Difference Between Rtd and Thermocouple

Introduction

A tedious conversion process is avoided by both an RTD and thermocouple sensors which results in getting temperature measurements in an easy manner. The basic difference between these two sensors lies in their manufacturing functions and operating principles.

What is RTD?

RTD stands for Resistance Temperature Detectors. This is an important type of sensor, widely used to measure temperature. The resistance of an RTD changes with changes in temperature (Wu, Luo & Liu, 2019). An increase in temperature causes the resistance of an RTD to increase as well. An RTD is not capable of generating output on its own. A film made of a wire coil or pure metal is available within ab RTD.

Figure 1: RTD

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What is a thermocouple?

In a thermocouple, two non-similar conductors are available. These conductors are used in producing voltage. This voltage is proportional to the differences in temperature. The resistance of the thermocouple decreases with the increase in temperature (Hwang, Cheon & Park, 2020). No such external form of excitation is required to measure temperature with a thermocouple.

Figure 2: Thermocouple

RTD sensors: Advantages and disadvantages

Advantages

• Low drift
• Reasonable linearity, stable, and high accuracy
• Applicable in precision

Disadvantages

• Current sour is needed
• Slow operation
• Signal conditioning is needed

Thermocouple sensors: Advantages and disadvantages

Advantages

• Usable in a hazardous environment
• Response rapidly with changes in temperature
• Thermal capacity is lower

Disadvantages

• Least expensive and least stable
• In converting voltage into a usable reading of temperature, a “substantial signal conditioning” system is needed

Sensor comparison between RTD and thermocouple

Different parameters	Thermocouple	RTD
Tip sensitivity	Excellent	Fair
Accuracy	Medium to good	Excellent
Response time	Medium to excellent	Good
Stability (Long-term)	Fair to poor	Excellent
Linearity	Fair	Good
Self-heating (Undesirable)	Excellent	Medium to excellent
Repeatability	Poor to fair	Excellent
Measuring range	-2700C to 23200C	-2400C to 6500C

Difference between RTD and thermocouple

Different characteristics	RTD	Thermocouples
Accuracy	The accuracy of an RTD temperature measurement is the highest. Around the temperature of ± 0.050C to ± 0.10C, an RTD provides the most accurate measurement of temperature (Kim et al. 2018).	A thermocouple’s accuracy is lower than an RTD. Around ± 0.20C to ± 0.50C temperature, the accuracy of this sensor can be measured properly.
Cost	An RTD is more expensive than a thermocouple.	Thermocouples’ cost is around one-third of an RTD’s cost. A regular adjustment and longer installation process lead this sensor to carry a long-term cost.
Sensitivity	An RTD sensor is more durable and faster in terms of sensitivity. Changes in temperature impact the sensitivity of an RTD.	A thermocouple takes a longer time than an RTD in reaching the thermal equilibrium. The presence of a cold junction point in a thermocouple impacts the sensitivity.
Single point measurement	An average value and entire surface value are considered while the calculation of temperature is done with an RTD (Goswami & Kumar, 2018). The available large elements are responsible to create difficulties in the measurement process.	The Design of a thermocouple provides the scope of narrowing down the measuring point to the specific and exact point. This point is named “naked tips”.
Drift	As designed, the sensor drift of an RTD is small. This helps in producing stable and accurate reading for a longer duration.	A high drift over time is found to be associated with thermocouples. The inhomogeneity of a conductor wire is the main reason behind this high drift. The inhomogeneity can be caused by mechanical damage and chemical exposure as well (Johra, 2020). Frequent adjustment and calibrations are mandatory and required within thermocouples.

Applications of thermocouple

Usage of thermocouples is easy in nature and that is why this is widely used while measuring temperature. Some of its most important and popular applications are −

• In hospital thermometers
• The diagnostic testing process for a diesel engine
• In the mechanism of temperature sensors
• Various industrial applications such as kilns and gas turbine

Application of RTD

An RTD is an important passive device, used in measuring a loss range of temperature. Its most important applications are as follows −

• In microelectronics
• Textile production process
• Plastic processing
• Grills and stoves
• Refrigerator
• Air conditioner
• Petrochemical processor