Every process depends on temperature. Whether for control or upset determination, temperature is a critical entity. The traditional method for measuring temperature is a thermometer. This gives a direct visual reference to the temperature, but can never be very precise and there is no way to get the information to a control system. So, rather than rely on the mark 1 eyeball, there are 4 sensor types that can give a digital temperature signal to a control system. These are thermistors, thermocouples, RTDs, and fiber optic temperature sensors.
* Thermistors are nonlinear and vary resistance with temperature. They are less accurate than RTDs. You do not find these in the industrial setting very often, but everywhere in consumer electronics.
* Thermocouples consist of a weld joint between two dissimilar metals. A very small voltage is generated between the two welded wires that vary with temperature. They can tolerate extremely high temperatures and can be used in high vibration environments. They are not as accurate as RTDs, but are very useful in the previously mentioned applications. Care must be taken with Thermocouples to keep extension wires made from the same metals as the thermocouple. If you extend sensor wire with standard copper wire, you will have created two more thermocouples at each extension joint thereby skewing your small voltage measurement.
* RTDs (Resistance Temperature Detectors) are similar to thermistors in that they vary their resistance with temperature, but they are very linear and accurate. These are the go-to sensors for most processes if the temperature is less than ~600°C and vibration isn’t exorbitant. They come with 2, 3, or 4 lead wires. The extra lead wires allow for compensation of resistance from the wiring and junctions.
* Fiber-optic Temperature Sensors are the last type and really are only used in specialized applications. They have the potential to measure temperature at multiple points in a reactor or other vessel and be small enough to fit in a small pipe well or conduit. Traditional thermocouple or RTD multi-points are cumbersome and thick with limited number of points possible due to bundle size.
With so many choices out there for measuring temperature, it can help to consult with myself or the fine folks at Jasper Engineering to choose the right fit for your process. 952-938-6504 | firstname.lastname@example.org