Thermometer difference??  Smoking 3 poke butts today and have been seeing this issue,,,  I've got a TelTru thermometer on my door, left side about 8" above grate level (see profile pic)...   I've also got a probe attached to the center of the grate about a foot from the fire box... I'm getting 75-95 degree differences between the 2, with the door mounted thermometer always higher. Is this normal?