I added a second thermometer to my six year old Weber Genesis 900 LP grill
(Home Depot 1000 with cast iron grates). I'm doing rotisserie coffee
roasting in it, and I wanted to compare temps in the center zone where the
roasting drum sits versus at the grill (which is where the stock thermometer
reads). I drilled a small hole with my Unibit (bought for the WSM temp mod)
in the center of the lid, about three and one-half inches down from the top.
The second thermometer also goes up to 700 F, where my Weber thermometer
pegs at 550.

I then ran a test of the difference in readings. During heat-up (all burners
on high) the gill runs about 125 F hotter than the center zone. However,
once the unit is fully heated (15-20 minutes) the temp difference is
negligible-- maybe 10 degrees. This is true at 250, 350, and 525 degrees.
The max temps reached in the grill as measured by the lid thermometer) are
just shy of 675 F (ambient about 55 that day). I had suspected it got close
to 700 at the grill, given the way it sears a steak.

For coffee roasting, I'll need to preheat a little longer than for grilling,
but I think the unit will give me good temp control and stability at the
range I need to run in.