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A thermometer will have no influence on the ability to go overnight without making adjustments. And I am not totaly convinced that a Guru will either, at least not on a Big Green Egg. I have done several dozen BGE overnighters and all but one time the needle was pegged at 250ish without moving. The other time the fire had gone out but dome temp was still at 200 when I checked in the morning. There are many stories of fires going out WITH a Guru in use also. Set it and forget it, sans Guru or grid probe....IMO.
FWIW.....when I did care about grid temp, I once experienced a 100deg differential betwwen grid/dome. All devices calibrated and double checked with Thermapen.
The best advice I ever got on this forum came from a long time member who said "To Hell with Grid Temp". I chunked the Maverick almost 3 years ago and haven't regretted it once.