[ELI5] How does a rice cooker “know” the timing to switch from cook to warm when there’s no more water in it?
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The rice cooker will heat the bowl. The water will absorb this heat and turn to steam, which the rice absorbs. Once there’s no more water left to absorb the heat, the bowl will quickly rise in temperature. Sensors in the rice cooker will detect this jump in temperature and switch off or switch to keep warm mode.
There is a thermometer that senses when the water has fully boiled away. The presence of water keeps the temperature at the boiling point. Once the water is gone the temperature rises, tripping the internal thermostat to switch from boil, to keep warm
While there is water the temperature will never exceed the boiling temp of water. This is the natural of boiling water (or anything) when you boil water the water *can not* get hotter then the boiling point until it has infact completely boiled.
This is one reason a lot of recipes will tell you to bring water to a boil. It’s an easy way to make sure you cook at a specific temperature. You totally could cook pasta at a lower temp. But good luck getting the timing right without a thermometer and constantly adjusting the temp.
Once all the water has boiled away the temperate will rise and the rice cooker knows to shut off. There are a lot of different types of temperature sensors so I cant tell you which ones a given model uses.
Honestly rice cookers are one of the coolest devices because of how deceptively simple they are.
Technology Connections on YouTube has a video that does a great job of concisely explaining how they work.
The classic/cheap rice cookers use a magnet to keep the electrical switch closed in the “Cooking” position.
The magnet heats with whatever is in the bowl.
As long as there is water, the temperature will stay at a maximum of 100°C (at sea level).
Once the water has evaporated, the temperature while rise above 100°C.
The magnet is made to loose its magnetic properties just above 100°C and it will release the switch back to the “Warm”.
When water gets to 100 C it turns to steam.
Turning water to steam takes energy, and if you put more energy in, it just makes more water turn to steam, rather than making the water hotter.
Rice is done when the water is boiled away, so they put energy in until all the water is boiled away, then that energy starts going into making the rice hotter.
Some rice cookers use a sensor to detect the increase heat.
But if you have one of those rice cookers with a tab that makes a clunk when you press it, it doesn’t really have a sensor.
What happens is that there’s a magnet put near the wire for power. When you pressed down the magnet stuck to the bottom and connected the power.
Now when a magnet gets too hot it stops being a magnet.
So when all the water boiled off, the magnet got too hot, stop being a magnet and then fell off, disconnecting the power.
Electromagnet that stops being magnetic at a certain temperature. After most of the water has boiled away, it can rise to the temperature.
the cheap ones have a spring in them. the combined weight of the rice and water in the inner pot compress the spring, allowing the bowl to contact the heating element and as the water evaporates from turning into steam, the weight lessens and the spring expands pushing the bowl away from the heating element.
It’s really simple. Basically there’s a heating element and a temperature sensor. Once the temperature of the rice reaches a certain temperature it’s done and stops cooking. It won’t reach that temperature before the rice finishes cooking
The cooker uses a magnet to trigger the heater. When the water is all gone the temp of the bowl rises over the curie (?) Temp of the magnet material amd it looses its magnetic property, which turns off the heater.
It really is an amazing little feat of engineering.
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