The temp of the grate will be much higher than the surrounding air, so if the probe is touching metal the thermometer will be reading really high temps. You will compensate by lowering the fire, resulting in a too cool cooker and a cook time measured in years. To dramatize this heat your smoker to 225*. Now open the cooker and wave a bare hand it around in there. Probably feels warm. Now take that same hand and grab a grate. Dang hot, isn't it!