The basic theory of the thermocouple effect is found from a consideration of the electrical and thermal transport properties of different metals. A measurement junction (this junction is formed of different metal) is exposed to the environment whose temperature is to be measured. The other "reference" junction (known as cold junction) are held at common. The "seeback voltage" then is proportional to the difference between junction temperatures. Certain standard configurations of thermocouples using specific metals (or alloys of metals) have been adoped, each type has its particular features. Such as range, linearity inertness to hostile environment sensitivity, and so on, and is chosen for specific applications accordingly.
A resistance-temperature detector (RTD) is a temperature transducer that is based on the principles, that is, metal resistance increasing with temperature. An estimation of RTD sensitivity can be noted from typical values of the linear fractional change in resistance with temperature. For platinum, this number is typically on the order of 0.0385/¢XC thus, a change of only 0.385 wiuld be expected for PT100 if the temperature is changed by 1¢XC.