What is Pt100?
A Pt100 is a platinum RTD — Resistance Temperature Detector whose nominal electrical resistance is 100 ohms at 0 °C. Instead of generating a thermoelectric voltage like a thermocouple, a Pt100 measures temperature through the predictable change in the electrical resistance of platinum.
In simple terms: as the temperature of a Pt100 changes, its resistance changes. A PLC, temperature controller, transmitter or precision measuring instrument measures that resistance and converts it into temperature using the standardized platinum resistance–temperature relationship.
Pt100 sensors are widely used in industrial process measurement, HVAC systems, machinery, tanks, pipelines, energy systems, laboratories, food production and many other applications where good stability and repeatability are important.
What does “Pt100” mean?
Pt identifies platinum as the sensing material, while 100 indicates a nominal resistance of 100 Ω at 0 °C.
For the common IEC platinum characteristic:
0 °C → 100.00 Ω
100 °C → approximately 138.51 Ω
The resistance increases as temperature rises.
This is fundamentally different from a thermocouple, which produces a millivolt-level thermoelectric EMF. A Pt100 is a passive resistance sensor and requires a measuring circuit to pass a controlled current through the element and determine its resistance.
Which standard defines industrial Pt100 sensors?
The principal international standard for industrial platinum resistance sensors and resistance thermometers is IEC 60751.
The current edition, IEC 60751:2022, specifies the resistance–temperature relationship and additional requirements for industrial platinum resistance temperature sensors and completed platinum resistance thermometers.
Temperature values in the standard are related to ITS-90 — International Temperature Scale of 1990.
What is the Pt100 temperature coefficient?
The common IEC Pt100 characteristic uses a nominal temperature coefficient of approximately:
α = 0.003851 °C-1
where:
α = (R100 − R0) / (100 × R0)
This corresponds to an average resistance change of approximately 0.385 Ω per °C between 0 °C and 100 °C.
It is important to call this an average coefficient rather than an exact linear slope. The Pt100 curve is slightly nonlinear, so the actual resistance increase per degree changes with temperature.
What about older α = 0.00392 Pt100 sensors?
Older technical documentation and specialized platinum thermometers may use a different characteristic, often referred to as Pt-392 or α ≈ 0.00392.
This is not the same resistance–temperature curve as the common IEC industrial Pt100 characteristic.
A measuring instrument configured for Pt-385 should therefore not be expected to read a Pt-392 sensor correctly. The sensor characteristic and instrument configuration must match.
What is the Pt100 temperature range?
IEC 60751 defines the platinum resistance–temperature relationship over a broad region extending approximately from -200 °C to +850 °C.
This does not mean that every commercial Pt100 probe is suitable for -200 to +850 °C.
The usable range of a complete sensor can be restricted by:
platinum element construction
lead-wire insulation
probe sheath material
internal filling materials
terminal head
process connection
environmental conditions
For example, the sensing element may tolerate a temperature that the cable insulation cannot. Always distinguish between the standard Pt100 reference range and the rated range of the complete probe assembly.
How does a Pt100 measure temperature?
The measuring instrument passes a small, controlled excitation current through the Pt100 and measures the resulting voltage drop.
The resistance can then be determined from Ohm's law:
R = V / I
The measured resistance is converted into temperature using the appropriate standardized Pt100 resistance–temperature function.
This means that an instrument does not merely need to “measure resistance.” It must support the correct Pt100 / IEC 60751 characteristic and the appropriate 2-, 3- or 4-wire connection method.
Is a Pt100 perfectly linear?
No. A Pt100 is considerably more linear than many thermocouple characteristics, but its resistance–temperature relationship is not a perfect straight line.
This is why the simple approximation:
Temperature ≈ (Resistance − 100) / 0.385
should only be regarded as a rough engineering estimate around a limited temperature region.
It should not be used as a precision Pt100 conversion formula over the complete range.
Callendar–Van Dusen equation
The standardized Pt100 curve is commonly represented using the Callendar–Van Dusen form.
For temperatures at and above 0 °C:
R(t) = R0[1 + At + Bt2]
Below 0 °C:
R(t) = R0[1 + At + Bt2 + C(t − 100)t3]
For the common IEC Pt-385 characteristic, the standard coefficients are approximately:
A = 3.9083 × 10-3 °C-1
B = -5.775 × 10-7 °C-2
C = -4.183 × 10-12 °C-4
The C term is used below 0 °C. PLC modules and digital temperature instruments normally perform this conversion internally without requiring the user to enter the equation manually.
Why was the old linear Pt100 table misleading?
A table created by adding exactly 0.385 Ω for every one-degree increase treats the sensor as perfectly linear.
The real IEC Pt100 characteristic is nonlinear. For example, the standard curve is approximately:
0 °C → 100.000 Ω
1 °C → approximately 100.391 Ω
100 °C → approximately 138.506 Ω
For accurate engineering or calibration work, use an IEC 60751 table, a validated calculator or the proper Callendar–Van Dusen equation rather than constructing a linear table manually.
What is the difference between RTD and Pt100?
RTD is the general sensor category. A Resistance Temperature Detector measures temperature through a predictable change in electrical resistance.
Pt100 is one specific platinum RTD with a nominal resistance of 100 Ω at 0 °C.
Therefore:
Every Pt100 is an RTD, but not every RTD is a Pt100.
RTDs can use different nominal resistances and, in some sensor families, different resistance materials. Industrial precision temperature measurement, however, is strongly associated with platinum RTDs because of platinum's stable and repeatable resistance behaviour.
Pt100 versus Pt1000
Both are platinum RTDs. The main nominal difference is their resistance at 0 °C:
Pt100 → 100 Ω at 0 °C
Pt1000 → 1000 Ω at 0 °C
The higher resistance of Pt1000 can make lead resistance a smaller percentage of the total measured resistance in some applications.
The receiving instrument must still be configured for the correct sensor. A Pt1000 connected to an input configured as Pt100 will not produce the correct temperature.
See What Is a Pt1000? for more detail.
Are all Pt100 sensors thin-film sensors?
No. This is another important correction to older descriptions of Pt100 sensors.
Pt100 sensing elements are commonly manufactured using two broad technologies:
Wire-wound platinum resistors
Thin-film platinum resistors
A wire-wound element uses fine platinum wire arranged on or within a suitable support structure.
A thin-film element uses a deposited platinum layer on a ceramic substrate.
Thin-film designs can provide compact size, fast response and economical production. Wire-wound designs can be advantageous where wider temperature capability, high stability or precision performance is required.
Neither construction is automatically “better” in every application.
What is a 2-wire Pt100 connection?
A 2-wire Pt100 is the simplest RTD connection. One conductor connects each end of the sensing element to the measuring instrument.
The disadvantage is that the measuring instrument sees both the sensor resistance and the resistance of the two lead wires:
Rmeasured = RPt100 + Rlead1 + Rlead2
Because a Pt100 changes by only a fraction of an ohm per degree, even a few ohms of lead resistance can produce a substantial temperature error.
Two-wire Pt100 connections are therefore most suitable for short cable runs or applications where high measurement accuracy is not required.
How does a 3-wire Pt100 work?
Three-wire Pt100 measurement is extremely common in industrial process systems.
The additional conductor allows the measuring circuit to estimate and compensate for much of the lead-wire resistance.
This method normally assumes that the relevant lead wires have approximately equal resistance.
If the leads have different lengths, conductor sizes, connection resistances or temperatures, some residual error can remain.
A 3-wire Pt100 should therefore be described as lead-resistance compensated, not as completely immune to lead resistance.
What is a 4-wire Pt100 connection?
A 4-wire Pt100 uses separate current and voltage-sensing conductors.
One pair carries the excitation current through the Pt100. The second pair measures the voltage directly across the sensing element.
Because the voltage measurement input has very high impedance, almost no current flows through the sense leads. Their resistance therefore has negligible influence on the measured voltage.
This is essentially a four-wire or Kelvin resistance measurement and provides the best lead-resistance rejection of the standard Pt100 connection methods.
2-wire, 3-wire and 4-wire Pt100 comparison
2-wire: simplest connection; lead resistance is included directly in the measurement.
3-wire: widely used in industry; compensates much of the lead resistance when the lead resistances are sufficiently balanced.
4-wire: separates excitation-current and voltage-measurement paths; provides the best compensation for lead resistance and is preferred for higher-accuracy resistance measurement.
Does Pt100 cable require special thermocouple alloys?
No. This is an important difference between a Pt100 and a thermocouple.
Thermocouple extension wiring must preserve a specific thermoelectric relationship. A Pt100 connection does not require thermocouple-alloy conductors.
However, lead resistance, conductor size, temperature rating, insulation, shielding and the 2-/3-/4-wire configuration can all affect Pt100 measurement performance.
How should Pt100 cable insulation be selected?
Cable insulation should be selected from the actual product specification rather than assuming one universal temperature limit for every PVC, PFA or fiberglass cable.
PVC can provide economical and flexible wiring for many normal-temperature applications but has limited high-temperature capability.
PFA, PTFE and related fluoropolymer insulation can provide useful high-temperature, chemical and moisture resistance depending on the cable design.
Fiberglass insulation can support significantly higher temperatures in suitable constructions but generally does not provide the same abrasion, moisture or immersion characteristics as sealed polymer insulation.
The actual cable manufacturer's temperature and environmental rating should always be checked.
What Pt100 tolerance classes are used?
IEC 60751:2022 defines temperature-dependent tolerance classes rather than one fixed error value for the entire temperature range.
For completed platinum resistance thermometers, commonly used classes include:
Class AA: ±(0.10 + 0.0017|t|) °C
Class A: ±(0.15 + 0.002|t|) °C
Class B: ±(0.30 + 0.005|t|) °C
Class C: ±(0.60 + 0.010|t|) °C
where |t| is the absolute value of temperature in degrees Celsius.
The tolerance therefore becomes wider as the temperature moves farther from 0 °C.
For example, Class A is not simply “±0.15 °C everywhere.” At 100 °C its tolerance is approximately ±0.35 °C.
Do Pt100 tolerance classes apply over the full -200 to +850 °C range?
No. Each tolerance class has a defined temperature range of validity, and the permitted range can also differ between wire-wound and thin-film constructions.
For completed thermometers under IEC 60751:2022, for example:
Class AA: wire-wound approximately -50 to +250 °C; film approximately 0 to +150 °C.
Class A: wire-wound approximately -100 to +450 °C; film approximately -30 to +300 °C.
Class B: wire-wound approximately -196 to +600 °C; film approximately -50 to +500 °C.
Class C: wire-wound approximately -196 to +600 °C; film approximately -50 to +600 °C.
The sensor datasheet should therefore state both the tolerance class and the valid temperature range.
Pt100 tolerance is not complete system accuracy
The IEC tolerance of the Pt100 element or completed thermometer does not include every error in the installed measurement system.
Additional effects can include:
lead-wire resistance
3-wire lead imbalance
measurement-current accuracy
self-heating
PLC or transmitter input accuracy
installation and immersion depth
thermal gradients
individual calibration
A Class A Pt100 therefore does not automatically make the complete system a ±0.15 °C system.
What is Pt100 self-heating?
A Pt100 requires an excitation current so its resistance can be measured. That current generates electrical power in the sensing element:
P = I2R
If the measuring current is too high, the sensor can warm itself slightly above the true process temperature.
This is called self-heating.
The resulting error depends not only on excitation current but also on how efficiently heat is transferred away from the sensing element.
A sensor in moving liquid can dissipate heat much more effectively than the same sensor suspended in still air.
Can a Pt100 be connected directly to a PLC?
Yes, if the PLC has an appropriate RTD / Pt100 input module.
A PLC RTD input module supplies the measuring current, determines the resistance and converts it into temperature using the configured Pt100 characteristic.
The supported connection method — 2-wire, 3-wire or 4-wire — should be checked in the PLC module specifications.
A Pt100 should not be connected directly to an ordinary 0–10 V input without a suitable resistance measurement circuit or transmitter.
Can Pt100 be converted to 4–20 mA?
Yes. A Pt100 can be connected to a temperature transmitter or sensor converter.
The transmitter measures the Pt100 resistance, calculates temperature and converts the result into a standard process signal such as 4–20 mA or 0–10 V.
This can be useful for long cable runs or where a standard PLC analog input is used instead of a dedicated RTD module.
Pt100 versus thermocouple
Pt100: measures temperature through resistance change.
Thermocouple: generates a thermoelectric EMF from a temperature distribution.
Pt100 sensors can provide excellent stability and measurement quality in many low- and moderate-temperature processes.
Thermocouples can extend to much higher temperatures and can be manufactured in extremely small, rugged and fast-response constructions.
A Pt100 does not require thermocouple-style cold-junction compensation, but lead-wire resistance must be handled correctly.
See Thermocouple vs Pt100 for the detailed comparison.
Can Pt100 measure surface temperature?
Yes, with a suitable surface-sensing construction.
Simply pressing an ordinary immersion probe against a metal surface does not necessarily produce an accurate surface-temperature measurement.
Contact area, mounting pressure, thermal compound, ambient air and heat conduction through the probe and cable can all affect the result.
For surface measurement, use a Pt100 construction designed to obtain reliable thermal contact with the surface.
Can Pt100 measure liquids and gases?
Yes. With a suitable sheath, process fitting and sealing system, Pt100 probes are widely used for liquid and gas measurement.
Common applications include tanks, pipelines, HVAC systems, boilers, food-processing equipment and industrial process lines.
For pressurized or corrosive processes, sheath material and, where required, a thermowell should also be evaluated.
How can a Pt100 fault be identified?
Common Pt100 faults include:
Open sensing element
Broken lead wire
Short circuit
Incorrect 3-wire connection
Unequal lead resistance
Wrong Pt100/Pt1000 instrument setting
Wrong α characteristic
Poor thermal contact with the process
At ordinary room temperature, a disconnected Pt100 will normally measure somewhat above 100 Ω. The exact resistance depends on the actual temperature.
A single multimeter measurement can confirm basic continuity and approximate plausibility, but it does not prove that the sensor remains correctly calibrated across its full temperature range.
What should be specified when selecting a Pt100?
A useful Pt100 specification should include at least:
Normal operating temperature
Minimum and maximum temperature
2-, 3- or 4-wire connection
Required tolerance class
Wire-wound or thin-film construction where relevant
Probe diameter and length
Sheath material
Cable insulation and length
Process connection
PLC or controller input type
Calibration requirement
Why do Pt100 prices vary?
The price of a Pt100 is not determined simply by the fact that it is a 100 Ω platinum sensor.
Probe diameter, length, sheath alloy, cable, connection head, tolerance class, transmitter, process fitting and calibration can all affect total cost.
See Pt100 Prices for the main cost factors.
Conclusion
A Pt100 is a platinum RTD with a nominal resistance of 100 Ω at 0 °C. The common IEC industrial characteristic uses approximately α = 0.003851 °C-1.
Its main strengths include good repeatability, stable resistance–temperature behaviour and high measurement quality through many low- and moderate-temperature industrial applications.
Reliable Pt100 measurement, however, depends on more than the sensing element. 2-/3-/4-wire configuration, lead resistance, excitation current, self-heating, instrument accuracy, probe installation and calibration all influence the final result.
For long cable runs, 3- or 4-wire measurement should normally be considered, and the complete probe — not only the platinum element — must be rated for the actual process environment and temperature.
Related Pt100 and temperature measurement topics
+ What Is a Thermocouple (T/C)?
+ Temperature Sensor Converters · Temperature Controllers · PLC Systems