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A friend complained about his new rice cooker. It's a very expensive model with programmable features and claimed to use "ai" for "perfectly cooked rice" every time. After a dozen or so attempts he called their help line and spent a half hour using their smartphone app that talked to the cooker. It passed the diagnostics with flying colors. He isn't happy.
Simple rice cookers tend to work well every time. They rely on two bits of physics that give clear signals. The first is how water responds as you heat it. The energy needed to raise the temperature of liquid water by one degree is very nearly the same from room temperature until it's right at the boiling point. Adding more heat at the boiling point just turns the water into water vapor - the remaining liquid water stays at 100°C.
The next clever bit was discovered by Pierre Curie who probably never thought of its rice cooker use.
The cooker I use has a spring at the bottom of the pot and a strong permanent magnet. You put rice and water into a smaller cooking pot and push it down a bit until the magnet attracts and grabs a piece of special metal. Then the heating element engages and cooking begins. The rice is cooked as the water heats and continues to cook as it boils. The rice temperature stays below 100°C until just enough of the water starts boiling. When most of the water has boiled away the rice temperature starts rising again. Now the discover of Madame Curie's husband takes over. At around about 105° C, the Curie temperature of the metal, the metal is no longer attracted by the magnet.1 The spring pushes up and turns off the heating element and a power light. On mine it is also connected to a mechanical bell and turns off a power light. The rice is always great.
I looked at a repair takedown of an expensive rice cooker. The construction was better, but it was complex with a microprocessor, control panel, barometer, humidity sensor, thermometer and Bluetooth chip (they have a smartphone app:-). Presumably it has some sort of table that rice is perfectly cooked when all of those signals reach a certain values, although it could be more complex. I'll stick with simple.
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1 Magnets lose their magnetism when heated at a specific temperature called the Curie Point. In this case the permanent magnet stays magnetic at 105° C, but the special metal used here suddenly loses its ability to be attracted by a magnet.
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