RTD Sensor

RTD temperature sensors work on the principle that the electrical resistance of a metal increases with temperature in a predictable way. As the temperature of the metal rises, its atoms vibrate more, making it harder for electrons to pass — this increases the resistance. By measuring this change in resistance, the corresponding temperature can be accurately calculated.

An RTD Resistance Temperature Detector senses temperature by monitoring variations in the electrical resistance of a metal, usually platinum.

The RTD sensor’s operating principle relies on the fact that resistance varies with temperature in a known manner, providing accurate and stable temperature readings.

Think about parameters such as temperature range, precision, environment (vibration, chemicals), response time, and installation type. Select materials and construction based on these.

RTDs are applied to steel, pharmaceuticals, food processing, petro, HVAC, aerospace, power plants, and industrial automation for accurate temperature measurement and control.

The Callendar-Van Dusen equation is used to define RTD resistance:
R(t) = R₀(1 + At + Bt² + C(t – 100)t³), where A, B, C are constants.

Common materials are platinum (most precise), copper, nickel, and nickel-iron alloys—selected on the basis of stability, linearity, and corrosion resistance.

Platinum is highly stable and has a large range; copper is economical but low-resistance; nickel is highly sensitive but non-linear.

Platinum RTDs generally function within the range of –200°C to +850°C, whereas copper and nickel versions possess lower temperature thresholds determined by their design and materials.

IEC 751 specifies tolerances for RTD:
Class A = ±(0.15 + 0.002×t)°C;
Class B = ±(0.3 + 0.005×t)°C;
There are other classes such as 1/3, 1/5 DIN which are for greater precision.

They are high-purity platinum RTDs in accordance with ITS-90 standards and used in metrology laboratories for precise and repeatable measurements.

RTDs are made up of a sensing element (wire or film), insulators, leads, and a protective cover. They can be constructed as thin-film, coil-wound, or mineral insulated.

This form employs platinum wire wound into a helix and placed inside a ceramic tube for support, suitable for precise lab and industrial use.

Platinum wire is wound over a mandrel and glass- or ceramic-covered here, providing improved vibration resistance and moderate accuracy.

RTDs operate on 2, 3, or 4-wire configurations. Additional wires ensure the elimination of lead resistance and ensure greater accuracy in measurements.

Simple configuration in which a single lead is connected to both ends of the element. It’s easy but has the effect of lead resistance being measured, which decreases the accuracy.

It’s the most popular industrial setup; it takes care of lead wire resistance if all leads have the same resistance.

Utilized in applications requiring precision, it totally removes lead resistance effects by sensing voltage along a known current path.

RTD wiring generally adheres to color codes: two red and one white for 3-wire; two red and two white for 4-wire configurations.

These RTDs are housed in compacted MgO within a metal sheath, thereby being vibration-resistant and flexible, and suited for harsh environment use.

Typical errors are lead wire resistance, insulation breakdown, self-heating, mechanical stress, and long-term calibration drift.

Conformity guarantees standardized performance from sensors; increased conformity indicates greater interchangeability without recalibration.

Sensitivity is a measure of how much the resistance varies per degree; greater sensitivity enhances measurement resolution and signal intensity.

High insulation resistance avoids shunting errors and insures the RTD’s readings are correct and not affected by leakage currents.

Current measurement induces minor self-heating. If not relieved, it causes errors. Reduced current or improved heat removal lessens the effect.

It establishes the speed with which the RTD responds to changes in temperature. Reduced time constants allow faster response in dynamic applications.

Repeatability guarantees that the RTD delivers the same output for a given set of circumstances, essential for process control and data logging to be reliable and consistent.

Long-term resistance to drift is represented by stability. Platinum RTDs exhibit excellent stability, particularly in harsh industrial environments.

Suitable packaging facilitates heat transfer, guards the element, and maintains precision and quick response in the desired environment.

These are robust RTD assemblies contained in protective sheaths and used for direct immersion or industrial installations within a thermowell.

Probe assemblies consist of the RTD sensor, sheath, lead wires, and mounting hardware to meet process connection specifications.

Flexible RTDs are thin, flexible sensors applied in curved or irregular surfaces, offering quick response and high accuracy in confined areas.

These RTDs are engineered for custom applications like surface mounts, embedded sensors, or flexible strip form in OEM equipment.

RTDs are employed in environments such as process industries, laboratories, pharmaceuticals, aerospace, energy, and HVAC, where accurate and consistent temperature regulation is necessary.

They offer excellent accuracy, enduring stability, wide temperature range, high repeatability, making them ideal for precise temperature control applications.

RTDs are pricier than thermocouples, respond more slowly, and are less applicable at extremely high temperatures (beyond 850°C).