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The science behind a rice cooker is fascinatingly clever, yet painfully simple. In short, boiling water temperature plateaus due to the latent heat of vaporiza

The science behind a rice cooker is fascinatingly clever, yet painfully simple. In short, boiling water temperature plateaus due to the latent heat of vaporization. When the water is gone the temperature of the pot starts to rise. The rice cooker uses this principle to control the switch. The switch is held by magnet. When the pot finishes boiling the heat rises and exceeds the curie point of the magnet, which means the magnet ceases to be magnetic, and the (no longer) magnetic switch is released turning the rice cooker off. Here’s a great video explaining how it works. https://youtu.be/RSTNhvDGbYI https://youtu.be/RSTNhvDGbYI

jrimbault 6y ago
It's basically the same physics involved in this toaster https://youtu.be/1OfxlSG6q5Y https://youtu.be/1OfxlSG6q5Y same channel. Thing heats and deforms (change properties), when it reaches temperature n, thing deforms just a bit too much, cuts electrical contact.
- bch 6y ago
  These are both clever, but different physics. The toaster uses metallic elasticity (and a subsequent distortion) due to heat, and the rice cooker uses the heat-induced disappearance of magnetism to disengage a switch. (I have one of those toasters, and they’re very pretty and charming to see work.)
jrootabega 6y ago
It's not the magnet's curie point; it's the curie point of the metal the magnet sticks to. And I guess it's not really the curie point, just the point where the force becomes weaker than the spring
- throwaway_pdp09 6y ago
  That sounds wrong. From wiki "In physics and materials science, the Curie temperature (TC), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties". What you're saying is that metals stop being attracted to magnets above a certain, rather low temperature. Possible but very hard for me to imagine.
  - vincnetas 6y ago
    Thats exactly whats happening. If magnet was to loose its magnetic properties, you would have one time use cooker with permanent "non magnet" ofter first batch. Metal above certain temperature also is not attracted by magnet.
  - amluto 6y ago
    If you take a permanent magnet, heat it above the Curie point, then cool it again, it’s no longer magnetized. So you would have a one-use rice cooker. Magnets stick well to ferromagnetic materials. If you heat a ferromagnetic object, e.g. a piece of steel, above Tc, it stops being a ferromagnet and magnets will stick to it much less firmly. And, when you cool it, it will once again be ferromagnetic.
  - throwaway_pdp09 6y ago
    Thanks @amluto, @vincnetas. I really didn't know that.
smnscu 6y ago
I knew it would be a video from that channel, haha. I discovered it recently and it's a treasure trove of fascinating, "How It's Made"-on-crack technology videos. Link for the lazy: https://www.youtube.com/channel/UCy0tKL1T7wFoYcxCe0xjN6Q https://www.youtube.com/channel/UCy0tKL1T7wFoYcxCe0xjN6Q
- spaceflunky 6y ago
  They’re funny videos because you’re like “man I can’t stand this guy’s cheesy jokes” cut to 12 videos later “I CANT WAIT TO SEE WHAT HE DOES NEXT!!!”
throw0101a 6y ago
> When the water is gone the temperature of the pot starts to rise. Water-to-rice ratios are important. America's Test Kitchen has a good video on the topic, especially if you're using a pot and not a rice cooker: * https://www.youtube.com/watch?v=JOOSikanIlI https://www.youtube.com/watch?v=JOOSikanIlI * https://www.youtube.com/watch?v=DJFU7ezipbg https://www.youtube.com/watch?v=DJFU7ezipbg The rice actually only 'needs' a 1:1 ratio, but you need to put in extra to deal with evaporation; brown rice needs more-extra because it takes longer to cook (the bran slows absorption), and the longer cooking time leads to more evaporation. TL;DR: ignore what's on the bag/box, and for every cup of rice add one cup of water (1:1, 2:2, 3:3), and then a fixed amount of extra overhead for evaporation.
- dogma1138 6y ago
  Rinsing the rice and letting it sit for 30-60min absorbing some moisture then a 1:1.25 ratio is by far the best method I’ve used. I also let it steam in its own heat for the last 10min or so of cooking since the temp won’t rise it won’t overcook the rice but it will prevent any rice from being undercooked or from the rice at the bottom form burning. If I don’t have the tome to rinse I usually increase the ratio slightly and cook it for 5min longer and let it sit covered for 5 additional minutes but the result will also be a much more stickier and starchy rice. People also for some reason stir rice which you should never do unless you are making glue or rice pudding (which tbh is the same recipe minus the sugar and flavorings).
  - hantusk 6y ago
    Also, don't add salt until the process is finished, as that also makes it more starchy
- gbear605 6y ago
  The traditional method in a lot of Asia (per what I’ve read online) is to put a finger on top of the rice pointing down, so that your fingernail is touching the rice, and to fill water up to your knuckle.
  - 6y ago
jhallenworld 6y ago
>In short, boiling water temperature plateaus due to the latent heat of vaporization. When the water is gone the temperature of the pot starts to rise. The rice cooker uses this principle to control the switch. This is true for a lot of appliances: water boiler, coffee percolator, drip coffee maker, etc. They have to be reliable enough that you can have a 1000W+ heater right next ABS plastic and not have problems with millions of units.

jrimbault 6y ago

It's basically the same physics involved in this toaster https://youtu.be/1OfxlSG6q5Y https://youtu.be/1OfxlSG6q5Y same channel. Thing heats and deforms (change properties), when it reaches temperature n, thing deforms just a bit too much, cuts electrical contact.

bch 6y ago
These are both clever, but different physics. The toaster uses metallic elasticity (and a subsequent distortion) due to heat, and the rice cooker uses the heat-induced disappearance of magnetism to disengage a switch. (I have one of those toasters, and they’re very pretty and charming to see work.)

jrootabega 6y ago

It's not the magnet's curie point; it's the curie point of the metal the magnet sticks to. And I guess it's not really the curie point, just the point where the force becomes weaker than the spring

throwaway_pdp09 6y ago
That sounds wrong. From wiki "In physics and materials science, the Curie temperature (TC), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties". What you're saying is that metals stop being attracted to magnets above a certain, rather low temperature. Possible but very hard for me to imagine.
- vincnetas 6y ago
  Thats exactly whats happening. If magnet was to loose its magnetic properties, you would have one time use cooker with permanent "non magnet" ofter first batch. Metal above certain temperature also is not attracted by magnet.
- amluto 6y ago
  If you take a permanent magnet, heat it above the Curie point, then cool it again, it’s no longer magnetized. So you would have a one-use rice cooker. Magnets stick well to ferromagnetic materials. If you heat a ferromagnetic object, e.g. a piece of steel, above Tc, it stops being a ferromagnet and magnets will stick to it much less firmly. And, when you cool it, it will once again be ferromagnetic.
- throwaway_pdp09 6y ago
  Thanks @amluto, @vincnetas. I really didn't know that.

smnscu 6y ago

I knew it would be a video from that channel, haha. I discovered it recently and it's a treasure trove of fascinating, "How It's Made"-on-crack technology videos. Link for the lazy: https://www.youtube.com/channel/UCy0tKL1T7wFoYcxCe0xjN6Q https://www.youtube.com/channel/UCy0tKL1T7wFoYcxCe0xjN6Q

spaceflunky 6y ago
They’re funny videos because you’re like “man I can’t stand this guy’s cheesy jokes” cut to 12 videos later “I CANT WAIT TO SEE WHAT HE DOES NEXT!!!”

throw0101a 6y ago

> When the water is gone the temperature of the pot starts to rise. Water-to-rice ratios are important. America's Test Kitchen has a good video on the topic, especially if you're using a pot and not a rice cooker: * https://www.youtube.com/watch?v=JOOSikanIlI https://www.youtube.com/watch?v=JOOSikanIlI * https://www.youtube.com/watch?v=DJFU7ezipbg https://www.youtube.com/watch?v=DJFU7ezipbg The rice actually only 'needs' a 1:1 ratio, but you need to put in extra to deal with evaporation; brown rice needs more-extra because it takes longer to cook (the bran slows absorption), and the longer cooking time leads to more evaporation. TL;DR: ignore what's on the bag/box, and for every cup of rice add one cup of water (1:1, 2:2, 3:3), and then a fixed amount of extra overhead for evaporation.

dogma1138 6y ago
Rinsing the rice and letting it sit for 30-60min absorbing some moisture then a 1:1.25 ratio is by far the best method I’ve used. I also let it steam in its own heat for the last 10min or so of cooking since the temp won’t rise it won’t overcook the rice but it will prevent any rice from being undercooked or from the rice at the bottom form burning. If I don’t have the tome to rinse I usually increase the ratio slightly and cook it for 5min longer and let it sit covered for 5 additional minutes but the result will also be a much more stickier and starchy rice. People also for some reason stir rice which you should never do unless you are making glue or rice pudding (which tbh is the same recipe minus the sugar and flavorings).
- hantusk 6y ago
  Also, don't add salt until the process is finished, as that also makes it more starchy
gbear605 6y ago
The traditional method in a lot of Asia (per what I’ve read online) is to put a finger on top of the rice pointing down, so that your fingernail is touching the rice, and to fill water up to your knuckle.
- 6y ago

jhallenworld 6y ago

>In short, boiling water temperature plateaus due to the latent heat of vaporization. When the water is gone the temperature of the pot starts to rise. The rice cooker uses this principle to control the switch. This is true for a lot of appliances: water boiler, coffee percolator, drip coffee maker, etc. They have to be reliable enough that you can have a 1000W+ heater right next ABS plastic and not have problems with millions of units.