In 2008 I bought a second refrigerator as a backup for my main refrigerator and to replace a chest freezer that consumed far too much electricity. The second refrigerator is in the basement and does not get regular use - it failed last year causing some frozen food to spoil before the problem was detected. Over the years I have accumulated a number of weather stations with outdoor sensors that can handle freezer temperatures. I had no difficulty connecting to the remote sensor inside the freezer one floor down and 12 meters/40 feet away. I can also display the min/max readings through the weather station.
Alerting is the next step. I run a small OpenHAB (https://openhab.org) home monitoring system that supports legacy Insteon equipment and newer Z-Wave devices. Z-Wave thermometers are expensive and have limited range compared to 433MHz radios. OpenHAB supports some 433MHz devices via RFXCOM or Jeelink transceivers that are not only expensive but hard to find in North America. Software Defined Radio (SDR) USB interfaces are cheaper and readily available but require more effort to integrate with OpenHAB. So much to do, so little time ...
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