The instant-read thermometer is a Digital Folding Thermometer, available in both analog and digital styles, that allows a cook to take instant temperature readings of a food. They're great for testing the doneness of a piece of meat or poultry while it cooks, but they're not meant to be left in during cooking. You simply insert the probe into the food, check the temperature, and then remove it.
Instant-read thermometers also can be used to measure the temperature of hot food in a steam tray or chafing dish, as well as cold items in a salad bar, and to measure how quickly a soup or sauce is cooling (to ensure that it does not spend excessive time in the temperature danger zone).
Oven dials are not a reliable measure of the real temperature inside your oven. When you set your oven to 350 F or 400 F, it could be anywhere from 25 to 50 degrees higher or lower than that. Even worse, your oven might have hot spots or cool spots—regions where the temperature is consistently higher or lower than other parts of the oven interior —which can affect baking times.
To find out, get yourself an oven thermometer, which you can place on the shelf (or hang by a hook from one of the oven racks). When you set your oven to 350 F, the thermometer will tell you whether it's actually at that temperature or not. Some amount of fluctuation is normal but if you find that your oven consistently reads 25 degrees hotter than what you set it to, you can adjust the temperature accordingly. No more blackened cookies!
A miscalibrated refrigerator can really cause problems. If your fridge doesn't get cold enough, pathogenic bacteria can grow. The temperature inside your refrigerator should be between 33 F and 40 F. But to make sure, get a refrigerator thermometer. It works the same way as the oven thermometer: stick it in the fridge and it'll display the temperature of the inside of your cooler. (Freezer thermometers also read the temperature of your freezer's interior, which should be 0 F.)