Data loggers are devices which record data to a non-volatile memory for analysis later on computer, we use temperature data loggers, these record temperature once every ten seconds and can hold over 16,000 readings (over 3 days worth of continuous recordings) these are particulary good for getting average temperatures which are important for understanding the locations normal conditions.
You can also see any sudden temperature changes on a computer after the vigil and these changes can be compared to manual readings and via the vigil notes for confirmation, to have two different meters getting similar reading is better than just one!
These are recommended for anyone who wants to do more scientific paranormal research