This past Christmas we bought a turkey with one of those plastic pop-up thermometer thingies in it. It got me to wondering: How do these things work, are they reliable, and can they be reused?--George
Although some people refer to them as timers, they are actually thermometers, as you note, since they indicate the temperature of the meat and not how long it has been cooking. If you look inside one of them, you will see the pop-up plunger, usually red plastic, inside the housing. The end of the plunger is held in place by a small glob of metal that is made to melt at a temperature very close to 165°F (74°C) plus or minus one or two degrees F (a half to one degree C). Once the metal blob has melted, a spring inside the device lifts the plunger up to indicate that the turkey is cooked.
While the temperature at which they will pop can be determined quite accurately, they have two shortcomings. First, they are fixed in length so they only penetrate a fixed distance into the meat. Since the breast of a turkey isn't always the same thickness, the sensor end of the pop-up may not be placed far enough in on a larger turkey, or may be too close to bone on a small one.
Also, they only measure the temperature at one spot on the bird. If it has cooked unevenly, or if it was frozen and thawed unevenly, the pop-up may give a wrong reading. As note in Roast Turkey Temperature, the meat of a turkey turns out best when cooked to 165°F (74°C) for the breast and between 175°F (80°C) and 180°F (82°C) for the thigh and drumstick.
For these reasons, it is recommended that you check the temperature in several places using an instant read thermometer to be sure that everything is cooked right.
As far as reusing them, while some people suggest that this can be done, it may be difficult to assure that the end of the plunger is properly fixed back into the metal so that it won't release too early. Since they come with the turkey, or can be purchased new for about a dollar each, the question is whether your health, and that of your family or friends, is worth the risk.