Difference Between RTD and Thermocouple

The two crucial devices that are used to measure temperature are RTD and thermocouple. Both of these are used as temperature sensors. The major difference between RTD and thermocouple lies in their principle of operation. An RTD uses a single metal whose variation in resistance predicts the variation in temperature. As against a thermocouple is a device that uses two metallic wires which generate a difference in voltage at the junction that corresponds to the change in temperature.

As we have already mentioned in the beginning that RTD and thermocouple are crucial temperature sensors. However, other than these two we also have a thermostat and thermistor that are used as temperature sensors.

Basically, temperature sensors are the devices that are used to measure the amount of heat energy generated in a system. And this is measured by sensing any physical change associated with that device or system.

Like in RTD the change in resistance of metal shows the change in temperature. While in thermocouple the variation in EMF signifies the temperature change, associated with the device. Here we will discuss the various other factors that differentiate RTD from thermocouple other than the principle of operation.

Content: RTD Vs Thermocouple

1. Comparison Chart
2. Definition
3. Key Differences
4. Conclusion

Comparison Chart

Basis for Comparison	RTD	Thermocouple
Operating Principle	Temperature changes with change in resistance.	Temperature changes with change in emf generated.
Response time	1 – 50s	0.1 – 10s
Cost	High	Low
Operating range	-200 to 600°C	-200 to 2000°C
Physical size	Large	Comparatively small
Sensitivity	Low	Quite high
Accuracy	More	Less
Self-heating	Exist	Not Exist
Stability	More	Comparatively less
Output	Linear	Non-linear
Applications	Used to measure engine temperature along with temperature of amplifier and oil temperature sensor, etc.	Measures temperature of chemical and petroleum plants and detects temperature of metals and aluminum for industrial purposes.

Definition of RTD

RTD is an acronym used for resistance temperature detector. It is a temperature sensor that allows determining the temperature by measuring the resistance of the electrical wire. This electrical wire acts as a temperature sensor. Basically, in this, there is a metallic wire present and with the increase in the amount of heat experienced by the metallic substance, there is a change (rise) in resistance due to variation in the hotness of the wire. However, the resistance of the wire falls when the amount of heat supplied to the wire decreases.

In this way, the change in resistance of the wire either increase or decrease signifies the temperature change and in this way variation in temperature through variation in resistance is predicted.

Generally, the metals whose resistance is known are used in the construction of RTD so that the change in value can be easily interpreted and recorded. The popularly used metals for constructing RTDs are copper, nickel, platinum, etc.

It is to be noted here that usually platinum is used as it shows stable resistance temperature characteristics for a wide operating range. While in the case of nickel, it becomes non-linear above 300° C.

Definition of Thermocouple

It is another electrical device that is used to sense the temperature but this time by using the voltage. The thermocouple is designed to generate the voltage due to the thermoelectric effect where the temperature is voltage-dependent. It is composed of two different electrical conductors that form an electric junction.

Basically, it operates in a way that two different metal wires when heated then the difference in the temperature generates emf in the circuit, and voltage produced is measured at the junction.

The change in temperature of the two wires changes the voltage at the junction.

It is noteworthy here that the different combinations of metals provide different temperature ranges and sensor characteristics. Some widely used metal pairs used for the construction of thermocouples are copper-iron, copper-constantan, antimony-bismuth. The various types of thermocouples are named E, J, K, B, etc.

It is also known as a thermoelectric thermometer.

Key Differences Between RTD and Thermocouple

1. An RTD utilizes the change in resistance of the metal to predict the change in temperature. While thermocouple is a thermoelectric sensor that uses the change in voltage/ emf to get the change in the temperature.
2. RTD generally operates in the range between -200 to 600° C. In contrast, a thermocouple offers even wider operating range than RTD i.e., usually -200 to 2000° C. Thus, thermocouple suits a variety of applications.
3. Thermocouples offer a response time of 0.1 to 10s which is better than the response time of RTDs ranging between 1 to 50s.
4. On the basis of sensitivity, thermocouples are said to be more sensitive than RTD. This is so because these react faster than RTD with the variation in temperature.
5. Thermocouples are generally cost-effective than RTD. Although the initial cost of RTD is a little less than thermocouple, as far as maintenance is concerned, the RTDs cost thrice the thermocouple. Thus, generally, RTDs are expensive than thermocouple.
6. The physical size offered by the thermocouple is smaller than RTD thus, offers ease of use.
7. For applications that require greater accuracy, RTD is preferred as they generate more accurate result than thermocouples.
8. The stability offered by RTD is quite higher than the thermocouple. This is due to the reason that RTD is designed to provide repeatability in the results for a longer time for the same input. Whereas the chemical change occurring in thermocouple causes drift in its reading.
9. The phenomenon of self-heating exists in RTD but is negligible in the thermocouple.
10. For RTD the graph between resistance and temperature is linear, leading to generate accurate temperate measurement. While in the case of thermocouple there is non-linearity in the generated output.

Conclusion

Thus, the above discussion concludes that both RTD and thermocouple have their own advantages and disadvantages and hence finds applications in the respective fields according to the needs.