Page 104 - Turkey Book from Meathead

You cannot trust the popup thermometer that comes inserted in the bird. The plunger that pops up is anchored in metal that is supposed to melt at a set temp, often at 185°F. At that temp a turkey breast is more particle board than party. That 20°F dierence is the dierence between succulent and sucky.
In November 2013 Consumer Reports tested popup thermometers and wrote that “three timers popped up when meat was still below that safe zone, one as low as 139.5°F. These low readings are a concern... Serving undercooked turkey means you risk sending your guests home with a nasty case of food poisoning. Our food safety experts recommend that cooks do not rely on these timers to tell whether their holiday bird is done.”
Worse still, if you stu your bird, it is not measuring the temp of the stung which is several inches further away from the heat than the tip of the popup.
What about that dial thermometer you have in the junk drawer? Take a look at the calendar. This is the digital age. Bi-metal dial thermometers were invented in the 1800s, and all but the most expensive models can most charitably be called “indicators,” not precision measuring instruments.
Cooking without a good digital thermometer is like driving without a speedometer, flying without an altimeter, building furniture without a tape measure, filling your tires without a pressure gauge, or repairing the reactor without a Geiger counter.