If you download your readings and notice a big difference in measurements, it's highly likely that the battery is getting to the end of it's life and has caused a corruption within the Logger.
The impact of often downloading the data is quite significant on battery life and can be reduced by turning off (hibernating) the Logger between uses.
You can check the average battery life of your Logger on our products page, by selecting your model number. On average this is between 2-3 years.
If your Logger is relatively new and you still have the same problem, please send us a diagnostics file.
-40 DEGREES CELSIUS READINGS
If your Logger only records temperatures of -40 degrees Celsius this could be a connection problem with the probe.