I would say if there are air gaps between the ultratherm and the surface of the T8 it wouldn't be radiating heat optimally. Also, how are you measuring the heat? On the surface of the substrate or directly on the inside of the T8 bottom? T8 has a bottom area of 4 feet by 2 feet and you have an 11 inch by 17 inch mat where the hottest spot is going to be the middle 75% or so and you could be measuring the heat at the outer edge of the heat mat's influence. So if the mat is on the say back 50% of the cutout of the UTH and you are measuring in the front 50% with the thermometer and you are doing so from the top of say 2 inches of substrate, you may not be measuring the accurate hot spot.
Just a thought but I could be terribly wrong. I doubt the probe is an issue so long as you have it fit properly in the cut out channel they make for them.
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