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I was talking to my father about his auto-off coffee maker and he mentioned that turning it on while empty is bad even though it will turn back off on its own because heating up while empty is worse than heating up until all the water evaporates.

His reason for this was

If filled, the heating element's temperature rises much slower; at the end it jumps from about 95 °C to 105 °C, then turns off. Without water, the temperature rises very quickly from 20 °C to 105 °C, and even though at this point the machine turns off, the heat keeps rising due to momentum, up to like 120 °C or so.

This seems very strange to me since as soon as no electric energy is available any longer, where would the energy for an an additional increase in heat come from? Certain parts of the heating system might get hotter, but as a whole, shouldn't it immediately start to cool off?

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Not momentum, no. It is just that the core element is much hotter than the sensor, and even when switched off, continues to supply heat as it cools. Additionally the heat loss from the device to the air is lower when there is no water to evaporate as steam.

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  • $\begingroup$ Ok, so this means that the core element does get hotter without water, but it doesn't mean it'll heat up further after the coffee maker turns off, right? $\endgroup$ – Chris G Mar 22 '18 at 7:32
  • $\begingroup$ Correct, the core element will not heat up when the device is off. $\endgroup$ – JMLCarter Mar 22 '18 at 7:36

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