When working with critical processes in chemical manufacturing, petrochemical processes, sugar and alcohol industries, pulp and paper industry, grain storage and transport, or food processing, one of the main challenges is ensuring accurate temperature measurement. The wrong sensor can lead to reduced performance, safety problems, or even equipment failure. That's why selecting the right technology for your application is fundamental to operational integrity. RTD is one of the most well-known, and we'll explain why.
What is a temperature sensor?
Unlike analog thermometers, which only visually display the thermal state, temperature sensors actively detect changes in the process and convert these variations into electrical output signals. These signals are transmitted to control systems (such as PLCs or DCSs), allowing operators to efficiently monitor and respond to any fluctuations.
Two of the most widely used devices in industry are Resistive Temperature Detectors (RTDs) and Thermocouples. Although both perform the function of measurement, they operate under distinct physical principles and have specific capabilities for different scenarios.
What is an RTD?
A resistance temperature detector (RTD), also known by its English acronym, is a sensor that bases its operation on the variation of the electrical resistance of a metal as a function of temperature. It essentially consists of a sensor element, connecting wires, and a support structure for positioning in the process.
RTDs are widely recognized for their exceptional accuracy and stability. Constructed with pure noble metals, such as platinum, their resistance varies in a predictable way. Platinum models, such as the PT100 (100 ohms at 0 °C) and the PT1000, meet stringent international standards, such as the IEC 60751 standard, ensuring consistent performance globally. They are particularly suitable for applications requiring high fidelity in a range that typically spans from -200 °C to 850 °C.
Main components of an RTD
Sensor element : This is the core of the device and can be manufactured from materials such as platinum, nickel, or copper. Platinum is the most widely used due to its high thermal stability, excellent measurement repeatability, and greater corrosion resistance. The element can be constructed as a wound wire or a thin film, depending on the level of precision and the industrial application.
Connecting wires : connect the element to the measuring instrument. They play a crucial role in signal integrity. They can be configured with two, three, or four wires, with the three- and four-wire versions being preferred industrially because they compensate for the resistance of the cables themselves.
Support structure : ensures mechanical stability and thermal protection for the sensor element, using materials such as ceramic or glass to withstand harsh environments.
Protective coating : the assembly is encapsulated in a stainless steel or Inconel sheath, protecting the sensor against mechanical damage, vibration, and corrosive media.
Thin-film RTDs : feature a microscopic layer of metal deposited on a ceramic substrate. They are compact, economical, and ideal for applications requiring fast response times and resistance to vibrations.
Wound-wire RTDs : These utilize a coil of ultra-fine platinum wire embedded in a ceramic or glass core. They offer the highest possible precision and stability, making them ideal for laboratory standards or processes with extremely high technical requirements.
The way the RTD is connected directly influences the quality of the data obtained:
- 2 wires: This is the simplest configuration, but the least precise, as the resistance of the connecting wires is added to that of the sensor, generating an error in the reading.
- 3 wires: uses a third wire to measure the line resistance and subtract it from the final calculation, eliminating distance errors.
- 4 wires: the most complex and expensive configuration, used in laboratories. It completely eliminates the impact of conductor resistance on both sides, producing results with maximum accuracy.
How do RTDs work?
The fundamental principle is predictability. The metallic material of the sensor alters its electrical resistance as the temperature changes. This rate of change is constant and documented. The use of extension wires allows this measurement to occur at a safe distance from the heat source. Although nickel and copper are options for lower temperature ranges and lower cost, platinum remains the gold standard for its longevity and resistance to oxidation.
Thermocouples operate differently: they are composed of two distinct metals joined at one end (measuring junction). When there is a temperature difference between this junction and the opposite end (reference junction), a proportional millivoltage is generated (Seebeck effect).
They are famous for their extreme robustness and ability to measure temperatures far beyond the limits of RTDs. A Type K (Chromium/Alumel) thermocouple, for example, is a workhorse of the metallurgical and furnace industries.
RTD vs. Thermocouple
Although both serve the purpose of electronic thermometry, the choice between an RTD and a thermocouple should be based on the following criteria:
Temperature Range : Thermocouples dominate high-temperature environments. While 90% of RTDs operate below 400 °C (reaching up to 850 °C in special models), certain thermocouples can monitor processes up to 2.500 °C in controlled atmospheres.
Cost : Thermocouples are generally cheaper to purchase initially. An RTD can cost two to three times as much. However, the RTD uses standard copper wiring for extension, while the thermocouple requires specific and more expensive compensation wires, which can balance the total installation cost.
Sensitivity and response : Thermocouples have a much faster thermal response. A model with a grounded or exposed junction reacts almost instantly to sudden changes, making it essential in overheating protection systems.
Accuracy and linearity : in this respect, RTDs are unbeatable. They offer accuracy of around 0,15 °C (Class A at 0°C – IEC 60751) and a near-linear response. Thermocouples exhibit an "S" curve and tend to suffer "drift" (loss of calibration) over time due to oxidation and chemical changes in the metal wires.
Stability : RTD readings are highly repeatable over long periods. If your application requires the sensor to maintain the same accuracy for years without constant recalibration, the RTD is the superior choice.
Where to buy RTD
When it comes to high-precision RTDs, Alutal is a benchmark in the Brazilian market. With over three decades of experience in temperature measurement, the company develops sensors for industrial applications that demand stability, durability, and compliance with technical standards.
Alutal's RTDs, such as the PT100-based models, are designed to ensure reliable readings even in environments with vibration, humidity, and abrupt temperature variations. Each batch undergoes testing in our own calibration laboratory, ensuring metrological traceability and consistent performance.
For industries seeking true precision in thermal control and wanting to reduce operational risks, investing in an Alutal RTD is betting on long-term reliability. Request a quote and bring more safety to your industrial process.
