Oh sure, blame the inanimate object . I learned a very long time ago to never place total trust in anything that has moving parts, which includes thermometers. I typically use a floating thermo and a lab thermo rubber banded together with the sensing bulbs parallel [and some of you thought you were anal]. The two meters register about a degree and a half different so I split the difference. I also do stir more than I should if HSA is taken into consideration. My point is that I suspect what takes place in my mash tun is pretty typical of what takes place in almost everyone else's mash tun, and that is that we're not achieving the degree of temperature control that we'd like to believe we are getting, and yet for most brewers the beer turns out quite drinkable, at the very least. If you can dial your mash temp in to within 1/2 a degree without busting a gut, cool. If not, don't sweat the petty stuff.
You should be able to get that down to 1 degree or less with enough gentle stirring.
Yep. I don't have trouble getting a uniform mash temp. I wonder about Visor's thermometer.
And FWIW, my brew day is never less than 6 hours and batch sparging hasn't significantly shortened it, but it has improved yields and is a lot less dickin' around than fly sparging.
I'm definitely not ever convinced that my thermometer reads exactly accurately (though Thermowerks makes nice products). But it reads accurately and consistently enough to help me make good beer. It's just that when I read that you're reading "10 or 15 degrees difference in areas only a few inches apart, although usually it's closer to 5 or so degrees", it generally makes me think it's one of two things: 1/ Mash not stirred thoroughly enough, which seems to not be the case here, or 2/ Thermometer inaccuracy. I meant no offense by it whatsoever, but I've brewed for over two decades and am able to reach a mash temp within a degree F in maybe 5 minutes of gentle stirring each time. I was just puzzled.