I picked up a relatively cheap bluetooth temperature/humidity monitor - "Govee Hygrometer Thermometer" - to keep an eye on our 12V refrigerator for when we're boondocking. It came in handy to determine what power setting our refrigerator could be on to maintain <40F, but not waste power by trying to run a lot cooler than necessary. Sure, a $1 mercury thermometer could be used, but I like gadgets! And the bluetooth temp/humidity monitor keeps a log of the data, so you can look back over the day, week, month, or even year by pulling up the app on your phone. Its not waterproof, so you would have to be careful using it in a cooler with ice - maybe put it in a plastic bag or vacuum seal it - but them the humidity wouldn't read right, which really doesn't matter for this purpose. Its currently $13.99 on Amazon.
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