RTD & Thermocouples

It is said that temperature is the most widely measured of the four process variables (pressure, temperature, level and flow). Electrical temperature instruments have been in use for much longer that electrical measuring solutions. Below are the two primary technologies, RTDs and Thermocouples. The following technical summaries are courtesy of Pyromation.

THERMOCOUPLES – Thermocouples are the most common, convenient, and versatile devices used to measure temperature. They convert units of heat into useable engineering units that serve as input signals for process controllers and recorders. A thermocouple consists of a welded ‘hot’ junction between two dissimilar metals – usually wires – and a reference junction at opposite ends of the parent materials. Heating the ‘hot’ junction in the working environment produces a temperature gradient which generates an Electromotive Force (EMF). The

EMF appears across the free ends of the thermocouple wires where it is measured and converted into units of heat calibration. Through selection of appropriate thermocouple wires and sheath components, thermocouples are suitable to be used in temperature ranges from ( 200 to 2316) °C [-328 to 4200] °F.

RESISTANCE TEMPERATURE DETECTORS – Resistance temperature detectors (RTD) accurately sense temperature with an excellent degree of repeatability and interchangeability of elements. The RTD is composed of certain metallic elements whose change in resistance is a function of temperature. In operation, a small excitation current is passed across the element, and the voltage, which is proportional to resistance, is then measured and converted to units of temperature calibration. Since most RTDs have a low initial resistance, often 100 ohms, and have a small change in resistance per unit of temperature range, the resistance of the lead wire is often compensated for with a three or four wire bridge configuration built into the measuring devices. By selecting the proper elements and protective sheathing, RTDs can operate in a temperature range of (-200 to 600) °C [-328 to 1112] °F.

There are almost limitless ways to package RTD and Thermocouple products. This includes industrial assemblies with head connections (both general purpose and hazardous location), RTD or thermocouples with plugs and jack connectors, numerous leadwire and termination options, transmitter options, the list goes on. There are configurations available for almost every market segment. Plastics Industries, Food and Beverage, Pharmaceutical, etc.  Colonial Instruments can help you select the proper technology and all of the various options necessary to get you a product that will measure accurately and last.