Technically speaking, a data logger is any device that can be used to store data. This includes many data acquisition devices such as plug-in boards or serial communication systems which use a computer as a real time data recording system. Most instrument manufacturers, however, consider a datalogger a stand alone device that can read various types of electrical signals and store the data in internal memory for later download to a computer.
The advantage of data loggers is that they can operate independently of a computer, unlike many other types of data acquisition devices. Data loggers are available in various shapes and sizes...
Thermocouples are the most widely used temperature sensors on the planet due to their combination of wide temperature capabilities, ruggedness, and low cost. Thermocouples are used in applications that range from home appliances to industrial processes, to electric power generation, to furnace monitoring and control, to food and beverage processing, to automotive sensors, to aircraft engines, to rockets, satellites and spacecraft.
When it comes to high temperatures, small size, fast response, high vibration or shock, most times you will find thermocouples providing the temperature measurements.