Introduction and Overview Temperature is a measure of a material's internal molecular activity. As the level of molecular activity rises@ the temperature of the material increases. Hot and cold are subjective@ qualitative descriptions of a change in molecular activity Temperature is often the most important of the common measurements because it is an indicator of process stream composition and product quality. It would be nice if we had online analyzers throughout the process but the fact of the matter is that most plants have infrequent@ offline lab analysis at best. In the chemical industry@ nearly all loops that are controlling the composition of a unit operation use temperature as the primary controlled variable. Even when online or at-line analyzers exist@ these are usually relegated to monitoring and the manual trim or optimization of temperature set points by supervisory or model predictive control. Temperature measurements are also essential for equipment protection and performance monitoring. Some examples of common unit operations that have a critical dependence upon tight (minimum variability) temperature control are: Bioreactors and fermentors Calciners and kilns Chemical reactors Columns (e.g.@ absorber@ distillation@ stripper) Condensers Crystallizers Dryers Evaporators Extruders Furnaces Superheaters and desuperheaters Vaporizers Over the years@ the need in the process industry for more consistent and accurate ways to describe temperature led to the invention of temperature- measuring devices@ or sensors. Sensors use standard@ universally recognized temperature scales. Because these scales rely on fixed points in nature (e.g.@ freezing point of water)@ they provide a way to describe temperature that is both objective and quantitative. The four temperature scales in use today are Fahrenheit@ Celsius (also called Centigrade)@ Kelvin@ and Rankine. In commercial applications@ Fahrenheit and Celsius are the most commonly used scales.