Learn what the difference is between RTD's and Thermocouples and what you need to know to get the most out of each.
The two most common ways to measure temperature for process control in automation are with Thermocouples and Resistance Temperature Detectors or RTDs for short. An RTDs is just a device that resistance changes over temperature in a predictable, linear way. They are great for applications up to around 600 degrees Fahrenheit. Because RTDs are typically made from a very fine wire wrapped around a ceramic core - or created using thin film technology - they provide accurate, repeatable results over the long term, but they are also fragile, have a long response time and are a bigger than thermocouples. For wider temperature ranges up to a couple thousand degrees You'll need to use a thermocouple. They create a small voltage across a junction of two dissimilar metals when exposed to a temperature gradient. This produces a less accurate reading and can drift over time. But it also means they don't have any ceramics so they are not fragile, they have a much faster response time and they are smaller. When we say fragile and refer to the size we're only talking about the element they are both packaged in the same sized probes and once it is packaged in that probe they are both very robust. Thermocouples are identified by letter. There's a whole bunch of these, but the most common ones are J, K and T which designate a specific temperature range. Thermocouple wire is color coded like this so you can simply pick one up, look at the wires and immediately tell which one it is. Don't use regular copper wire to extend thermocouples it will mess up the readings. Copper wire IS fine for RTDs, but you will want to make sure it is shielded and the shield is grounded at one end to reduce noise. RTD's come in a variety of types, with the most common being a Pt100. Its made from platinum that has been calibrated to be 100 ohms at 0 degrees C. AutomationDirect's PLCs also handle a wide variety of RTDs including Pt1000, jPt100, Cu10, Cu25, and Ni120 to name a few. Once you have selected your temperature sensor, just connect it to your PLC's thermocouple or RTD input module. AutomationDirect's temperature measurement modules convert that temperature directly into memory. Which means you don't have to write any code at all to get the temperature. Check out the other videos in this series for more helpful tips and live step by step demos of exactly how to use Temperature sensors with AutomationDirect PLCs. If you need help, please don't hesitate to contact AutomationDirect's free, award winning, tech support during regular business hours they will be happy to help you. And don't forget the forums! There are lots of folks there that love to share their years of experience just don't post any support questions there, the forums are not regularly monitored by tech support.