Resistance sensors in industrial systems are one of the fundamental components of industrial temperature measurement systems. In practice, these are most often RTD sensors, primarily Pt100 and Pt1000, which are used in automation, the energy sector, HVAC, and the food and chemical industries. In all areas where temperature affects process stability, product quality or the safety of the installation.
From a technical point of view, the issue is only seemingly straightforward. Simply choosing between a Pt100 and a Pt1000 does not in itself solve the problem. The quality of the measurement is determined by the entire system: accuracy class, probe design, connection method, cable length, the presence of a transducer, the design of the protective sheath and the operating conditions at the site. It is precisely here that errors arise, which are later mistakenly attributed to the sensor itself.
What are resistive sensors and how do they work?
A resistance sensor utilises the relationship between temperature and the resistance of the sensing material. In industrial temperature systems, platinum is the standard choice because it offers stable, repeatable and well-documented characteristics. As the temperature rises, the resistance of the sensing element increases, and the measurement electronics convert this change into a temperature value.
In industrial practice, the designations Pt100 and Pt1000 are most commonly encountered. The number in the name indicates the nominal resistance at 0°C. For Pt100, this is 100 Ω, and for Pt1000, 1000 Ω respectively. Both types operate on the same principle, but behave differently throughout the measurement circuit, particularly when the length of the leads, the quality of the connections and the manner of interaction with the controller input or transmitter are taken into account.
Why has platinum become the industry standard?
In industrial systems, it is not enough for a sensor to simply ‘give a rough indication of the temperature’. Repeatability, long-term stability and predictable performance across a wide operating range are essential. This is precisely why platinum has remained the material of choice in professional temperature metrology and in industrial RTD sensors for many years.
This is particularly important where temperature is used not only for monitoring but also for process control. In a dryer, heat exchanger, heating system, refrigeration system or production line, an error of just a few degrees can result in unnecessary energy consumption, process instability or a false alarm. For this reason, a resistance sensor should not be treated as a simple addition to the system, but as a measuring element on which the quality of decisions made by the automation system depends.
Pt100 and Pt1000 – differences that matter in design
The simplest comparison is that a Pt100 has a resistance of 100 Ω at 0°C, whilst a Pt1000 has ten times that value. This is true, but from a design perspective, what matters more is what this means in practice. The higher nominal resistance of the Pt1000 reduces the relative impact of the resistance of wires and connections. As a result, in some new distributed systems, building installations and less-than-ideal installation conditions, the Pt1000 is simply more tolerant of installation imperfections.
However, the Pt100 remains deeply entrenched in traditional industrial automation. In many plants, it remains the basic standard, well supported by transducers, measuring equipment and older control systems. Therefore, the choice between Pt100 and Pt1000 should not be based on fashion or the simple slogan ‘newer is better’, but on actual operating conditions and system architecture.
| Parameter | Pt100 | Pt1000 |
|---|---|---|
| Resistance at 0°C | 100 Ω | 1000 Ω |
| How it works | Platinum RTD | Platinum RTD |
| The effect of cable resistance | relatively larger | relatively smaller |
| Typical working environment | traditional industrial automation | new layouts, longer lines, distributed systems |
| Compatibility with transducers | very wide | wide |
The list may seem innocuous, but in practice it can lead to maintenance issues down the line. If the system is to operate for years in conditions involving high vibration, contamination, moisture and numerous connection points, the choice of sensor type alone should be part of a broader design decision. This is where it is easiest to make the mistake of treating Pt100 and Pt1000 as fully interchangeable solutions.
Construction of the measuring element – thin-film and wire-wound
Technical documentation often distinguishes between thin-film and wound components. For the investor, this is usually just a single line in the product data sheet, but for the designer and automation engineer, it makes a difference.
The thin-film process involves applying a thin layer of platinum to a ceramic substrate. Such a cartridge is usually smaller, withstands vibrations well, and is ideal for applications where compact dimensions and mechanical robustness are key. In contrast, a wire-wound element is better suited to applications where stability and accuracy over a wider temperature range are the priority.
It’s not that one type is always better. The point is that both are suitable under different conditions. If the system is operating on a machine subject to high levels of vibration, a thin-film sensor is often the more sensible choice. If long-term stability and precision are more important, the wound-type sensor may have the advantage. The Pt100 symbol alone does not indicate this difference, yet it is precisely this difference that can later become apparent during operation.
2-, 3- and 4-wire connections – it’s very easy to ruin a good measurement here
The way in which resistive sensors are connected in industrial systems has a direct impact on the results. This is not merely an installation detail, but one of the key parameters of the measurement circuit.
In a 2-wire system, the resistance of the wires is added to the resistance of the sensor itself. Whilst this may be acceptable for short distances and simple applications, in industrial installations this approach quickly reveals its limitations. The longer the line, the greater the error. In many cases, this error can completely negate the benefit of using a more accurate sensor.
The 3-wire configuration is very common today, as it allows for the effects of the leads to be compensated for, provided their resistance is similar. It offers a sensible balance between cost and measurement quality. The problem is that, in real-world scenarios, this assumption is not always perfectly met. Differences in lead lengths, cross-sections or the quality of connections can interfere with the compensation effect.
A 4-wire configuration delivers the best metrological results, as it virtually eliminates the influence of the leads on the measurement. In critical processes, this solution simply makes sense. Not because it looks more ‘professional’, but because it minimises one of the most common causes of discrepancies between the theoretical value and the actual measurement taken on site.
| How to connect | Advantage | Restriction | Typical application |
|---|---|---|---|
| 2-wire | simplicity and low cost | a significant effect of cable resistance | simple installations, short sections |
| 3-wire | a good balance between cost and accuracy | requires cables with similar resistance | most industrial applications |
| 4-wire | the highest accuracy | higher cost and more complex cabling | measurements requiring a high degree of reliability |
The key takeaway is simple. If a circuit is to operate in a process that is genuinely sensitive to temperature, cutting corners on the wiring often ends up costing more than doing it properly from the start.
Accuracy classes and tolerances in accordance with the standard
One of the most common oversimplifications in product descriptions is the claim that a Class A sensor is accurate, whilst a Class B sensor is less so. This is true, but without specific figures, it doesn’t tell us much. In the standard for industrial platinum sensors, the tolerance depends on temperature and increases as the distance from the reference point increases.
This means that the sensor error is not a fixed value. It is therefore not possible to accurately assess the quality of the measurement based solely on a single general specification listed in the data sheet. One must consider the accuracy class, temperature range and operating conditions of the entire system.
| Class | A typical tolerance relationship |
|---|---|
| AA | ±(0.1 + 0.0017· |
| A | ±(0.15 + 0.002· |
| B | ±(0.3 + 0.005· |
| C | ±(0.6 + 0.01· |
The table itself shows that the sensor’s accuracy is not independent of the operating temperature. This is particularly important in industrial applications, where the device does not operate at a constant temperature, but is instead subjected to a wide range of temperatures, heating and cooling cycles, and sometimes dynamic conditions as well.
From a design perspective, the most sensible approach is to select the accuracy class in conjunction with an analysis of the entire measurement chain. If the controller input, cables and installation introduce a greater error than the sensor itself, purchasing a more expensive, higher-grade component does not yield a commensurate benefit.
Temperature sensor – an often underestimated component of the system
In modern industrial installations, a resistance sensor is very often used in conjunction with a temperature transmitter. It is the transmitter that converts the resistance signal into a standard signal, such as 4–20 mA or HART, and ensures a stable transmission to the control system.
This is particularly important because, in industrial environments, the problem lies not only with the sensor itself but with the entire signal path. Long cable runs, inverters, densely packed equipment cabinets, motors and electromagnetic interference can effectively disrupt the measurement. In such conditions, a well-chosen transducer is not a premium accessory, but a component that genuinely improves the system’s resilience. From a maintenance perspective, it is also a more convenient solution. It is easier to diagnose a current signal, easier to distinguish between sensor error and transmission error, and easier to maintain measurement consistency in more complex installations.
Does the installation method determine whether the sensor measures the process itself or merely an approximation of it?
Many articles on RTDs devote too much attention to the electrical aspects and too little to the mechanical aspects of installation. Yet the immersion length, the sheath diameter, the sheath material, the installation location and thermal contact with the medium all have a direct impact on the measurement result and response time.
An incorrectly selected immersion length leads to flattened readings or a delayed response. Poor thermal contact with the sheath reduces measurement quality, even if the element itself is highly accurate. An unsuitable sheath material, in turn, can shorten the sensor’s service life in a harsh process environment. This is where the difference between a sensor selected from a catalogue and one selected on a technical basis becomes apparent. In industrial applications, the measuring element itself is just the beginning. Equally important is how and where it has been installed.
Resistance sensors in industrial systems versus thermocouples
A comparison between RTDs and thermocouples is inevitable, as both solutions are used in the same systems, but they perform well in different scenarios. Resistance sensors excel where good stability, repeatability and accuracy are required within typical industrial ranges. Thermocouples have the advantage at very high temperatures and in some more extreme process applications.
In practice, this means that RTDs perform well in process automation, HVAC systems, the power industry, the food industry and many other industrial applications. They are ideal wherever reliable measurement within a moderate to medium temperature range is required. A thermocouple becomes the natural choice when the temperature range clearly exceeds the comfortable operating range of an industrial platinum sensor. There is no single universal winner here. It is simply a matter of selecting the right technology for the process. And this is precisely what should be the focus of any serious technical analysis.
The most common mistakes when selecting resistive sensors
In practical design, most problems do not stem from defects in the sensors themselves, but from incorrect assumptions. A very common mistake is selecting a sensor based solely on its name and accuracy class, without analysing the connection method and cable length. A second mistake is ignoring the impact of the mechanical aspects of the installation. A third is omitting a signal conditioner where the noisy environment clearly requires one.
An error also occurs when a system is designed on the assumption that laboratory conditions will be directly replicated on the production floor. In real-world installations, it is not just accuracy ‘on paper’ that counts, but also stability, resilience to operational errors and long-term reliability.
Manufacturers’ specifications contain a wealth of limit values, classes and parameters. This information is essential. However, it is equally important to take a realistic view of what happens on site. A temperature sensor will only function correctly if the entire measurement circuit has been designed as a coherent system, rather than a haphazard collection of randomly selected components.
How should one choose the right resistive sensor for a system?
The most sensible approach starts with the process itself. You need to define the temperature range, the required accuracy, the rate of change, the medium, and the mechanical and environmental conditions. You also need to determine whether the temperature is used solely for monitoring purposes or whether it directly influences the control system. Only then can you select the type of RTD, its class, the connection method, the sheath and the transmitter.
In simpler systems with short cable runs and moderate requirements, the Pt1000 can prove to be a convenient and reliable choice. In traditional industrial automation, with its wide range of compatible equipment, the Pt100 remains a very strong standard. If the process is demanding and the result must be truly reliable, the importance of a 4-wire system, proper installation mechanics and a high-quality transducer increases.
This is important because the cost of choosing the wrong sensor usually goes beyond the price of the sensor itself. Far more costly are the subsequent downtimes, unstable control, false alarms and the time wasted trying to identify the cause of the problem in an area that was unnecessarily simplified at the design stage.
Summary – Resistive sensors in industrial systems
Resistance sensors in industrial systems are a mature, proven and widely used solution, but they only perform as expected when viewed in a broader context than simply through the lens of the Pt100 or Pt1000 designation. The quality of the measurement is determined by the entire system: the material of the sensing element, the accuracy class, the connection method, the length of the leads, the presence of a transducer, and the correctness of the mechanical installation.
The biggest mistake is that the resistance sensor is often treated as a minor piece of equipment. In reality, it is one of those components that can determine the stability of the process. In a well-designed system, the RTD is not a random addition. It is a carefully selected measuring component designed to operate reliably for years to come.
