The key difference between RTD and thermocouple is that RTDs function by detecting alterations in the electrical resistance of a metal wire, typically platinum, as temperature fluctuates. With rising temperatures, the resistance of the RTD likewise increases while, Thermocouples produce a voltage gradient resulting from temperature disparities between two dissimilar metals at their junction. This voltage is directly correlated with the temperature.
What is RTD?
RTD stands for Resistance Temperature Detector, a type of temperature sensor that determines temperature by measuring the resistance of an electrical wire. This wire functions as the temperature sensor, where its resistance changes with variations in temperature. Essentially, as the metallic substance heats up, its resistance increases, and conversely, it decreases when the heat supplied diminishes.
The fluctuation in resistance of the wire, whether it increases or decreases, indicates a change in temperature, allowing for the prediction of temperature variations based on resistance changes.
Typically, metals with well-known resistance characteristics are utilized in constructing RTDs to facilitate easy interpretation and recording of changes. Commonly used metals for RTD construction include copper, nickel, and platinum.
It’s worth noting that platinum is often preferred due to its stable resistance temperature characteristics across a wide operating range. In contrast, nickel tends to exhibit non-linear behavior above 300°C.
What is Thermocouple?
The thermocouple is another electrical device employed for temperature sensing, utilizing voltage as its measuring parameter. It operates on the principle of the thermoelectric effect, where temperature influences voltage generation. Comprising two distinct electrical conductors forming an electric junction, it functions by generating voltage when subjected to varying temperatures.
Essentially, when two dissimilar metal wires are heated, the temperature difference between them induces electromotive force (EMF) in the circuit, with the voltage produced being measured at the junction.
Notably, different combinations of metals yield distinct temperature ranges and sensor characteristics. Common metal pairs utilized in thermocouple construction include copper-iron, copper-constantan, and antimony-bismuth. Various types of thermocouples, such as E, J, K, B, etc., are differentiated based on their metal combinations.
Additionally, it’s referred to as a thermoelectric thermometer.
RTD vs Thermocouple
The primary difference between RTD and thermocouple is given below:
| Feature | RTD | Thermocouple |
|---|---|---|
| Operating Principle | Temperature changes with change in resistance. | Temperature changes with change in EMF generated. |
| Measurement Method | Measures resistance of metal wire. | Measures voltage generated at the junction of two dissimilar conductors. |
| Response Time | Typically 1 – 50 seconds. | Typically 0.1 – 10 seconds. |
| Cost | Generally higher. | Generally lower. |
| Operating Range | Typically -200 to 600°C. | Typically -200 to 2000°C. |
| Physical Size | Larger. | Comparatively smaller. |
| Sensitivity | Lower. | Higher. |
| Accuracy | Typically higher. | Typically lower. |
| Self-Heating | Exists. | Does not exist. |
| Stability | Typically more stable. | Typically less stable. |
| Output | Linear. | Non-linear. |
| Common Applications | Engine temperature, amplifier temperature, oil temperature sensor, etc. | Chemical and petroleum plants, industrial temperature sensing, etc. |