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Thermocouples vs. RTDs

Selecting the right temperature sensors for each application requires a close examination of the requirements.

Imagine you are an instrumentation engineer looking at the prints for a new plant, and you see the many locations marked out for temperature transducers. You wonder, what exactly has been specified to fill each space? Thermocouples, because of the lower cost and familiarity, or RTDs, for the high accuracy and linearity?

Temperature transducer selection affects many aspects of the design and installation of the equipment in the plant such as:

•The type of wire that needs to be run.

  • •The type of instrument that will be in the control room on the other end of that wire.

  • •Whether there will be local junction boxes with terminal strips or transmitters. And, if so, what type of transmitter is required.

  • •Whether any special piping considerations need to be made to install or protect the sensor or to provide the required response time.

These are just some of the details, and the actual sensor selection and design has not even been discussed yet. Looking at the big picture can help narrow down the choices in a logical way. Look first at survival of the sensor and then the finer points such as any performance requirements.

Resistance Elements & RTDs

Resistance elements come in many types conforming to different standards, capable of different temperature ranges, with various sizes and accuracies available. But they all function in the same manner: each has a pre-specified resistance value at a known temperature which changes in a predictable fashion. In this way, by measuring the resistance of the element, the temperature of the element can be determined from tables, calculations or instrumentation. These resistance elements are the heart of the RTD (Resistance Temperature Detector). Generally, a bare resistance element is too fragile and sensitive to be used in its raw form, so it must be protected by incorporating it into an RTD.