Your PT100 Sensor Isn’t Reading the Wrong Temperature – Your Wiring Might Be

Most of us assume that if a PLC displays the wrong temperature, the temperature sensor must have failed.

In many cases, however, that is not the real problem.

Engineers often replace the PT100 sensor, recalibrate the system, or even replace the PLC input module, only to discover that the temperature reading is still incorrect.

The real cause may be cable resistance, the wiring method, cable length, electrical noise, or even selecting the wrong type of temperature sensor in the first place.

To understand why this happens, we first need to step back and ask a more fundamental question:

What types of temperature sensors are available, and what are the key differences between them?

In this article, we’ll explore the most common temperature sensing technologies, explain how each one works, discuss where each technology is best suited, and examine why RTD sensors, particularly PT100 and PT1000, have become the industry standard for industrial automation, medical equipment, HVAC systems, and other precision measurement applications.

At first glance, all temperature sensors appear to perform the same task – measuring temperature. In reality, however, the differences between them are significant. Some technologies are designed for temperatures exceeding 1,000°C, others deliver exceptional accuracy and long-term stability, while some are chosen primarily for their low cost or ease of integration.

Among these technologies, RTD sensors stand out because they combine excellent accuracy, outstanding long-term stability, and near-linear response over a wide temperature range. For this reason, RTDs have become the preferred choice for industrial automation, medical equipment, HVAC systems, laboratory instrumentation, and other applications where measurement reliability is just as important as the temperature range itself.

Technology	Operating Principle	Typical Temperature Range	Accuracy	Long-Term Stability	Linearity	Typical Applications
Thermocouple	Generates a voltage between two dissimilar metals (Seebeck Effect)	-200°C to +1800°C	Moderate	Good	Non-linear	Industrial furnaces, turbines, engines, high-temperature processes
RTD (PT100 / PT1000)	Platinum electrical resistance changes with temperature	-200°C to +850°C	Very High	Excellent	Nearly linear	Industrial automation, medical equipment, laboratories, HVAC, process control
NTC Thermistor	Electrical resistance decreases as temperature increases	-50°C to +150°C	High (within a limited range)	Moderate	Non-linear	Consumer electronics, battery management, household appliances, basic control systems
Digital Temperature Sensor	Semiconductor sensor with digital output	-55°C to +125°C (typical)	High	Good	Linear (after calibration)	IoT devices, embedded electronics, PCB monitoring, smart systems

Each sensing technology has its own strengths and limitations. Choosing the right temperature sensor is not simply a matter of selecting the most accurate device, but of selecting the technology that best matches the application’s environmental conditions, temperature range, accuracy requirements, and system architecture.

When long-term stability, repeatability, and measurement accuracy are critical, RTD sensors, particularly PT100 and PT1000, are often the preferred solution. In the following sections, we’ll explore why.

What Is an RTD Sensor?

Among all temperature sensing technologies, RTD (Resistance Temperature Detector) sensors are widely regarded as the preferred choice whenever high accuracy, long-term stability, and repeatable measurements are required.

Their operating principle is relatively simple: the electrical resistance of a material changes as its temperature changes. By accurately measuring that resistance, the system can calculate the corresponding temperature.

Although RTD sensors can be manufactured from several different metals, almost all industrial RTDs are made from platinum. Platinum offers an exceptional combination of chemical stability, corrosion resistance, predictable temperature characteristics, and the ability to maintain its accuracy even after thousands of heating and cooling cycles.

As a result, when engineers refer to an RTD, they are usually referring to a platinum-based sensor such as a PT100 or PT1000.

What Do PT100 and PT1000 Mean?

This is where many engineers encounter their first misconception.

The number in the sensor’s name does not indicate its temperature range, accuracy, or quality.

Instead, it represents the sensor’s electrical resistance at 0°C.

PT100 – 100 Ω at 0°C
PT1000 – 1000 Ω at 0°C

Both sensors are based on exactly the same measurement principle and use the same sensing material – platinum. The primary difference is their nominal resistance, which directly affects how the measurement system responds to external influences.

As temperature increases, the resistance of both sensors rises in an almost perfectly linear manner. However, because a PT1000 has ten times the resistance of a PT100, it maintains a resistance value that is approximately ten times higher throughout its operating range.

This seemingly simple difference has a significant impact on measurement performance. In applications with long cable runs or electrically noisy environments, the higher resistance of a PT1000 makes the measurement less sensitive to cable resistance and electrical interference, resulting in improved overall system accuracy.

Figure 2: Although PT100 and PT1000 sensors are based on the same operating principle and use the same sensing material – platinum – their nominal resistance differs by a factor of ten. As a result, both sensors exhibit nearly identical temperature-resistance characteristics, but the PT1000 is significantly less affected by cable resistance and electrical noise, making it particularly well suited for applications with long cable runs.

Why Do Both PT100 and PT1000 Exist If They’re Both Made of Platinum?

At first glance, there seems to be very little difference between them.

Both are platinum-based RTD sensors, both comply with the IEC 60751 standard, both offer excellent accuracy and long-term stability, and both are suitable for a wide range of industrial applications.

So why were two different versions developed?

The answer lies not in the sensor itself, but in the way the measurement system reads its resistance.

When a controller, PLC, or temperature transmitter measures an RTD, it is actually measuring electrical resistance. However, the sensor is not the only component in the circuit. The connecting cables also introduce their own electrical resistance.

In a system using a PT100, even a small increase in cable resistance can introduce a noticeable measurement error.

With a PT1000, the sensor’s nominal resistance is ten times higher. As a result, the same cable resistance represents a much smaller percentage of the total measured resistance, significantly reducing its impact on the temperature reading.

This is one of the main reasons why many modern systems, particularly those with long cable runs between the sensor and the controller, favor PT1000 sensors.

That said, this does not mean that PT100 is outdated or less accurate. In fact, PT100 remains the industry standard in many manufacturing plants, laboratory instruments, and legacy control systems because of its broad compatibility with existing PLCs, transmitters, and measurement equipment.

Ultimately, choosing between a PT100 and a PT1000 is not about selecting the “better” sensor. It is about selecting the sensor that best fits the requirements of the measurement system.

PT100 vs. PT1000 Comparison

Feature	PT100	PT1000
Nominal Resistance at 0°C	100 Ω	1000 Ω
Operating Principle	Platinum RTD	Platinum RTD
Linearity	Excellent	Excellent
Long-Term Stability	Excellent	Excellent
Sensitivity to Cable Resistance	Higher	Lower
Suitability for Long Cable Runs	Good	Excellent
Compatibility with Existing Equipment	Excellent	Good
Typical Applications	Industrial automation, laboratories, legacy systems	Modern control systems, HVAC, industrial automation, medical equipment

As you can see, the difference between the two sensors is not their quality, but rather how they interact with the overall measurement system. In many cases, the difference between a stable measurement and one that produces recurring errors has little to do with the sensor itself and much more to do with cable length, environmental conditions, and wiring configuration.

This leads us to one of the most common mistakes in RTD system design: choosing the wrong wiring configuration.

To understand why, let’s look at how 2-wire, 3-wire, and 4-wire RTD connections can produce very different measurement results – even when exactly the same PT100 sensor is used.

Why Does the Number of Wires Matter?

One of the most common misconceptions is that a PT100 sensor is simply a two-terminal device, much like an ordinary resistor.

In reality, when the sensor is installed several meters away from the controller, the connecting cables become part of the measurement circuit.

Every meter of cable adds a small amount of electrical resistance. Over long cable runs, that additional resistance can accumulate and introduce temperature errors of several degrees.

The higher the required measurement accuracy, the more significant the effect of cable resistance becomes.

This is precisely why three different RTD wiring configurations were developed.

2-Wire PT100 Sensor

The 2-wire configuration is the simplest and most economical way to connect a PT100 sensor.

In this configuration, the measurement system reads the total resistance of the circuit, including:

The resistance of the PT100 sensor
The resistance of the first cable
The resistance of the second cable

Because the controller cannot distinguish between the sensor’s resistance and the resistance of the connecting wires, any change in cable length, ambient temperature, or conductor characteristics directly affects the measured temperature.

For this reason, a 2-wire PT100 is generally recommended only when:

The cable run is very short.
Measurement accuracy is not critical.
Lower cost is more important than maximum accuracy.

3-Wire PT100 Sensor

When higher measurement accuracy is required, most industrial control systems use a 3-wire RTD configuration.

In this arrangement, a third wire is added, allowing the PLC or temperature transmitter to compensate for the resistance of the connecting cables.

Provided that the two current-carrying conductors have the same length and wire gauge, the measurement system can almost completely eliminate the effect of cable resistance on the temperature reading.

This is why the 3-wire RTD has become the industry standard for industrial automation over the years.

It delivers significantly better accuracy than a 2-wire connection while avoiding the additional complexity and cost associated with a 4-wire configuration.

4-Wire PT100 Sensor

When the highest possible measurement accuracy is required, such as in calibration laboratories, medical equipment, research facilities, or precision test systems, a 4-wire RTD configuration is the preferred solution.

In this configuration, one pair of wires supplies a precisely controlled current through the sensor, while a second pair independently measures the voltage across the sensing element.

Because virtually no current flows through the voltage-sensing leads, their resistance has almost no influence on the measurement.

For this reason, the 4-wire connection is considered the most accurate method of measuring RTD sensors and is commonly referred to as a Kelvin measurement.

However, this level of accuracy comes at the cost of more sophisticated instrumentation, additional wiring, and higher installation costs. As a result, 4-wire RTDs are typically reserved for high-precision applications rather than standard industrial control systems.

Which Sensor Should You Choose?

Now that we understand the differences between PT100 and PT1000, as well as the significance of the various wiring configurations, the obvious question is: which option should you choose for your next project?

As with most engineering decisions, the answer is: it depends on the application.

If you are working with an existing system that already uses PT100 input modules, there is usually little reason to redesign the architecture. A 3-wire PT100 provides excellent accuracy and has been the standard choice in industrial automation systems for decades.

When designing a new system, however, especially where the sensor is installed far from the controller or in an electrically noisy environment, a PT1000 may be the better choice. Its higher resistance reduces the relative impact of cable resistance and can improve overall measurement reliability.

Where maximum accuracy is required, such as in calibration laboratories, medical equipment, or precision test systems, a 4-wire RTD is generally the preferred solution because it virtually eliminates the effect of lead resistance.

There is no single sensor that is always “the best.” The right choice is the one that matches the system requirements, cable length, required accuracy, and available measurement equipment.

Why Didn’t Replacing the Sensor Solve the Problem?

Let’s return to the engineer from the beginning of the article.

He replaced the PT100 sensor.

The reading was still wrong.

The problem was not the sensor.

The problem was the measurement system.

In many cases, temperature errors are caused by cable resistance, the wiring configuration, oxidized contacts, loose connectors, or even an RTD input that has been configured incorrectly in the PLC.

Before replacing a temperature sensor, it is worth asking a few simple questions:

Is the sensor a PT100 or PT1000?
Is it connected in a 2-wire, 3-wire, or 4-wire configuration?
Has the cable length changed?
Is the cable type suitable for the application?
Is the PLC input configured for the correct sensor type?
Is there excessive resistance in the connectors or terminals?

Many temperature measurement problems can be resolved at this stage without replacing the sensor at all.

Case Study – When the Sensor Wasn’t the Problem

Consider an industrial heating system that uses an LTP-TP-SS-PT sensor to measure the temperature of a liquid inside a tank.

The sensor is installed approximately 40 metres from the control cabinet and connected to the PLC through a copper cable with a total conductor length of approximately 80 metres for the outgoing and return paths.

During operation, the system operator notices that the displayed temperature is several degrees higher than the actual temperature. Naturally, the sensor becomes the first suspect.

However, laboratory testing confirms that the sensor is functioning correctly.

The LTP-TP-SS-PT is available with either a PT100 or PT1000 sensing element, depending on the selected version. It is offered in Class A or Class B accuracy, features a 316 stainless steel probe body and IP67 sealing, and is suitable for temperatures from -55°C to +220°C, with operation at the maximum temperature limited to 30 minutes.

So what actually caused the error?

Assume that the sensor was connected in a 2-wire configuration using a 0.5 mm² copper cable.

The typical resistance of such a conductor is approximately 0.036 Ω per metre.

In this example:

Distance between the PLC and the sensor: 40 m
Total conductor length, outgoing and return: 80 m
Total cable resistance: approximately 2.9 Ω

A PT100 changes by approximately 0.385 Ω per °C.

Therefore:

2.9 Ω ÷ 0.385 Ω/°C ≈ 7.5°C

This means that the cable resistance alone could create a measurement error of approximately 7.5°C, even though the sensor itself is completely accurate.

Replacing the sensor would not solve the problem. A new sensor would produce almost the same error because the additional resistance is introduced by the wiring, not by the sensing element.

The correct solution is to change the measurement configuration. A 3-wire connection allows the PLC to compensate for cable resistance, while a 4-wire connection is preferred when maximum accuracy is required.

For a new system, a PT1000 version may also be considered because the same cable resistance represents a much smaller proportion of the sensor’s total resistance.

💡 Engineers often replace the sensor when a temperature reading appears incorrect. In systems with long cable runs, however, the real cause is frequently the wiring configuration, not the sensor itself.

Conclusion

PT100 and PT1000 RTD sensors are among the most accurate and reliable temperature measurement technologies available today. However, even the highest-quality sensor cannot compensate for poor system design.

Choosing the correct sensor type, wiring configuration, and installation method often has a greater impact on measurement accuracy than the sensor’s published specifications.

In many systems, the difference between stable, reliable measurements and recurring temperature errors begins long before the sensor is installed. It starts with sound engineering decisions.

Choosing the Right PT100 Sensor Construction

Selecting the sensing element is only part of the design process. Equally important is choosing the mechanical probe construction that matches the installation method, operating environment, and measurement requirements.

The same PT100 sensing element can be integrated into a variety of probe designs, each optimized for specific applications.

Ring Terminal – Surface Temperature Measurement

Ring terminal sensors are bolted directly onto a metal surface such as a motor housing, heatsink, electronic enclosure, or battery pack. They provide fast thermal response without penetrating the system.

Typical applications: Motors, heatsinks, electronic assemblies, battery packs, and power electronics.

Tubular Probe – Liquids, Gases, and Process Measurement

This is the most common probe design for measuring the temperature of liquids, gases, and industrial processes. Typically manufactured from stainless steel, tubular probes provide accurate and reliable measurements even in demanding environments.

Typical applications: Tanks, HVAC systems, laboratories, medical equipment, and industrial process control.

Screw-In Probe – Pipes and Pressurized Vessels

Where permanent thermal contact is required, threaded probes can be installed directly into pipes, heat exchangers, or pressure vessels. This design provides excellent mechanical stability and efficient heat transfer between the process and the sensing element.

Typical applications: Hydraulic systems, industrial machinery, engines, and energy systems.

IP67 Sealed Probe – Harsh Environments

Where moisture, oil, dust, vibration, or outdoor exposure is expected, an IP67-rated probe provides reliable long-term performance under challenging operating conditions.

Typical applications: Industrial automation, HVAC, outdoor equipment, agricultural machinery, and marine applications.

Pipe Clamp Sensor – Non-Invasive Pipe Temperature Measurement

When it is undesirable or impossible to penetrate the pipe, a clamp-on sensor provides a practical alternative. It attaches directly to the pipe surface, allowing quick installation without interrupting the process.

Typical applications: Heating systems, cooling systems, water distribution, HVAC installations, and energy management.

Ultimately, selecting a PT100 sensor involves more than choosing between PT100 and PT1000. The mechanical construction must also match the installation method, operating environment, and application requirements. Choosing the right combination of sensing element and probe design ensures stable, accurate, and reliable temperature measurements for years to come.

Common Mistakes When Selecting and Installing PT100 Sensors

Even the highest-quality PT100 sensor cannot deliver accurate measurements if the overall system is not properly designed. In our experience, many temperature measurement problems are caused not by the sensor itself, but by avoidable design and installation mistakes.

1. Using a 2-Wire Connection for Long Cable Runs

This is probably the most common mistake. In a 2-wire configuration, cable resistance is added directly to the sensor resistance and can introduce temperature errors of several degrees, particularly when the sensor is located far from the PLC. In these situations, a 3-wire or 4-wire connection is generally recommended.

2. Confusing PT100 with PT1000

Both sensors use platinum sensing elements and often appear mechanically identical. However, their nominal resistance differs by a factor of ten. Connecting a PT100 to a PT1000 input, or vice versa, will immediately produce incorrect temperature readings.

3. Incorrect PLC Input Configuration

Most PLCs and temperature transmitters allow the user to configure both the RTD type and the wiring method. Even if the correct sensor is installed, selecting the wrong configuration-for example, PT100 instead of PT1000, or 2-wire instead of 3-wire can result in significant measurement errors.

4. Using an Unsuitable Cable

Long cable runs, undersized conductors, poor-quality connectors, or corroded terminals all add unwanted resistance to the measurement circuit and reduce accuracy. Industrial installations should use appropriately sized, high-quality cables designed for instrumentation applications.

5. Installing the Sensor in the Wrong Location

Even the most accurate temperature sensor cannot provide meaningful data if it is installed where it does not represent the actual process temperature. Mounting a sensor too close to a heater, against the wall of a tank, or in an area with localized airflow can produce misleading measurements.

6. Choosing the Wrong Probe Construction

Not every PT100 probe is suitable for every application. Surface mounting, pipe installation, immersion measurement, outdoor environments, and process control each require different mechanical designs, sealing levels, and materials. Selecting the correct probe construction is just as important as selecting the sensing element itself.

7. Replacing the Sensor Before Troubleshooting the System

When a temperature reading appears incorrect, the first instinct is often to replace the sensor. In many cases, however, the real cause is cable resistance, wiring configuration, poor connections, or incorrect PLC settings. A systematic inspection of the measurement system can often resolve the problem without replacing the sensor.

Ultimately, selecting a PT100 sensor is about much more than choosing a sensing element. A properly designed measurement system-from the sensor type and wiring method to the probe construction and installation location-is what determines whether temperature measurements remain accurate, stable, and reliable throughout the system’s lifetime.

🧩 Further Reading – Measurement as a System

This article is part of an engineering series exploring how reliable measurement depends on proper system design rather than on a single sensor component.

Before diving deeper into industrial temperature sensing, you may also find the following articles in the series useful:

Frequently Asked Questions (FAQ)

What is the difference between PT100 and PT1000?

Both are platinum-based RTD (Resistance Temperature Detector) sensors that operate using the same measurement principle. The main difference is their nominal resistance at 0°C: a PT100 has a resistance of 100 Ω, while a PT1000 has a resistance of 1000 Ω. As a result, the PT1000 is less sensitive to cable resistance and is particularly well suited for applications with long cable runs.

Is a PT100 more accurate than a PT1000?

No. When both sensors are manufactured to the same accuracy class, such as Class A, their measurement accuracy is very similar. The choice between them depends primarily on the measurement system, cable length, and the control equipment being used.

When should I choose a PT100 sensor?

A PT100 is commonly used in industrial automation, laboratory equipment, and existing control systems where PLCs and temperature transmitters are already designed to support PT100 inputs. It has been an industrial standard for many years.

When is a PT1000 the better choice?

A PT1000 is often preferred in new system designs, particularly when the sensor is located a considerable distance from the controller or when electrical noise is a concern. Its higher resistance reduces the relative effect of cable resistance and improves measurement reliability.

What is the difference between 2-wire, 3-wire, and 4-wire RTD connections?

In a 2-wire configuration, cable resistance is added directly to the sensor resistance and can introduce measurement errors. A 3-wire configuration allows most PLCs and temperature transmitters to compensate for cable resistance, making it the standard solution in industrial automation. A 4-wire configuration provides the highest measurement accuracy using Kelvin measurement, where cable resistance has virtually no effect on the result.

Can I replace a PT100 with a PT1000 without modifying the system?

Usually not. The PLC, temperature transmitter, or measurement device must support PT1000 inputs and be configured accordingly. Connecting the wrong sensor type will result in incorrect temperature readings.

Can a long cable really affect temperature accuracy?

Yes. In a 2-wire installation, cable resistance alone can introduce temperature errors of several degrees, even when the sensor itself is functioning perfectly. For long cable runs, a 3-wire or 4-wire configuration is recommended, or alternatively a PT1000 sensor.

Can PT100 and PT1000 sensors be customized?

Yes. RTD sensors are available in a wide range of configurations, including different cable lengths, Class A or Class B accuracy, various probe constructions, IP protection ratings, and customized electrical connections to suit specific application requirements.

Key Terms

RTD (Resistance Temperature Detector)

A temperature sensor that measures temperature by monitoring changes in the electrical resistance of a metal. In most industrial applications, the sensing element is made from platinum because of its excellent stability and accuracy.

PT100

A platinum RTD sensor with a nominal resistance of 100 Ω at 0°C. It is one of the most widely used industrial temperature sensors and is commonly found in automation systems, laboratory equipment, and process control applications.

PT1000

A platinum RTD sensor with a nominal resistance of 1000 Ω at 0°C. It operates using the same principle as a PT100 but is less affected by cable resistance, making it well suited for long cable runs.

Class A / Class B

Accuracy classes defined by the IEC 60751 standard for platinum RTD sensors. Class A sensors provide tighter accuracy tolerances than Class B sensors and are preferred in high-precision applications.

IEC 60751

The international standard that defines the electrical characteristics, temperature-resistance relationship, accuracy classes, and performance requirements for platinum RTD sensors.

2-Wire Connection

A wiring method in which the same two conductors carry both the measurement current and the sensor signal. The resistance of the connecting wires becomes part of the measured resistance and can introduce temperature errors.

3-Wire Connection

The most common RTD wiring configuration in industrial automation. A third conductor enables the measurement system to compensate for cable resistance, significantly improving measurement accuracy.

4-Wire Connection (Kelvin Measurement)

The most accurate RTD wiring method. Two wires supply the measurement current, while two separate wires measure the voltage across the sensing element. This configuration virtually eliminates the effect of cable resistance.

Cable Resistance

The electrical resistance of the wires connecting the RTD sensor to the measurement system. In 2-wire systems, cable resistance can introduce significant temperature measurement errors, especially over long distances.

Self-Heating

A small increase in the sensor’s temperature caused by the measurement current flowing through the sensing element. Excessive measurement current can cause the sensor to indicate a temperature slightly higher than the actual process temperature.

Response Time (t90)

The time required for a temperature sensor to reach 90% of its final reading following a sudden change in temperature. Shorter response times enable the sensor to track rapid temperature changes more effectively.

IP67

An ingress protection rating indicating that the sensor is completely protected against dust and can withstand temporary immersion in water under the conditions specified by the standard.

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