Tech Talk: (5) Temperature Measurement Basics

Accurate and repeatable temperature measurement requires that the Instrument Design Engineer understands the various methods available, their performance and limitations. This paper addresses the basics of temperature measurement as commonly applied in the process industries. RTD elements are normally constructed of platinum (Pt), copper or nickel, with Pt being the preferred material in the process industries. This material has a predictable and reproducible change in resistance with temperature. Typically, the change in resistance with temperature is made use of by having the RTD in one arm of a direct current (DC) Wheatstone bridge to obtain a DC voltage output proportional to the change in temperature. The added junctions must, in this event, be at the same temperature as all 'cold' junctions in the circuit, allowing a third metal such as copper leads to be added when connecting measuring instruments.