How to choose between RTDs, thermistors and thermocouples

Introductions

RTDs use a high-precision resistor, typically made of platinum, whose resistance increases with temperature. They offer high accuracy and stability thus are commonly used in industrial applications and typically come in 2, 3, or 4-wire configurations. Thermocouples consist of a junction between two different metal alloys, generating a voltage that varies with temperature. They are versatile, cost-effective, and can measure a wide range of temperatures. Common types include J, K, T, and N, each defined by the metals used and their specific temperature range and sensitivity. NTC thermistors are resistors that decrease in resistance as temperature increases. They are highly sensitive and respond quickly to temperature changes, making them suitable for applications requiring rapid detection of temperature variations.

Thermocouple, Platinum Resistance & Thermistor Comparison

How to choose between RTD Sensors and Thermocouples

RTD Sensors: Resistance Thermometers (RTDs) use a high-precision resistor, typically platinum, which increases in resistance as temperature rises. The widely adopted Pt100 standard has a resistance of 100.0 Ohms at 0°C and changes by 38.50 Ohms from 0 to 100°C. Platinum resistors provide highly stable and accurate temperature measurements. RTDs are usually 2-wire devices but can be extended to 3 or 4 wires for enhanced accuracy.

Thermocouples: Thermocouples consist of a junction between two different alloys and a two-wire extension. The EMF generated between the hot junction and a reference junction creates a stable and repeatable temperature measurement. Various thermocouple types (e.g., J, K, T, N) are defined by the alloys used, each displaying distinct EMF characteristics.

RTD’s are, generally: 

• More expensive 

• More accurate 

• Highly stable (if used carefully) 

• Capable of better resolution 

• Restricted in their range of temperature 

• Stem, not tip sensitive 

• Rarely available in small diameters (below 3mm)

Thermocouples are, generally: 

• Relatively inexpensive 

• More rugged 

• Less accurate 

• More prone to drift 

• More sensitive 

• Tip sensing 

• Available in smaller diameters 

• Available with a wider temperature range 

• More versatile

Conclusion

Each sensor type has its strengths and is chosen based on the specific requirements of the application, such as accuracy, temperature range, response time, and cost.

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