The thermocouples and Resistance Thermometer Detectors (RTD) are the sensors that are used to measure heat in scales like Fahrenheit and Kelvin. These instruments are used in a wide variety of applications and environments, and there are often difficulties in choosing between RTDs and thermocouples. They have their own advantages and disadvantages which make them suitable for certain conditions and circumstances.
These two temperature sensors are widely used across various industries, but each of them has specific features and advantages, as is described below.
Resistance Thermometer Detector (RTD)
An RTD is a sensor where the resistance changes with changes in the temperature. For instance, resistance increases with increasing sensor temperature. The relationship between resistance and temperature is well established and repeatable over time.
An RTD is a passive device, which produces no output itself. External devices are used for measuring the sensor’s resistance by transferring a small electrical stream to the sensor for voltage generation.
Thermocouples
Thermocouples are temperature sensors that use two different metals in the sensor to produce a voltage that can be read to determine the local temperature. Different combinations of metals can be used in building the thermocouples to provide different calibrations with different temperature ranges and sensor characteristics.
Unlike most of the other temperature measurement methods, the thermocouples are self-powered and do not require any external excitation form.
Major differences between thermocouple and RTD
Measuring range
Thermocouples have the main advantage of their range. Some RTD sensors are restricted to temperatures up to 400-500 ° C and sometimes higher, whereas some thermocouples can be used to test above 1400-1800 ° C, making them suitable for a wide variety of applications. Some thermocouple can be used at up to 2,500 ° C.
Accuracy
RTDs are more preferable over thermocouples because their measurements are more precise and repeatable. The RTD’s can provide high accuracy and considered to be the most reliable approach if a measurement value of temperature is to be about ± 0.05 to + /- 0.1 ° C.
In comparison, the precision of thermocouples is down to approximately ± 0.2 to ± 0.5 ° C. However, some of the thermocouple models can correspond to RTD accuracy. Linearity, repeatability and stability are the main variables that can impact sensor accuracy.
Sensitivity
RTD and thermocouples are designed to be long-lasting and react quickly to temperature changes. Thermocouples are faster and usually takes longer to achieve thermal equilibrium. This is mainly because there is a cold junction compensation, which is not as fast as the heat junction at the sensor tip when the temperature changes. Generally, thermocouples are inherently prone at tips, whereas RTD elements are isolated from their sheaths.
Drift or sensor stability
RTD sensor drift is small due to its design; it provides reliable readings for longer periods than can thermocouples. In comparison with RTD sensors, a thermocouple has a relatively high drift over time caused by homogenous conductor wires resulting from heat and chemical exposures or by mechanical damage such as bending, tugging and squeezing during operation. As a result, frequent calibrations and modifications for thermocouples are obligatory.
Cost
Generally, thermocouples installation is cheaper than RTDs due to their cheaper manufacturing process. An RTD with the same temperature and design will often cost two or three times more than a thermocouple. RTD installation saves money as cheaper copper wire is used. However, these savings are not sufficient to offset the higher price of the device.