I have 5 of the Hagen floating glass therms which I use to check things. 2 read 1 degree higher than the other 3 so just split the difference. I also have a couple Big Alert Digital units and all match each time I check. I use them to calibrate my controller probes.
On a side note, I run my tank @ 78.5ish. I'd be hesitant to run a controller at your 84F. Hard on some fish and many corals. Plus depleted O2.
This will also give you more algae and cyano issues.
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