Industrial temperature transmitter measures temperature through sensors like Resistance Temperature Detector (RTD). An RTD measures temperature by correlating the electrical resistance of a material to temperature changes. It operates on a simple principle where the electrical resistance of the material increases linearly as the temperature rises. RTDs are widely used for precise temperature measurement in industrial and laboratory settings, especially in applications requiring high accuracy and long-term stability between -200°C and 850°C. They are commonly applied in applications such as HVAC systems, food processing, pharmaceuticals, chemical manufacturing, etc.
Working Principle
An RTD operates based on a simple but highly dependable principle: the electrical resistance of certain metals increases with temperature. By measuring this change in resistance, the RTD can accurately determine the temperature of its environment.
The most commonly used RTD type is the Pt100, which is made of platinum and has a resistance of 100 ohms at 0°C. Platinum is preferred due to its excellent stability, repeatability, and predictable response to temperature changes.
The relationship between temperature and resistance in an RTD is nearly linear and can be expressed using the following equation:
R T = R 0 ( 1 + α T )
Where:
R T = the resistance at temperature T
R 0 = the resistance at 0 °C
α = the temperature coefficient of resistance (for platinum ≈ 0.00385 °C⁻¹)
T = the temperature in °C
As temperature increases, the resistance of the platinum element also increases in a predictable manner. This characteristic allows for precise temperature measurement.
Construction of an RTD Probe
A RTD probe is a mechanical assembly consisting of a sensing element, a conductor wire, and a wire termination (either on a terminal block or connector). This assembly is housed within a protective or mounting support to protect their fragile internal sensing elements from harsh environments, moisture, and vibration. The choice of housing material depends on the operating temperature, corrosion resistance, and responsiveness required.
Wiring Configurations
A RTD probe functions by changing resistance with temperature, usually in a 2, 3, or 4-wire configuration, with 3-wire being the most common industrially. Transmitters convert this low-level resistive signal into standardized, robust outputs (4–20 mA, 0–10 V) or digital protocols (HART/PROFIBUS) to enable long-distance signal transmission, reduce electrical noise, and improve measurement accuracy.
- 2-wire: Simple and cost-effective, but includes lead wire resistance, which can reduce accuracy.
- 3-wire: The most commonly used setup in industrial applications. It compensates for lead resistance and improves accuracy.
- 4-wire: Provides the highest accuracy by completely eliminating the effect of lead resistance. Typically used in laboratory or high-precision environments.
