Electric kettle that shuts off when boiling

[HeLiXe]

Electric kettle that shuts off when boiling...

How does it do it?!

I bought one this weekend and I think it is really cool...but I wonder how it does it. My guess is that it has some pressure sensor that detects the difference when the water is boiling...but I really wonder about the details of it even if that is the case. Does anyone know about it?

Here is a picture of the kettle:

 

[HeLiXe]

OR maybe there is some sensitive switch in the handle that is activated by the steam?

 

[turbo]

pressure switch or thermocouple. Anything higher than 100C has to be steam at low pressure.

 

[Danger]

Intriguing question. I know that I read up on the subject once, but I can't remember it. It used to be that I assumed a thermostat to be involved, but that wouldn't work unless the appliance was regional-specific. Boiling point varies with altitude, so a kettle calibrated for Denver wouldn't even think of boiling in Miami. The pressure increase, however small, might do the trick. I know that mine won't trip if the lid is open, which I consider an argument in favour of that theory. The best thing that I can recommend is to visit "HowSuffWorks" on the net. It's one of my favourite sites. Anything from a tennis ball to a neutron bomb is dissected and explained, with cool pictures. I'll look it up my own self later on, but for now I have other things on my mind.

edit: Turbo, how the hell is it that you always manage to sneak in ahead of me?

 

[turbo]

edit: Turbo, how the hell is it that you always manage to sneak in ahead of me?

There's an alien culture that has showed me how to "slip" time just a bit (they won't show me how to go WAY back). It's an "edge" given to friendly humans.

 

[HeLiXe]

There's an alien culture that has showed me how to "slip" time just a bit (they won't show me how to go WAY back). It's an "edge" given to friendly humans.

I think I bought the kettle from them

Thanks so much Danger and turbo!

 

[dlgoff]

I'm thinking it just has a timer that is set to a time that will guarantee a full pot gets boiled.

 

[HeLiXe]

I'm thinking it just has a timer that is set to a time that will guarantee a full pot gets boiled.

The only problem I can think of if they manufactured it that way, is the use of it at higher altitudes...people would really get annoyed if the pot kept shutting off before the water boiled. But really it is cool, as soon as the water comes to a boil it shuts off immediately!

 

[dlgoff]

The only problem I can think of if they manufactured it that way, is the use of it at higher altitudes...people would really get annoyed if the pot kept shutting off before the water boiled. But really it is cool, as soon as the water comes to a boil it shuts off immediately!

Well maybe it's got an altimeter that adjusts the timer.

But if it shuts off immediately then it's probably not a timer.

 

[Proton Soup]

perhaps it monitors temperature rise, and shuts off when delta-T reaches zero.

 

[Evo]

I'm thinking it just has a timer that is set to a time that will guarantee a full pot gets boiled.

No, I have a couple, as soon as it comes to a full boil it shuts off. About 30 seconds for 2 cups, up to 1-2 minutes for a full pot.

 

[dlgoff]

No, I have a couple, as soon as it comes to a full boil it shuts off. About 30 seconds for 2 cups, up to 1-2 minutes for a full pot.

Can you take it apart and let us know what type of sensor it's using?

 

[mgb_phys]

They mostly use a bimetalic switch, most of them invented by this guy http://en.wikipedia.org/wiki/John_C._Taylor_(inventor )

 

[BobG]

Just about all of them have an overtemp fuse, which a bimetalic switch would be appropriate for, but water boils at different temperatures depending on the altitude.

I think it has to be a pressure switch, since there's definitely a pressure change when the water boils - which is why the whistling tea kettles whistle when the water boils. Except the opening for the steam is partially blocked for the whistling tea kettles, allowing pressure to build up.

Check the opening of the spout. If it even has a light flap covering the spout, then there should definitely be enough pressure to activate a pressure switch (and maybe even if the spout is open provided you started with enough water). The kettle just has to heat the water faster than the steam can escape.

The danger would be if you turn the pot on with so little water that it doesn't build up enough pressure to activate the switch. Some of the better pots have a sensor to detect the pot is dry as a safety measure while others just assume having the kettle turn off as soon as the water boils is good enough, since the thermal fuse will open if the temperature gets too high.

The switch to turn off the pot when it's dry would be more interesting. Unless it's really just the thermal fuse. As long as there is water in the pot, the water can't get above boiling and it will carry off excess heat from the metal or glass. The only way the metal or glass can get much above boiling is if the pot is dry. In other words, both variations could work the same way, except they give it different names.

 

[Proton Soup]

cutoff when dry is already a common feature in coffee pots. when the water is all perked out, the element will overheat, which should be simple enough to control.

 

[edward]

It is one of those mysteries in life that does not have an earthly answer.

It is like my egg cooker that uses less water to cook 7 eggs than it dows to cook one.

http://www.google.com/products/cata...log_result&ct=image&resnum=2&ved=0CDAQ8gIwAQ#

 

[lisab]

Mine clicks twice: once when it reaches boiling, then again about 15 minutes later when it cools down (after it's empty). Both clicks sound very similar.

 

[Evo]

Mine clicks twice: once when it reaches boiling, then again about 15 minutes later when it cools down (after it's empty). Both clicks sound very similar.

It doesn't turn off when it starts to boil?

 

[lisab]

It doesn't turn off when it starts to boil?

Oh yes, the first click is when it turns off, about 10 seconds after it starts boiling. The second click...I have no idea what it means, except the kettle has cooled off a lot.

 

[edward]

Wiki says that they use a bi-metalic switch that bends and cuts off the electricity when the steam warms it up. That explains lisa's double click.

 

[HeLiXe]

Just about all of them have an overtemp fuse, which a bimetalic switch would be appropriate for, but water boils at different temperatures depending on the altitude.

I think it has to be a pressure switch, since there's definitely a pressure change when the water boils - which is why the whistling tea kettles whistle when the water boils. Except the opening for the steam is partially blocked for the whistling tea kettles, allowing pressure to build up.

Thanks BobG, this is what I was first considering. The spout is small and uncovered, but there is a lid covering the remainder of the opening of the kettle.

 

[HeLiXe]

Mine clicks twice: once when it reaches boiling, then again about 15 minutes later when it cools down (after it's empty). Both clicks sound very similar.

I haven't heard a second click with mine, but I'll listen for it next time.

 

[HeLiXe]

They mostly use a bimetalic switch, most of them invented by this guy http://en.wikipedia.org/wiki/John_C._Taylor_(inventor )

Thx for the info mgb

 

[HeLiXe]

perhaps it monitors temperature rise, and shuts off when delta-T reaches zero.

I will not even pretend as if I know what delta-T is *googles it*

 

[lisab]

I will not even pretend as if I know what delta-T is *googles it*

Final temp - initial temp, often you'll see it like T_f - T_i

 

[Topher925]

Wiki says that they use a bi-metalic switch that bends and cuts off the electricity when the steam warms it up. That explains lisa's double click.

Yes, this is pretty much it. The ones I have seen use a bimetalic switch (aka mechanical thermostat) placed in the top part of the handle above the water. The switches don't activate at 100C but more like at 85C since it takes time for the steam to heat the switch from ambient temp.

One thing to note, is if the switch triggers at 85C it doesn't matter what altitude you are at as almost all habital places in the world don't have boiling points lower than 90C.

 

[Jimmy Snyder]

I think it has a microphone that detects the whistle when it blows. But if not, then a motion detector that can tell when the bubbles break the surface of the water.

 

[HeLiXe]

I think it has a microphone that detects the whistle when it blows. But if not, then a motion detector that can tell when the bubbles break the surface of the water.

 

[HeLiXe]

Final temp - initial temp, often you'll see it like T_f - T_i

Thank you!

And thanks also Edward...I thought I thanked you yesterday but see I didn't...please forgive me @_@, Thanks Topher. And thanks to anyone else I have not thanked!

 

[RonL]

Does anyone can food ? the lid is sealed and when it cools it will click when the seal is complete. I think it is a heat flex type of switch in the water pot.

 

[litup]

Maybe it contains a vibration sensor, the thing does rumble a bit when the water boils. That or the pressure sensor gets my vote. It also has to have by law I think, an over temp switch, the bimetallic unit already mentioned. When that switch heats up too much due to low water, the energy flowing inside the switch causes it to bend when passing a certain temperature, say 120 degrees C, which would never happen if there was water inside, then it bends away from a contact opening the circuit. The best way would be to have a manual reset so a person interacts with it in such a way as it doesn't just recycle itself off and on if left both plugged in with no water and unattended. A simple bi-metallic switch is not smart enough by itself to keep it from just cycling off and on indefinitely when unattended. But if it activates a second circuit that totally shuts down the system, it would never come on again unless you hit a reset button. That keeps people from getting burned or starting fires.

 

[Proton Soup]

Wiki says that they use a bi-metalic switch that bends and cuts off the electricity when the steam warms it up. That explains lisa's double click.

i guess this means there's a lot more heat transferred from steam at the same temperature for this to work. my thermodynamics is pretty rusty, and i never liked it to begin with.

Final temp - initial temp, often you'll see it like T_f - T_i

more or less. but I'm thinking more in terms of something that moves with time, so that you look at the slope of the T(t) curve.
so like delta_T = T(t₂) -T(t₁).

but it sounds like edward dug up the right answer. it'd be simple, cheap, and durable.