. Many microcontrollers do this easily. The cost is that the conversion is specific to one part number.
When to choose an NTC thermistor
- Electronics, battery packs, chargers, motors and household appliances working between about −40 and 125 °C.
- Low cost and a small part matter more than interchangeability.
- A simple divider and the microcontroller’s own ADC have to be enough.
- Tolerance of a few tenths of a degree near one temperature is acceptable, or each unit is calibrated.
When to choose a PT100 or PT1000
- Process plant, HVAC, laboratory and pharmaceutical measurements where sensors are replaced in the field.
- A wider range, from about −200 to 600 °C.
- Traceable accuracy against a standard and long-term stability.
- Industrial transmitters and PLC cards, which accept PT100 and PT1000 directly.
Common mistakes
Assuming all “10k 3950” NTCs are the same. The Beta value is an approximation; makers’ tables differ. Use the table of the part you fit.
Sizing the divider for the wrong temperature. The best resolution is where the NTC resistance equals the series resistor. If the important range is 60 to 90 °C, a 10 kΩ resistor chosen for 25 °C wastes most of the ADC.
Two-wire PT100 over long cable. The leads add about 0.39 Ω per degree of apparent temperature; use three or four wires or a PT1000.
Ignoring self-heating in still air. A sensor in still air cools much more slowly than one in liquid, so the same current gives a larger error.
Frequently asked questions
Is a thermistor more accurate than an RTD?
Near one temperature, an NTC can resolve smaller steps because it changes about twelve times more per degree. Over a wide range, and between different sensors, a PT100 is more accurate: its curve and tolerance classes are defined by IEC 60751, while an NTC’s accuracy depends on its R25 and B tolerances and on how well the firmware models its curve.
What does 10k 3950 mean on an NTC thermistor?
The resistance is 10 kΩ at 25 °C, and the B value, which describes how steeply the resistance falls with temperature, is 3950 K. By the Beta equation such a part is about 33.6 kΩ at 0 °C and 3.59 kΩ at 50 °C; the maker’s table gives the exact values.
Can I replace a PT100 with an NTC thermistor?
Not directly. The input must be reconfigured, because a PT100 rises from 100 Ω while an NTC falls from thousands of ohms, and the conversion must use the NTC’s own curve. Most industrial PLC and transmitter inputs accept PT100 and PT1000 but not NTCs.
What is the temperature range of an NTC thermistor?
Typically about −50 to 150 °C for standard epoxy or glass parts; check the datasheet, since some glass-encapsulated types go higher. A PT100 covers −200 to 850 °C under IEC 60751, with tolerance classes up to 600 °C.
Why is my NTC reading wrong at high temperature?
Usually because the conversion uses the Beta equation far from 25 °C, or because the divider resistor was chosen for room temperature and the ADC resolution is poor at the hot end. Use Steinhart–Hart fitted to three points across your range, or the maker’s table, and size the divider for the range that matters.
Should I use a PT100 or a PT1000?
The curve is the same; a PT1000 has ten times the resistance. That makes lead resistance and self-heating ten times less important, so a PT1000 suits two-wire connections and battery-powered electronics. A PT100 is the long-standing default for industrial transmitters and four-wire precision measurement.