When you heat your oven, the temperature isn't always as consistent as you'd like it to be. It likely has hot and cold spots, which are easy enough to map out. However, your oven's temperature might be off to begin with.
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Epicurious points out that a simple oven thermometer can help discover how hot your oven actually gets when you set it to, say, 175C. It might actually be 150C or 190C, for example. Especially for baking, temperature is crucial, so you want to know if your oven is heating up properly. Some newer ovens come with instructions on how to calibrate for the right temperature. Epicurious explains:
If you have a newish oven and still have the manual, it's possible, even easy, as the manual will explain — with electric ovens and with some gas ovens, calibrating the temperature can be a pretty quick, straightforward process: you press a few buttons (electric) or you use a screwdriver to fidget with the temperature knob (gas). You have to have an oven thermometer first, though, to know that there's a problem.
If you don't have instructions on how to calibrate, you'll want to at least be aware of the temperature difference so you can adjust accordingly. For more info, head to the link below.