If you've been in the appliance industry for any period of time, you've likely been told that oven thermometers are not accurate. I hate to tell you, but it's just not true. Oven thermometers are typically quite accurate, and the problem is more so with the user. Let me explain.
Here we have five different brands of thermometers, all varying in price by just a few dollars. While they were not the cheapest that I could get my hands on, they were certainly not the most expensive. I've placed all five thermometers on a baking sheet in the center of the oven, and now it's time to preheat. After 10 minutes have gone by, the oven is now reaching its set temperature of 350 degrees. Here we can see that all of our thermometers are reading well below the temperature that the oven was set for. The reason behind this is that the oven will heat the air faster than any object placed inside of it, including its cavity. To correct our mistake, we closed up the oven door and let them bake for another 30 minutes. Taking a second look, we can see that our oven thermometers are all now reading almost exactly 350 degrees. In order for these thermometers to be accurate, they need to go through the initial preheat as well as 20 to 30 minutes of additional cook time. Please take into consideration that, over time, cooking oils and greases will build up in your thermometer, taking them out of calibration. In which case, they should be replaced.