This is something i have been close attention to lately. I spent a little coin on a high end inferred thermometer (RTEMP25PB). I have noticed some interesting things with mediums holding different temps. As far as the eggs? IDK, but the mediums yes. Straight Perlite for instance is much cooler than a Perlite\Peatmoss mix or Vermiculite\Perlite mix. Ssme incubators no cold spots same breed of eggs. The reason being is the additions to the Perlite hold in and insulate the heat with the spikes of the incubators and the straight Perlite not insulating well i think...I have noticed a temp. differance of as much as 3-4d!