Thermocouples (TCs) and resistance temperature detectors (RTDs) are the most widely used temperature sensors in automation and process control. They are found embedded in motors, valves, turbines, bearings, and a host of other devices. Most smart instruments such as flowmeters, pressure transmitters, and level transmitters also have an embedded temperature sensor-used to correct the primary measured variable or for process control.
When used alone, temperature sensors are often installed in thermowells, which are inserted into tanks, vessels, and pipes. A thermowell protects the sensor from the environment, but it slows the response time and degrades the accuracy. Installing sensors in a thermowell is a different subject, and will not be covered here. Instead, this article discusses direct immersion sensors; i.e., TCs and RTDs that are directly inserted and exposed to the process without the protection of a thermowell.
Theories of Operation
A thermocouple consists of two wires of dissimilar metals, joined at both ends-at the reference point outside the process (cold junction) and at a junction at the point of measurement (hot junction). The metals react differently to temperature changes and generate an electromotive force (EMF) voltage based on the temperature differential between the junctions (the Seebeck effect). A resistance temperature detector is based on the principle that electrical resistance in a wire increases with temperature.
In both cases, the sensor is wired to a transducer or signal conditioner that has been calibrated to accept the input voltage or resistance, calculate the correct temperature, and output it as a 4-20 mA, mV, or digital signal to an automation system.
The above is pretty basic, taught in every Instrumentation 101 class. But it poses the first question to consider when choosing a sensor: How do you wire the sensor to the transducer, signal conditioner, or automation system? Such devices and systems, being electronic in nature, need to be mounted in a reasonably safe location, away from high temperatures.
TCs must be wired with thermocouple extension wire, which is the same as the wire used in the TC. For example, a Type K TC uses a wire of nickel-chromium connected at the sensing junction to a wire of nickel-alumel. Extension wire must be the same composition, that is, one nickel-chromium and one nickel-alumel wire. In general, longer runs of extension wire are discouraged, as the wires act as an antenna, making the measurement more susceptible to electromagnetic and radio frequency interference. Cost may also be an issue when dealing with long extension wires, especially ones with exotic materials (e.g., Type R TCs). In certain cases, a compensation cable made up of a less expensive material with similar EMF properties to the TC can be used.
RTDs, on the other hand, can be wired with standard cable for much longer distances; however, they are typically limited by issues related to self-heating errors. In both cases, the extension wiring must be shielded against electrical noise in the plant. Note that TC extension wire is more susceptible to noise than RTD cable.