When you have a temperature measurement application, one of the main problems is selecting the appropriate temperature sensor. Temperature sensors and thermocouples are used in various applications – their object and versatility are entirely different.
RTD sensors
The RTD works on the principle that the electrical strength of metals increases with heat, including the hotter metals, and reduces when the metals get colder. RTD, therefore, are temperature sensors that are used to measure the change of local temperature by changing the electrical resistance of metals. A finely coiled platinum wire wrapped around the ceramic core or a thin platinum plate into an element substratum is present in every RTD sensor element.
The element is relatively fragile and is therefore generally placed inside a sheet to protect it. RTD sensor elements consist of materials with resistance to different temperatures. In other words, if the temperature fluctuates, the material has a predictable resistance change. It is the change used to determine the current temperature. RTDs are generally accepted as one of the most precise available temperature sensors. They provide excellent short-term, long-term stability and repeatability, as well as very high accuracy.
Thermocouple Sensors
A thermocouple is an electric sensor commonly used in industrial applications, such as control of machinery or instrumentation. It contains two copper wires isolated by a thin layer of metal. One end of a thermocouple acts as an active sample, the other as an inactive collector. A current is generated if a thermocouple detects a change in temperature across a junction.
Due to the large variety of thermocouples available on the market, users must know the right type for their application to measure temperatures. Most professional engineers prefer to use printed circuit board (PCB) thermocouples to improve accuracy and reliability. There are also numerous thermocouples available on the market, ideal for microwave and laser temperatures measurement.
RTD vs Thermocouple
The main reason for the RTD vs. Thermocouple debate is that they are far better than the others. Thermocouples are designed to monitor the temperature continuously, while RTDs are only for temporary monitoring and recording. Because RTDs and thermocouples have different designs, it is impossible to determine whether a resistance temperature detector or thermocouple sensor is the best choice for a given application because they have specific properties.
Measuring Range
The main advantage of thermocouples is their range of temperature. Although individual thermocouples can be used to quantify in the 1,400 – 1800 °C range, most RTD sensors are restricted to temperatures up to 400 – 500 °C, which makes them suitable for an enormous range of applications.
Cost
Thermocouples are usually cheaper than RTD sensors since most thermocouples cost from half a third to one-third of an RTD sensor. Thermocouples require regular adjustments and calibration, which adds to the product’s long-term cost and the more extended installation and installation times.
Sensitivity
Thermocouple sensors usually have a faster response time because of their point of contact temperature changes. It takes longer to achieve thermal balance. This is mainly due to the cold junction compensation that does not react to a temperature change once the sensor tip has the heat junction. In contrast, the RTD sensor is more durable and responds quickly to changes in temperature.
Stability
The stability of RTD is much higher than the thermocouple sensor. The reason is that the RTD sensor is designed to ensure repeatability in the results at the same input for a long time. The chemical change in thermocouples causes its readings to drift. The RTD sensor has excellent stability and low shift quantities.
Response time
The response time of the thermocouple is better than the RTD sensor. In 0.1 to 10s, thermocouples respond, while RTDs fluctuate between 1 and 50s. The grounding of RTDs cannot, therefore, be slower than the thermocouple. Due to their design, thermocouples can be placed within a tight sheath of diameter than the RTD sensor.
Final Thoughts
Temperature measurement and control begin with a proper sensor selection. By integrating the temperature signal conditioners upstream of the primary regulator, the sensor signal was made practical, and exceptionally precise Thermocouples and RTDs are valuable instruments for industrial process temperature measurement. RTD’s give their temperature range higher than thermocouples because platinum is a more stable material than most thermocouples. RTD also uses standard device wire to interface with the estimation or control gear.
Thermocouples are cheaper than RTD sensors. They are more solid and can be used at higher temperatures in high vibration or mechanical shock applications. Thermocouples can be made smaller than most RTD’s, so they can fit into a particular application.
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