Thermal Comparison

A fab produces a batch of 8 ASICs. Exactly one of them is highly resistive (defective) and draws less current, making it run slightly cooler, while the other 7 are identical and run hotter. You have a thermal comparator that can compare the heat of two groups of chips simultaneously to see which group is cooler, or if they are equal. What is the minimum number of comparisons needed to guarantee finding the defective chip?