ELI5- how do rice cookers know how long to cook the rice for no matter the different quantities
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The amount of time a rice cooker operates is often based on the amount of water which is added.
Rice cookers are extremely simple. They contain a piece of metal which is magnetic but will become non-magnetic when heated above the boiling temperature of water. That metal is used to complete an electric circuit that powers the heating element, and is exposed to the water within the rice cooker. The cooker will heat the water which will never exceed the boiling point while there is water left, and when the water runs out the temperature inside will begin to rise. That causes the metal to stop being magnetic which releases the electrical circuit which powers the heating element, completing the cooking of the rice without it being too wet and before it starts to burn.
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I know this one!!!
A thermostat set at 213°F, because water can not physically reach that temp, it is all boiled off, the rice cooker turns off when the rice reaches 213°f.
The same way your kettle does, there is a thermal switch which pops open at a given temp.
The amount of “free” water remaining in the pot controls the overall temperature in the pot similarly to how any pot with water in it will never be hotter than the boiling point of water until all the water has evaporated.
Once the water is absorbed by the rice or evaporates the required temp threshold can be reached and the switch pops.
I feel like the simple answer is:
“Because they have a thermoswitch that shuts them off when their temperature exceeds roughly 100C, which can only happen when all of the water has boiled away.”
My rice cooker isn’t a very good one so it shuts off at some temperature long past 100C, leaving me with crispy/dried out rice at the bottom.
Technology connections explains it perfectly. This guy does an amazing job at explaining so many different topics from microwaves to the USA power system. Cannot recommend enough.
It doesn’t. It just knows when the metal strip has reached over 100 degrees c and releases a spring loaded switch.
It’s actually incredibly clever, and deceptively simple. They have a tiny metal strip that acts as a magnet until it reaches 100 degrees celcius (the boiling point of water); hotter than that, it stops being magnetic.
Now, liquid water will generally be under 100 degrees Celsius… unless you live in a pressurized tank or smth. That means the bottom of the pot will always be under 100 degrees while there’s water in there. But when the water is all absorbed by the rice, and not in contact with the pot, the bottom can get hotter than 100.
They’ve placed that metal strip at the bottom of the pot. It completes the circuit to the heating element; connected by a magnet. This means when you turn the machine on, the heating element stays on until the water all gets absorbed. Once it does, the pot gets hotter than 100, the magnet stops working, the strip disconnects, and the circuit breaks; turning off the heat (this also typically flips the switch you used to turn it on; so the heat doesn’t turn back on when the rice cools)
This is effectively all that time math for rice is for; trying to guess when all that water is absorbed, and when to cut the heat. That tiny magnetic strip does all that work for you. All you have to do is add the right amount of water for the rice (ask Asians about the knuckle method for that one)
That said, there are more modern cookers with electronic heat sensors that are… well, they’re cool, but they don’t have that same kick that an OG one does.
I took apart and repaired one for my mother in law. Very simple thermal regulation circuit that trips a solenoid. Basically as the rice cooks the water gets absorbed and then gasses off so at some point the temperature spikes causing the magnetic coil to release turning off the cooked. In her case a resistor burnt out. Easy fix.
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