Measuring RTD Output: Application Methods and Equipment

Contents

1. Introduction
2. Bridge Measurement Systems
3. RTD Wiring Configurations
4. Measuring Differential Temperatures
5. Potentiometric (Direct Reading) Methods
6. Modern Instrumentation for RTDs
7. RTD Wiring Quick Reference Table

RTDs (Resistance Temperature Detectors) produce higher signal levels than thermocouples, making them ideal for high-accuracy industrial temperature monitoring. For example, a 100 Ω RTD energized at 1 mA yields approximately 1 Ω per °C (about 0.385 Ω/°C for Pt100), corresponding to ~5 mV/10°C—substantially more than a thermocouple.

Accurate temperature sensing depends on precise resistance measurement, achieved by either:

• Bridge circuits (voltage imbalance across a reference network)
• Potentiometric methods (measuring voltage across a known current source)

Bridge Measurement Systems

Bridge circuits, such as the Wheatstone bridge, compare the RTD's resistance to known resistors. The voltage imbalance correlates with temperature.

Two main types:

• Balanced Bridges: Used in labs; the bridge is manually or electronically rebalanced.
• Fixed Bridges: Widely used in industrial settings; the imbalance voltage is amplified and scaled to temperature.

RTD Wiring Configurations

Modern instruments are built to handle thermocouple signals more reliably, accurately, and flexibly.

Two-Wire RTD

• Simplest method.
• Lead wire resistance adds error.
• Suitable for short runs (<100 m).

Figure 3.1: Two-Wire RTD Bridge Setup

Three-Wire RTD

• Standard in industrial applications.
• Assumes matched lead wires; cancels resistance error.
• Good for up to 500 m runs.

Figure 3.2: Three-Wire RTD Bridge Setup

Four-Wire RTD

• Highest accuracy.
• Fully compensates for lead resistance.
• Ideal for long cable runs (up to 1 km).

Figures 3.3 and 3.4: Four-Wire RTD Wiring Options

Measuring Differential Temperatures

Measure the difference between two locations using:

• Two RTDs in opposing bridge arms.
• Four-wire setup preferred for precision.

Figures 3.5 and 3.6: Differential RTD Configurations

Potentiometric (Direct Reading) Methods

Measure the difference between two locations using:

• Two RTDs in opposing bridge arms.
• Four-wire setup preferred for precision.

Figures 3.7: Potentiometric Four-Wire Setup

Modern Instrumentation for RTDs

Contemporary RTD systems offer simplified, accurate readings:

• Digital Indicators & Recorders: Often self-balancing and linearized.
• Transmitters (4–20 mA): Improve noise immunity and long-distance signal transmission.
• Amplifiers: Provide signal conditioning for monitoring and control.

RTD Wiring Quick Reference Table

Wiring	Accuracy	Distance	Notes
2-wire	Low	~100 m	Simple; includes lead resistance
3-wire	Medium	~500 m	Balanced; common in industry
4-wire	High	~1km	Full compensation; best for precision

Further Reading

RTD vs Thermocouple – Choosing the Right Sensor
Explore the features and characteristics of thermocouples and RTDs

RTD Output Tables
View Resistance versus Temperature tables for all Pt100 sensors.

What are the RTD colour codes?
Explore RTD colour codes and wiring configurations.