I always use my dome thermo; however, if you're putting a lot of meat on, then the dome temp may be MUCH lower than the grate level. Remember, you've got all that cold meat between the fire and the dome. That's going to act like a great big insulator! A 100* temp difference is not unusual!
Also, as already pointed out, the dome thermo can actually penetrate into your meat on the top grate! This can also be confusing the first time it happens!
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