I use two kinds of monitors that I already had but stopped using them for my wine cabinet(after realizing it was ridiculously stable)
My favorite is like everyone else an Oregon Scientific...but I use the Starck model w/ matching sensor... I picked it up a couple years ago for the design, atomic clock and the full weather station that it has...I just wish I picked up more sensors back then since they are hard to find..I think they are being phased out (the Starck medium music clock w/ remote sensor is on sale directly from oregon scientific for a ridiculous price right now

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The base uses either 4AA or the power adapter, the sensor uses 2AAA and is extremely accurate, can handle 3