How do RTDs work?

Key Components of an RTD

1. Sensing Element:
   The core of an RTD is the sensing element, typically made of a pure metal such as platinum, nickel, or copper. Among these, platinum is the most widely used due to its excellent stability, repeatable resistance-temperature characteristics, and resistance to chemical corrosion. The element is designed in various forms, including thin-film, wire-wound, or coiled designs, depending on the required sensitivity, range, and application environment.
2. Lead Wires:
   The sensing element is connected to the measurement instrument through lead wires. These wires play a crucial role in transmitting the electrical signal generated by the resistance changes in the sensing element. Depending on the application and accuracy requirements, RTDs may be configured with two, three, or four lead wires. Three- and four-wire configurations are preferred for industrial applications as they minimize errors caused by lead wire resistance.
3. Support Structure:
   To ensure the durability and proper functioning of the sensing element, it is mounted on a support structure. This structure provides mechanical stability and protection, ensuring that the sensing element remains precisely positioned within the process. Common support materials include ceramics and glass, which can withstand high temperatures and harsh environments without affecting the RTD’s performance.
4. Protective Sheath:
   In many industrial applications, the RTD is enclosed in a protective sheath made from stainless steel, Inconel, or other robust materials. This sheath safeguards the sensor against mechanical damage, vibration, and exposure to corrosive or high-pressure environments.

Types of RTDs

RTDs come in various types, each designed for specific applications, environments, and performance requirements. Platinum RTDs dominate industrial and scientific settings due to their accuracy and stability, while nickel and copper RTDs offer cost-effective solutions for specific environments. Similarly, wire-wound, thin-film, and coiled-element constructions provide options for durability and response time, and the choice of two-, three-, or four-wire configurations allows for tailored accuracy levels.

1. Types Based on Sensing Element Material

The material of the sensing element significantly influences the RTD’s performance, including its accuracy, temperature range, and stability.

a. Platinum RTDs

Platinum is the most widely used material for RTDs due to its stability, linear resistance-temperature relationship, and wide operating range (-200°C to 850°C). These RTDs are often standardized (e.g., PT100 or PT1000), where PT100 indicates 100 ohms resistance at 0°C.

• Advantages: High accuracy, excellent repeatability, and chemical inertness.
• Applications: Industrial process control, laboratories, and aerospace.

b. Nickel RTDs

Nickel RTDs offer good sensitivity and a lower cost compared to platinum. However, their non-linear resistance-temperature relationship limits their use to a narrower range (-60°C to 180°C).

• Advantages: Cost-effective and suitable for moderate temperatures.
• Applications: HVAC systems and refrigeration.

c. Copper RTDs

Copper RTDs are known for their excellent linearity and affordability. However, they are limited to lower temperatures due to copper’s susceptibility to oxidation (-200°C to 150°C).

• Advantages: High conductivity and cost-effectiveness.
• Applications: Power plants and electrical systems.

2. Types Based on Construction

The construction of an RTD determines its durability, response time, and suitability for specific environments.

a. Wire-Wound RTDs

These RTDs use a fine wire of the sensing material wound around a ceramic or glass core. The winding can be open or encapsulated.

• Advantages: High accuracy and excellent long-term stability.
• Disadvantages: Susceptibility to mechanical damage and slower response time.
• Applications: Precision laboratory measurements and industrial processes.

b. Thin-Film RTDs

Thin-film RTDs are created by depositing a thin layer of platinum onto a ceramic substrate. The layer is then etched into a precise pattern.

• Advantages: Compact size, fast response time, and cost-effectiveness.
• Disadvantages: Slightly lower stability and maximum temperature compared to wire-wound RTDs.
• Applications: Electronics, automotive, and medical devices.

c. Coiled-Element RTDs

In coiled-element RTDs, the sensing wire is loosely wound and encapsulated within a protective sheath. This design allows the element to expand and contract freely, reducing stress and improving durability.

• Advantages: High accuracy, durability, and resistance to mechanical stress.
• Applications: High-vibration environments and industrial applications.

3. Types Based on Configuration

The configuration of the RTD determines its connection to measurement devices and suitability for specific applications.

a. Two-Wire RTDs

This is the simplest configuration, where two wires connect the RTD to the measurement device. However, the resistance of the lead wires introduces errors.

• Advantages: Simple and cost-effective.
• Disadvantages: Reduced accuracy due to lead resistance.
• Applications: Non-critical measurements.

b. Three-Wire RTDs

Three-wire RTDs add a third lead wire to compensate for lead resistance. This configuration balances accuracy and complexity.

• Advantages: Improved accuracy over two-wire designs.
• Applications: Industrial temperature monitoring.

c. Four-Wire RTDs

The four-wire configuration eliminates lead resistance errors entirely by using separate circuits for current and voltage measurement.

• Advantages: Highest accuracy.
• Applications: Laboratory and precision industrial applications.

Understanding these distinctions helps you select the right RTD for your needs, balancing factors like cost, accuracy, and environmental requirements. By choosing the appropriate type, you can achieve reliable and precise temperature measurements crucial for operational efficiency and safety.

How do they work?

RTDs are designed to ensure precise and repeatable temperature versus resistance characteristics. The type of material used in RTD construction will determine the limits on the temperature the RTD can be exposed to. The sensing element in an RTD is an electrical resistor that changes the resistance value as the temperature changes. The change in resistance with temperature happens at a predictable rate and can be measured to provide the temperature of a material of process.

The sensing element in an RTD typically contains either a coil of wire or a substrate with an etched film of platinum. The use of extension wires attached to the sending element allows the electrical resistance to be measured from a distance away from the process or material being measured. A protective sheath (normally stainless steel) houses the sensing element. Platinum is typically the material of choice in an RTD as this allows for a larger temperature range and it is a more stable material. Nickel and copper can also be used in RTD construction if a lower temperature range is required. Table 1. Outlines usable temperature ranges for each material.

For RTD construction, there are two methods most used. The most common is to place the RTD element and attached wires into a metal tube with a closed end. The tube is packed with a vibration dampening and/or heat transfer material, alumina powder in most cases, and seal the open end with a silicone, epoxy or ceramic cement.

An alternate construction method is the use of a mineral insulated metal sheath (MIMS) cable. The RTD element is inserted into a drilled hole and attached to nickel or copper wires insulated by Magnesium Oxide (MgO). The end is also insulated with MgO and welded closed. The other end has extension wires attached prior to sealing.
Once the temperature range and application has been determined, Pyrosales will select the most suitable materials and method of construction of the RTD.

2, 3 or 4 wire configuration

2-wire construction is the least accurate. 2-wire RTDs tend to be used with short lead wires or where close accuracy is not required.
3-wire construction is the most popular choice for industrial applications, whilst 4-wire construction is used in laboratories where close accuracy is imperative.

There are many options to consider when choosing the correct RTD element for your requirements:

1. Temperature rating
2. Tolerance, accuracy, and interchangeability
3. Time response
4. Distance from control or measuring equipment

Features and Benefits

• Accuracy. A special process combines strain-free construction with full winding support for dependable, accurate readings in standard RTD elements, and in flat film elements where platinum is etched onto a substrate.
• High signal-to-noise output. Increases the accuracy of data transmission and permits greater distances between the sensor and the measuring equipment.
• Interchangeability. Strain-free construction and precision trimming allow elements from different lots to be substituted without recalibration.
• Sensitivity. Self-heating is minimised, allowing precise measurement. The temperature coefficient (alpha) is carefully controlled at the industry standard while insulation resistance values exceed IEC-751 standards.
• Standardisation. Elements are available to meet or exceed the requirements of various standardisation agencies. IEC-751 standard tolerance classes A and B respectively are very well suited for industrial applications. Tolerance classes up to 1/10th DIN can also be supplied where higher accuracy is demanded
• Physical and chemical stability over a wide temperature range.  Pyrosales use a highly-controlled manufacturing process. Standard elements are built to resist mechanical vibration and shock, however, where there is high exposure to mechanical vibration, specially manufactured military specification thick film RTD elements can be supplied to suit the application.
• Repeatability. All elements exceed IEC-751 repeatability values even after long exposure to temperatures within the operating range.

Disadvantages of Resistance Temperature Detectors (RTDs) 

While RTDs are highly accurate and reliable for many applications, like any technology they have disadvantages. This includes low sensitivity, higher cost, lack of point sensing, susceptibility to shock and vibration, and complex wiring requirements. You should carefully evaluate these factors against the demands of your specific application. For environments requiring point sensing, rapid response, or extreme durability, alternative sensors like thermocouples may be more appropriate. Nonetheless, when precision and long-term stability are critical, RTDs remain a leading choice.

1. Low Sensitivity

RTDs exhibit lower sensitivity compared to some alternatives, such as thermocouples. While they provide highly accurate measurements, the change in resistance per degree of temperature is relatively small. For instance, a standard platinum RTD (PT100) has a resistance change of only 0.385 ohms per degree Celsius. This low sensitivity necessitates the use of precise instrumentation to detect and interpret these changes accurately. In environments where rapid or minor temperature fluctuations need to be detected instantly, this can be a drawback.

2. Higher Cost than Thermocouples

One of the most significant disadvantages of RTDs is their higher cost compared to thermocouples. The use of high-purity materials, such as platinum, and the intricate manufacturing processes involved in producing RTDs contribute to their expense. For applications where budget constraints are a priority, particularly in large-scale deployments requiring numerous sensors, thermocouples may be a more economical choice.

3. No Point Sensing

RTDs measure temperature over the length of their sensing element rather than at a single, precise point. While this is beneficial for obtaining an average temperature, it can be a limitation in applications requiring pinpoint accuracy, such as detecting hot spots or measuring rapid temperature gradients. Thermocouples, in contrast, are better suited for point sensing, making them ideal for localized temperature measurement.

4. Susceptibility to Shock and Vibration

RTDs are more prone to damage from mechanical shock and vibration compared to other temperature sensors. The delicate nature of the sensing element, particularly in wire-wound RTDs, makes them susceptible to breakage or deformation under harsh operating conditions. In environments such as industrial machinery or aerospace applications, where vibrations are common, thermocouples or thermistors may offer greater durability.

5. Requirement for Three- or Four-Wire Operation

To ensure accurate measurements, RTDs often require three- or four-wire configurations to compensate for lead wire resistance. This adds complexity to the installation process and increases the cost of wiring and instrumentation. While two-wire RTDs are simpler, they are less accurate due to the influence of lead wire resistance on the measurement. For applications where simplicity and cost-effectiveness are priorities, this additional wiring requirement can be a significant drawback.

Other Considerations

• Temperature Range: Although RTDs operate across a broad temperature range (-200°C to 850°C), thermocouples can withstand much higher temperatures, making them more versatile in extreme environments.
• Response Time: RTDs generally have slower response times compared to thermocouples due to their larger thermal mass and construction.

Pyrosales can help you make informed decisions and optimise the performance and cost-efficiency of your temperature measurement systems.

Applications

RTDs are extensively used in industries like oil and gas, pharmaceuticals, and food processing, where accurate temperature measurement is essential. 

RTDs are suitable for many applications across a wide range of industries including stoves and grills, textile production, food processing, plastics, petrochemical and for temperature measurement of air, gases and liquids. They are also found in laboratory settings, HVAC systems, and semiconductor manufacturing due to their reliability and precision.

By leveraging their robust design and predictable behavior, RTDs provide a dependable solution for temperature monitoring and control in diverse applications. Use an RTD when you require accuracy and stability and where accuracy must extend over a wide temperature range.