A Resistance temperature detector (RTD) or a thermocouple? Both can be used to measure temperatures and are suitable in a broad range of applications. This often leaves our customers with the dilemma of choosing what temperature sensor best suits their needs. Before we make that decision it is vital to understand the difference between the two sensors.
What is an RTD Sensor?
Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. This element is usually quite fragile, so it is often placed inside a sheathed probe as protection. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature.
What is a Thermocouple?
Thermocouples consist of two dissimilar metal wires, the two wires are welded at one end, creating a junction. This is where the temperature is measured. This is then connected to a thermocouple thermometer or other thermocouple-capable device at the other end. When the Junction experiences a change in temperature it creates a voltage, which can be referenced to calculate the temperature.
Thermocouples are versatile temperature sensors and are commonly used on in a range of applications – from a regular thermocouple found on utilities and regular appliances to an industrial usage thermocouple. Because Thermocouples have such wide range of models and technical specifications, it is important to understand its basic structure, how it works, and its ranges as to better determine what is the right type and material of thermocouple for your application.
What is best for me?
There are four factor that should be taken into consideration when deciding what temperature sensor is best for your application.
- Temperature: If process temperatures are between −200 to 500 °C, an industrial RTD is the preferred option. Thermocouples have a range of −180 to 2,320 °C. So for temperatures above 500 °C Thermocouples are your only choice of contact temperature measurement devices.
- Response time: If the process requires a very fast response to temperature changes—fractions of a second as opposed to seconds —then a thermocouple is the best choice.
- Size: A standard RTD sheath is 3.175 to 6.35 mm in diameter; sheath diameters for thermocouples can be less than 1.6 mm.
- Accuracy and stability requirements: If a tolerance of 2 °C is acceptable and the highest level of repeatability is not required, a thermocouple will serve. RTDs are capable of higher accuracy and can maintain stability for many years, while thermocouples can drift in shorter periods.
Both RTD’s and Thermocouples have their own advantages and disadvantages and are better suited to certain applications. It is essential to take temperature, response time, size, accuracy and stability requirements into consideration when choosing between the two. Thermocouples are generally cheaper and more durable, while RTD’s produce better and more reliable measurements. Due to their accuracy RTD’s are slowly replacing the use of thermocouples in many industrial applications below 500°C.
D&N have been manufacturing high quality temperature sensors since 1980 and are happy to lend our expertise to assist you in choosing the right sensor for your specific needs.