Thermocouple

A device used in measuring temperature by a combination of dissimilar metals conjoined at a single end.  Once the portion of this conjoined metals is exposed to different temperature environment, a thermal EMF is generated.  This EMF signal is converted to temperature, in ◦C or ◦F, by a thermometer in accordance to the type of prescribed metal combination.

The thermocouple design comes with a vast number of categories and is usually related to temperature and application.  The design offers a temperature range from -200 ◦C to 1800 ◦C.  And it may vary from basic design (a pair of extension grade thermocouple cable) to a highly complexed protection tube design for special applications.
Various types of thermocouples available are types K, J, N, T, E, B, R and S.

Resistance Temperature Detector (RTD)

A device used in measuring temperature that changes its resistance value in accordance to changes in temperature.  RTDs  are known for its stability, accuracy and repeatability.   
The RTD element is made from pure metals, typically platinum, nickel or copper, which  has an accurate  temperature – resistance relationship that is used as a temperature indication. 

RTDs are most sought after due to its accuracy in sensing temperature changes.  It is relatively immune from electrical noise (magnetic field), and therefore  is suitable for harsh applications, such as, electrical motors, generators, and other high-voltage equipments.

Thermometer & Probe