Resistance Thermometer (RTD): Construction, Working Principle, Equation, Advantages & Applications

Introduction

Resistance thermometers, also known as Resistance Temperature Detectors (RTDs), are highly accurate temperature sensors. Most RTD elements are made of fine platinum, nickel, or copper wire wound around a ceramic or glass core. These materials are selected because their electrical resistance varies predictably with temperature. Since RTD elements are delicate, they are often enclosed in protective probes.

The temperature is measured by detecting the change in electrical resistance of the sensing material. Platinum RTDs are the most widely used because of their long-term stability, wide temperature range, and high accuracy. They are gradually replacing thermocouples in many industries.

An RTD probe consists of a resistance element, protective sheath, lead wires, and termination. The sheath protects the sensing element from moisture, vibration, and environmental conditions.

Construction of a Resistance Thermometer

A typical platinum RTD consists of a platinum coil wound on a mica cross-frame structure. The entire assembly is placed inside an evacuated stainless-steel tube to minimize unwanted heating effects. Using mica between the platinum coil and the tube improves insulation and stability. The purity of the platinum is essential and is checked using the ratio R₁₀₀ / R₀, which should be greater than 1.390 for pure platinum.

Working Principle of a Resistance Thermometer

RTDs work on the principle that the electrical resistance of a metal changes with temperature. The sensing element—usually platinum—is wound or deposited on a ceramic or glass core.

As temperature increases, the resistance of the metal increases in a predictable manner. This change in resistance is measured using a Wheatstone bridge circuit and converted into temperature.

Desired Properties for RTD Sensing Materials

Large and predictable change in resistance with temperature
High stability and repeatability
High linearity over a wide temperature range
Fast response to temperature changes
Minimal drift with age

Platinum is preferred because it provides excellent linearity, high stability, and can operate reliably from -260°C to +1100°C.

Wheatstone Bridge Operation

A Wheatstone bridge detects changes in resistance in the RTD. When the temperature changes, the resistance of the sensing element (R_s) changes, causing an imbalance in the bridge. This imbalance produces a voltage that is calibrated to indicate the corresponding temperature.

Resistance Thermometer Equation

The temperature–resistance relationship of an RTD is given by:

R_t = R₀ (1 + αt + βt² + γt³ + … )

A simplified version is:

R_t = R₀ (1 + αt + βt²)

For Pure Platinum

α = 3.94 × 10⁻³ /°C
β = -5.8 × 10⁻⁷ /°C²

The platinum temperature coefficient (t_pt) is defined using the resistance at 0°C, 100°C, and temperature t. The difference between true temperature and platinum temperature is given by the constant δ, which ranges between 1.488 to 1.498. A lower value of δ indicates higher purity. Platinum RTDs typically operate from -100°C to 650°C.