Check - http://www.temperatures.com/ - they have a lot of temp sensor info
If I had a biolab I would monitor the freezers quite closely with multiple sensors in different places. (at least 2)
- to see trends in the temperature which can predict upcoming trouble, and
- the sensor can become encapsulated in a block of ice while other parts are not cold enough.
Add 1)
If I see a rise during the day climbing up and up it can indicate an alarm during the night (I dont like alarms especially not in the night 
Add 2)
happened to my previous food freezer...