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In: Technology
I assume you are talking about an oven that displays its current temperature (rather than just having a light that indicates it is heating). Typically, these ovens are set up to only show a real current temperature (estimate) while they are pre-heating or cooling down. Once they have reached their set temperature, they will keep displaying that temperature constantly, even though in reality ovens never maintain a perfectly constant temperature. See, ovens are on a thermostat cycle, just like (probably) the heating system in your home. If the temperature drops below a set point, the heating elements are switched on until the temperature is back to a second set point. For instance, if your oven is set to 400 degrees, then it might switch the heating on (or to a higher setting) when the temperature drops below 390, and switch off again (or to a lower setting) when it climbs above 410 (these numbers are made up – I have no idea how close together the set points typically are for ovens).
In theory, your oven could report all these temperature variations on its display, but my guess is this is deemed too confusing for the typical user. When switched on, we expect ovens to be either pre-heating or in set-temperature mode. That’s how ovens used to work, after all, before the digital age. So, to indicate that the oven is in steady-state operation, they made a design choice to have it keep displaying the set temperature.
It’s possible that the oven would issue a warning, or indeed switch to displaying the current temp, if the temperature dropped by a lot. But some drop in temperature is expected when opening the oven door, and so within this expected range it likely won’t do anything, except switch on the heating to compensate.
It’s worth realizing too that, while (some) hot air does escape when opening the oven, the oven itself is still hot. That is, the walls, heating elements, grills and baking trays, cooking vessels, etc. all retain their heat for a longer time. The air inside the oven doesn’t actually carry all that much heat energy, as air has a pretty low heat capacity per volume. So once you close the oven door again, the air heats up more quickly than if you started heating your oven from scratch, due to all the hot thermal mass that is still in the oven (and indeed, one way to better stabilize your oven temp is to put some more thermal mass in there, like a pizza stone or some ceramic cooking vessels).
If you want to keep track of the actual temperature fluctuations in your oven, you can buy an oven thermometer. Generally, bigger, more expensive ovens are better at maintaining a constant temperature, compared to smaller, more basic models.
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