The majority of the time these elements are made up of coiled wire that is spun around glass or ceramic. This material has a predictable change in resistance which is directly related to the change in temperature, because of this relationship it becomes simple to figure out the temperature based on the resistance.
RTD sensors can come in a variety of designs and forms all of which offer certain advantages such as greater stability, accuracy, and repeatability according to the application in which they are placed. Many of these different RTD sensor designs offer far more advantages than a thermocouple would offer in the same application. For a RTD sensor to function properly though, the wire that is wrapped around the ceramic or glass core must be kept completely contaminant-free. This wire can be made up of a number of different materials all of which have their own set of benefits, so choosing a wire material should be based on your requirements and application setting.
Platinum wires, for example, exhibit a very linear relationship between resistance and temperature, are resistant to vibrations, and have minimal differential expansion. Because of these benefits platinum wire may be the best choice for your application, but it is also costly. So if your application does not need all of the benefits platinum offers, a simple copper or iron wire may be better suited for your application.
Knowing your application and what you need from your RTD sensor is perhaps the most important step in the process. When looking at RTD sensors it is also important to note that there are two main groups in which all RTD sensors are categorized, immersion sensors and surface sensors. Immersion sensors are installed in your application in a way that ensures proper contact with the process medium, keeping external influences to a minimum.
Immersion sensors are often used to measure fluid or gas temperatures. Surface sensors, on the other hand, are generally utilized when it is not possible to totally immerse the sensor in the material or object of interest. These sensors require additional insulation to ensure accuracy, but they get the job done when an immersion sensor cannot. So whatever your needs may be, it is important to find a trusted RTD sensor manufacturer to get the job done right.