Cannot determine doneness
Kitchen thermometers remove the guesswork from cooking; it is the most reliable tool to ensure the safety and doneness of most types of food. In most cases, you must cook food items at a temperature high enough to kill harmful bacteria. For the aspect of “doneness”, on the other hand, you cook food to your desired taste (appearance, texture, juiciness, etc.). In this case, you need to use the conventional meat thermometer and NOT your infrared thermometer. Why? As said earlier, infrared thermometers read the surface temperature of the food and not the internal temperature.
Many home cooks depend on the visible indicators (color changes) of the food in their cooking. However, appearance and color are not reliable indicators for either safety or doneness of the food. A food thermometer is still the most accurate tool to determine the two, as it is the only tool you can use to tell if harmful bacteria within the food have been destroyed.
While doneness is primarily subjective, food safety experts emphasize the importance of increasing internal temperature up to a certain point (different types of food have different internal temperature requirements) to ensure the destruction of harmful pathogens. Thus, you must only use visual indicators for doneness after the food has reached its safe internal temperature.
Poultry, for example, should reach an internal temperature of 165˚ F for a minimum of 15 seconds to be safe for consumption. A hamburger patty should reach at least 155˚ F for 15 seconds to be safe. This is where you need traditional meat thermometers.
Not ideal for boiling water and other liquids
Again, infrared thermometer sensors read the surface temperature of a specific object. In the case of boiling water, it will simply measure the temperature of the steam.
Moreover, to avoid mistakes when measuring the temperature of liquid food, such as soups, make sure to stir the pot first so you can have an even temperature in your pot.
Not for shiny steel pans
Infrared thermometers cannot provide accurate temperature readings on shiny steel pans. They are, however, ideal for skillets, cast iron, and regular baking pans.