A Pt100 measures temperature through its electrical resistance. At 0 °C, the sensor element has a nominal resistance of 100 Ω. As the temperature rises, the resistance also increases. The connected measuring instrument or temperature transmitter then converts the measured resistance into a temperature value.
The problem is that the Pt100 element is not the only component with electrical resistance. Connecting cables, terminals, plug connections and transition points also contribute to the measured total resistance. Particularly in a 2-wire circuit, this additional resistance can simulate a temperature that is significantly too high.
Whether a 2-, 3- or 4-wire circuit is required therefore depends not only on the Pt100 itself. Cable length, conductor cross-section, accuracy requirements, transmitter and connection conditions must be considered as a complete measuring chain.
Table of Contents
- How does a Pt100 measure temperature?
- Why does lead resistance distort the measured value?
- Calculating the temperature error caused by lead resistance
- How does a 2-wire circuit work?
- How does a 3-wire circuit work?
- How does a 4-wire circuit work?
- Direct comparison of 2-, 3- and 4-wire circuits
- Influence of cable length and conductor cross-section
- Why cable temperature also matters
- Residual error in a 3-wire circuit
- Extending or replacing the sensor cable
- When is a temperature transmitter useful?
- Direct Pt100 connection to a PLC and control cabinet
- Screening, EMC and potential equalisation
- Typical connection errors
- Systematically testing a Pt100 measuring chain
- Practical example: 30-metre cable to the PLC
- Which connection type is suitable for the application?
- Which products are suitable?
- Conclusion
- Frequently asked questions
How does a Pt100 measure temperature?
A Pt100 is a platinum resistance thermometer. The designation consists of:
- Pt: platinum sensor element,
- 100: nominal resistance of 100 Ω at 0 °C.
Within the characteristic range commonly used in industry, the resistance increases by approximately 0.385 Ω per kelvin or degree Celsius near 0 °C.
Typical approximate resistance values are:
| Temperature | Pt100 resistance |
|---|---|
| −100 °C | approx. 60.3 Ω |
| 0 °C | 100.0 Ω |
| 50 °C | approx. 119.4 Ω |
| 100 °C | approx. 138.5 Ω |
| 200 °C | approx. 175.9 Ω |
The evaluation instrument supplies a small measuring current through the Pt100 and determines the resulting voltage drop. The electrical resistance and subsequently the temperature are calculated from this value.
The measuring current must be sufficiently low to prevent the sensor element from heating up through its own power dissipation. This effect is known as self-heating and must be considered in addition to the lead-resistance error.
Why does lead resistance distort the measured value?
Every electrical conductor has resistance. This depends on factors including:
- cable length,
- conductor cross-section,
- conductor material,
- cable temperature,
- contact resistance at terminals and connectors.
In a simple 2-wire measurement, the measuring instrument cannot distinguish which part of the measured resistance comes from the Pt100 and which part comes from the connecting cables.
The measuring instrument measures:
Rmeasurement = RPt100 + Rlead 1 + Rlead 2
As a higher resistance is interpreted as a higher temperature, the measuring instrument indicates a value that is too high when additional lead resistance is present.
The lead-resistance error can be particularly critical with:
- long cable routes,
- small conductor cross-sections,
- high ambient temperatures,
- narrow process tolerances,
- differential-temperature measurements,
- reference and calibration applications.
Calculating the temperature error caused by lead resistance
Near 0 °C, the temperature error of a Pt100 can be estimated as follows:
Temperature error ≈ additional resistance / 0.385 Ω per °C
If both connecting leads have a resistance of 0.5 Ω each, the additional total resistance is:
0.5 Ω + 0.5 Ω = 1.0 Ω
The approximate temperature error is therefore:
1.0 Ω / 0.385 Ω per °C ≈ 2.6 °C
This means that a Pt100 at an actual temperature of 20.0 °C could indicate approximately 22.6 °C in a 2-wire circuit.
The calculation using 0.385 Ω per °C is a practical approximation. The exact gradient of the Pt100 characteristic changes across the temperature range. The complete characteristic according to IEC 60751 must be used for precise calculations.
How does a 2-wire circuit work?
In a 2-wire circuit, the Pt100 is connected to the measuring instrument using two conductors. The measuring current and voltage measurement use the same leads.
Advantages of the 2-wire circuit include:
- low wiring requirements,
- low cable costs,
- simple connection technology,
- suitable for very short cables,
- suitable where only low accuracy is required.
The main disadvantage is that the resistance of both leads is added in full to the Pt100 resistance.
A 2-wire circuit may be suitable when:
- the sensor is connected directly to a transmitter inside the connection head,
- the internal connection is only a few centimetres long,
- a Pt1000 is used instead of a Pt100 and the relative influence of the cable is sufficiently small,
- the lead resistance can be compensated manually in the evaluation instrument,
- the permissible measurement deviation is significantly greater than the lead-resistance error.
However, a configured lead-resistance correction compensates only for a known and stable resistance. Temperature changes, additional terminals or replacement of the cable can cause a new error.
How does a 3-wire circuit work?
The 3-wire circuit is particularly widespread in industrial temperature measurement. Two conductors are connected to one side of the Pt100 and one conductor to the other side.
The evaluation electronics use an additional measurement to compensate for the lead resistance mathematically.
The compensation works reliably when:
- all three conductors have the same cross-section,
- all conductors are approximately the same length,
- the conductors are made from the same material,
- they are exposed to comparable temperatures,
- the contact and transition resistances are similar.
The 3-wire circuit offers a good compromise between:
- accuracy,
- wiring requirements,
- cable costs,
- practical suitability for industrial applications.
It is typically suitable for:
- machine and plant engineering,
- process-temperature measurement,
- heating and cooling systems,
- temperature measurement on motors and bearings,
- Pt100 input cards in PLC systems,
- DIN-rail and head-mounted transmitters.
How does a 4-wire circuit work?
In a 4-wire circuit, two conductors supply the measuring current and two additional conductors measure the voltage drop directly across the Pt100.
Because of the high input resistance of the measuring instrument, only a very small current flows through the two separate voltage-sensing leads. Their own resistance therefore causes practically no relevant voltage drop.
This allows the measuring instrument to determine the resistance of the Pt100 almost independently of the connecting leads.
Advantages of the 4-wire circuit include:
- very little influence from lead resistance,
- compensation even with unequal lead resistances,
- high accuracy with long cables,
- good suitability for reference and laboratory measurements,
- reliable measurement of small temperature differences.
Disadvantages include:
- greater cable and wiring requirements,
- four suitable input terminals are required,
- not every PLC card or transmitter supports 4-wire technology,
- more terminal points and greater space requirements.
A 4-wire circuit is particularly useful for:
- calibration and reference measurements,
- narrow temperature tolerances,
- long connecting cables,
- changing ambient temperatures along the cable,
- high-accuracy differential-temperature measurements,
- unknown or asymmetrical lead resistances.
Direct comparison of 2-, 3- and 4-wire circuits
| Criterion | 2-wire | 3-wire | 4-wire |
|---|---|---|---|
| Number of conductors | 2 | 3 | 4 |
| Lead resistance | Included in full in the measured value | Largely compensated when the conductors are equal | Almost completely compensated |
| Sensitivity to conductor asymmetry | Yes | Yes | Very low |
| Wiring requirements | Low | Medium | High |
| Typical accuracy | For short cables and simple tasks | For many industrial measurements | For precision and reference measurements |
| Typical application | Short internal connection | Machines and process plants | Laboratories, calibration and high-accuracy measuring chains |
Influence of cable length and conductor cross-section
The electrical resistance of a copper cable can be estimated using:
R = ρ × length / conductor cross-section
For copper at room temperature, a resistivity of approximately 0.0175 Ω × mm²/m can be used.
In a 2-wire circuit, twice the one-way cable length must be considered because the measuring current flows to the sensor and back again.
| One-way cable length | Conductor cross-section | Total lead resistance | Approximate Pt100 error |
|---|---|---|---|
| 5 m | 0.5 mm² | approx. 0.35 Ω | approx. 0.9 °C |
| 10 m | 0.5 mm² | approx. 0.70 Ω | approx. 1.8 °C |
| 20 m | 0.5 mm² | approx. 1.40 Ω | approx. 3.6 °C |
| 50 m | 0.5 mm² | approx. 3.50 Ω | approx. 9.1 °C |
| 20 m | 1.0 mm² | approx. 0.70 Ω | approx. 1.8 °C |
The table shows that a larger conductor cross-section reduces the resistance. However, where high accuracy is required, it does not replace a suitable 3- or 4-wire circuit.
Why cable temperature also matters
The resistance of a copper cable increases with its temperature. A cable that runs partly through a hot machine room or alongside a pipe can therefore have a different resistance from the value measured during commissioning at room temperature.
In a 2-wire circuit, this change directly affects the indicated temperature even though the temperature at the Pt100 remains unchanged.
In a 3-wire circuit, the influence is largely compensated as long as all conductors are exposed to similar temperature conditions. If individual wires are routed separately or a terminal has increased contact resistance, a residual error occurs.
The 4-wire circuit is the least sensitive to such changes.
Residual error in a 3-wire circuit
3-wire compensation assumes that the relevant lead resistances are equal. In practice, differences can result from:
- different conductor cross-sections,
- different cable lengths,
- poor or corroded terminals,
- different plug connections,
- partially replaced cables,
- different ambient temperatures.
If two compensated conductors differ by 0.1 Ω, for example, this can produce an approximate residual error of 0.26 °C:
0.1 Ω / 0.385 Ω per °C ≈ 0.26 °C
This error may be insignificant for basic process monitoring. However, it may already be relevant for calibration or a narrow product tolerance.
Extending or replacing the sensor cable
An existing Pt100 cable must not be extended arbitrarily without checking the connection type and the effects on the measuring chain.
The following should be considered when extending the cable:
- the same conductor cross-section for all wires,
- the same conductor material,
- suitable temperature resistance,
- clean and corrosion-protected terminals,
- suitable screening,
- correct assignment of the conductors,
- updating the documentation.
In a 3-wire circuit, all three conductors should be extended together. If only one wire is changed using a different cross-section or an additional terminal point, the compensation may be impaired.
With preassembled precision sensors, the connecting cable should not be shortened or extended without the manufacturer’s approval. The cable may form part of the calibrated or specified sensor assembly.
When is a temperature transmitter useful?
With long cable routes, it can be useful to convert the Pt100 signal into a robust standard signal close to the sensor.
A head-mounted or field transmitter can, for example:
- evaluate the Pt100 using 2-, 3- or 4-wire technology,
- linearise the Pt100 characteristic,
- scale the measuring range,
- detect sensor breaks and short circuits,
- galvanically isolate the signal,
- provide a 4–20 mA, HART or digital output signal.
The sensitive resistance-measuring circuit therefore remains short. A standard signal that is less sensitive to cable resistance is then transmitted over the long route to the PLC.
Example:
- Pt100 connected to the head-mounted transmitter over 50 cm using a 3-wire circuit,
- transmitter measuring range: 0 to 200 °C,
- output: 4–20 mA,
- long current loop from the transmitter to the control cabinet.
However, a transmitter does not solve errors caused by incorrect sensor positioning, poor thermal contact or an unsuitable thermowell. It improves only the electrical signal conditioning.
Direct Pt100 connection to a PLC and control cabinet
Many PLC systems have dedicated Pt100 or RTD input cards. Before connection, the following must be checked:
- supported sensor type,
- 2-, 3- or 4-wire input,
- permissible resistance and temperature range,
- terminal assignment,
- sensor current,
- lead-break and short-circuit detection,
- galvanic isolation,
- required configuration.
A 3-wire card cannot automatically evaluate a 4-wire Pt100 as a 4-wire circuit. Depending on the input card, one conductor may have to remain unused or a defined bridge may have to be installed.
Conversely, a 2-wire Pt100 must not be connected to a 3- or 4-wire input without checking the requirements. Any necessary bridges and terminals must correspond to the manufacturer’s connection diagram.
Screening, EMC and potential equalisation
A Pt100 provides a comparatively small resistance signal. Interference from frequency converters, motor cables, contactors or solenoid valves can affect the measurement.
The following measures are useful for a low-interference installation:
- twisted conductors,
- suitable screened sensor cable,
- separate routing of power and measuring cables,
- short unscreened cable sections,
- clean terminal and plug connections,
- a consistent earthing and screening concept,
- distance from frequency converters and motor cables.
The screen must not be used as a Pt100 measuring conductor. Protective earth, signal ground and cable screen should also not be connected together arbitrarily.
A galvanically isolated temperature transmitter may be useful where significant potential differences exist.
Typical connection errors
A 3-wire Pt100 is connected as a 2-wire sensor
The lead resistances are not compensated and increase the indicated temperature.
Conductors of the same colour are assigned incorrectly
The measuring circuit is interrupted or the evaluation instrument detects a short circuit or implausible resistance.
A required terminal bridge is missing
With certain 2-wire connections to universal inputs, one measuring channel remains open.
A bridge is installed on the wrong terminals
The Pt100 is partly short-circuited or the compensation measurement does not function.
Different conductor cross-sections are used
The resistances in the 3-wire circuit are no longer symmetrical.
A corroded terminal increases the contact resistance
The temperature indication drifts or jumps depending on vibration and moisture.
The input is configured for Pt1000 instead of Pt100
The indicated temperature is completely implausible.
Lead compensation is applied twice
A measured value that has already been compensated is additionally altered by a manual offset.
Pt100 and thermocouple are confused
The two sensor types operate according to completely different measuring principles and require different inputs.
Systematically testing a Pt100 measuring chain
- Check the sensor type: Pt100, Pt1000 or another resistance sensor?
- Determine the connection type: Are two, three or four effective conductors present?
- Check the terminal diagram: Do the conductors match the input of the measuring instrument?
- Measure the de-energised sensor: Check the resistance using a suitable ohmmeter.
- Test each conductor separately: Compare continuity and resistance of every wire.
- Check transition points: Inspect terminals, connectors and extensions.
- Check the evaluation instrument: Are the Pt100 characteristic, number of conductors and measuring range configured correctly?
- Simulate a defined resistance: Test the input using a suitable Pt100 or resistance simulator.
- Perform a comparative measurement: Compare the sensor with a reference at a stable temperature point.
- Assess the installation: Check immersion depth, heat conduction and thermowell.
A simple resistance measurement using a handheld multimeter is only an initial plausibility check. For an accurate assessment, the measuring current, connection type and measurement uncertainty of the test instrument must be considered.
Practical example: 30-metre cable to the PLC
A Pt100 measures the temperature of a hydraulic tank. The sensor is located 30 metres from the control cabinet. A copper cable with a conductor cross-section of 0.5 mm² is used.
Option 1: 2-wire circuit
The approximate resistance of both leads is:
2 × 30 m × 0.0175 Ω mm²/m / 0.5 mm² = 2.1 Ω
The resulting temperature error is approximately:
2.1 Ω / 0.385 Ω per °C ≈ 5.5 °C
The indicated temperature is therefore approximately 5.5 °C too high, regardless of the actual sensor tolerance.
Option 2: 3-wire circuit
All three conductors have the same cross-section and length and are routed in the same cable. The PLC largely compensates for the common lead resistance.
The remaining error depends mainly on conductor asymmetry and contact resistance. This solution is sufficient for operational temperature monitoring of the hydraulic tank.
Option 3: Local transmitter
Alternatively, a head-mounted transmitter is installed directly at the sensor. The Pt100 is connected using a short 3-wire circuit. A 4–20 mA signal is transmitted from the transmitter to the PLC.
This solution provides additional advantages:
- short Pt100 measuring circuit,
- robust transmission to the control cabinet,
- simple scaling in the PLC,
- sensor-break detection,
- easy current-loop testing.
Either a direct 3-wire circuit or a local transmitter is selected for the application. A 2-wire connection is unsuitable because of the calculated error.
Which connection type is suitable for the application?
| Application | Recommended connection type |
|---|---|
| Short internal connection inside the connection head | 2-wire may be sufficient |
| Basic temperature monitoring with a short cable | 2- or 3-wire after assessing the error |
| Industrial process measurement connected to a PLC | Usually 3-wire |
| Long cable with conventional process accuracy | 3-wire or local transmitter |
| High accuracy or narrow tolerance | 4-wire |
| Calibration and reference measurement | 4-wire |
| Different conductor temperatures or asymmetrical leads | 4-wire |
| PLC has only a 4–20 mA input | Pt100 with temperature transmitter |
At least the following information should be available for selection:
- temperature range,
- required overall accuracy,
- Pt100 tolerance class,
- cable length,
- conductor cross-section,
- ambient temperature of the cable,
- 2-, 3- or 4-wire input of the measuring instrument,
- direct connection or transmitter,
- installation conditions and sensor design,
- vibration, moisture and EMC conditions,
- calibration and documentation requirements.
Which products are suitable?
Resistance thermometers and Pt100 sensors
The resistance thermometers / Pt100 sensors category includes different sensor designs for machines, pipes, vessels and process plants.
Depending on the version, available products include:
- Pt100 and Pt1000 sensors,
- 2-, 3- and 4-wire circuits,
- cable, screw-in and insertion sensors,
- resistance thermometers with connection heads,
- versions with protection tubes or thermowells,
- sensors for hygienic and hazardous-area applications,
- customised probe lengths and process connections.
The connection type should be defined together with the measuring range, accuracy, cable length and evaluation instrument.
Temperature transmitters and accessories
The temperature transmitters and accessories category includes devices for signal conditioning and integrating Pt100 sensors into PLC and control systems.
Depending on the device, available features include:
- Pt100 and Pt1000 inputs,
- 2-, 3- and 4-wire connections,
- head-mounted and DIN-rail installation,
- freely configurable temperature ranges,
- sensor-break and short-circuit detection,
- galvanic isolation,
- 4–20 mA, HART, Modbus or other output signals.
For long cable routes, a transmitter installed close to the Pt100 is frequently more suitable than transmitting the resistance signal directly to the control cabinet.
Testing and troubleshooting
A suitable resistance or temperature simulator is required to test a Pt100 input. It can inject defined Pt100 values and verify the indication or PLC scaling.
If the temperature transmitter has a 4–20 mA output, the downstream current loop can be tested using the UPS4E current-loop calibrator.
However, the UPS4E does not simulate Pt100 resistance. For a complete test, the Pt100 input and current output must be assessed separately or together using a suitable temperature simulator.
Conclusion: Lead resistance is part of the complete Pt100 measuring chain
In a 2-wire circuit, the resistance of both connecting leads is added directly to the Pt100 resistance. Even a few metres of cable can therefore cause a temperature error of several degrees.
The 3-wire circuit largely compensates for lead resistance as long as all conductors are as electrically and thermally equal as possible. It is therefore the best compromise between accuracy and wiring requirements for many industrial temperature measurements.
The 4-wire circuit separates the current supply and voltage measurement. This means that unequal lead resistances are also almost completely compensated. It is particularly suitable for reference measurements, long cables and narrow tolerances.
For very long cable routes, a local temperature transmitter can be useful. The sensitive Pt100 signal is then carried only over a short distance and converted into a robust standard signal directly at the measuring point.
The correct connection type must be selected together with the sensor, cable, transmitter, PLC input and accuracy requirement. A high-quality Pt100 cannot provide precise temperature measurements if its connecting cable is incorrectly designed or wired.
Frequently asked questions about Pt100 sensors in 2-, 3- and 4-wire circuits
What is the difference between a Pt100 with 2, 3 and 4 wires?
With two wires, the entire lead resistance is included in the measured value. Three wires allow compensation when the lead resistances are largely equal. Four wires provide the most accurate measurement independently of the lead resistances.
How large is the temperature error caused by 1 Ω of lead resistance?
Near 0 °C, an additional resistance of 1 Ω corresponds to a Pt100 temperature error of approximately 2.6 °C.
Why does a 2-wire Pt100 usually indicate too high a temperature?
The additional resistance of the outgoing and return leads is measured as part of the Pt100 resistance and is therefore interpreted as a higher temperature.
Is a 3-wire circuit always fully compensated?
No. Compensation requires the lead resistances to be as equal as possible. Different cross-sections, cable lengths, temperatures or contact resistances produce a residual error.
When is a 4-wire measurement required?
It is useful for high accuracy requirements, reference measurements, long cables, small temperature differences or asymmetrical lead resistances.
Can I connect a 3-wire Pt100 to a 2-wire input?
In principle, one conductor can remain unused. However, the measurement then operates as a 2-wire circuit and has no lead-resistance compensation.
Can I connect a 2-wire Pt100 to a 3-wire input?
This is possible with some devices using a defined terminal bridge. The exact wiring must be taken from the connection diagram of the evaluation instrument.
Does the conductor cross-section matter?
Yes. A larger cross-section reduces the lead resistance. Where high accuracy is required, however, the correct 3- or 4-wire circuit remains decisive.
Why does the error change with the ambient temperature?
The resistance of a copper cable increases with its temperature. In a 2-wire circuit, this also changes the indicated Pt100 temperature.
Is a Pt1000 better suited to a 2-wire circuit?
The relative influence of the same lead resistance is lower with a Pt1000 than with a Pt100. Nevertheless, the accuracy, input device and cable length must be checked.
Can a transmitter eliminate the lead-resistance error?
A transmitter can evaluate the Pt100 using a 3- or 4-wire circuit and convert the signal directly at the measuring point. This keeps the sensitive resistance-measuring circuit short.
How can a Pt100 input be tested?
Defined resistance values are injected using a suitable Pt100 or resistance simulator. The indication, scaling and lead-break detection are then checked.
Which information does ICS Schneider require for selection?
The required information includes the temperature range, accuracy requirement, Pt100 tolerance class, connection type, cable length, conductor cross-section, process connection, sensor design, available PLC or transmitter input and requirements concerning the degree of protection, hazardous-area version and calibration.