How does an electric tea kettle know when to turn off?

28 June 2017 | How Does it Work?, Video | 14 comments

The age old question: How does your electric tea kettle know when to turn off? Is it electronic, or something simpler?

It turns out the key is a bimetallic strip. At a certain, carefully adjusted temperature, the bimetallic strip will bend just enough such that right when the water starts to boil, the bent strip will trigger the mechanism that flips the switch to the OFF position.

It doesn’t matter how much water is in the pot. Once the water is boiling, the temperature in the base will be at a certain point. That temperature point is the trigger threshold for the bimetallic strip. More or less.

That sounds complicated, but watch the video. It’s pretty simple!

14 Comments

Jacques on 28 June 2017 at 15:33

Thanks Scottie.

And what about tea kettle that you can chose your temperature like 80C, 85C, 90C and so on.

Do they have many kind of switches or many kinds of metals?
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- Scottie on 28 June 2017 at 21:26
  
  Probably a variable “off switch”, like in an old-fashioned thermostat. But, I’ve never even seen a kettle like that before!
  Reply
  - Diana Weaver on 22 October 2022 at 20:29
    
    My kettle did not turn off, now it won’t work as the water boiled completely out.
    
    How can I repair it?
    Reply
    - Scottie on 22 October 2022 at 21:10
      
      You probably have to take it apart and replace the bimetallic strip – assuming you can even figure out exactly which one to get and where to get it!
      Reply
DOUG JONES on 1 August 2017 at 04:49

Thanks Scottie,

I used my automatic kettle for years at sea level where I bought it. it switched off at just the right time about 10 seconds after the water boiled, which of course was at 212 deg f. it never switched off too early – it always waited until after the water was boiling.

Now I use the same kettle in Denver at 5000 ft altitude where boiling point is about 202 deg f. Yet the kettle still switches of when the water boils (although I have noticed it takes longer to switch off). Since liquid water can’t heat beyond boiling point, how does the same kettle switch of at 202 deg f?

Thanks
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- Scottie on 1 August 2017 at 10:09
  
  Aha! Interesting question. The kettle I use (a little bit above sea level) turns off right after boiling occurs. If the same kettle is used at high altitude, the pressure is lower so the boiling temp is also lower as you noted.
  
  So, one might figure then that the kettle is turning off at the same temp, but will boil a bit longer as the temp rises in the base where the bimetallic strip is usually located. Even though the water is boiling, it will still absorb heat until the water is all boiled away as water vapor… and then the kettle will probably melt or catch on fire. 🙂
  
  Also, the bimetallic strip probably isn’t designed to turn off at boiling temperature at sea level, but a different (lower) temp since the strip is in the base of the kettle underneath the heating element. In other words, the strip is not “detecting” the temp of the water, but rather the temp in the base of the kettle. That’s my guess, anyway!
  Reply
David Boring on 31 December 2019 at 15:20

I suspect that the primary thing carrying the heat to the bimetallic strip is water vapor. When the concentration of heated vapor heats the strip to the point where on side of the strip expands, the strip bends. I’ll bet that that trip temperature of the strip is slightly below 200F. It doesn’t get heated enough until the air surrounding it is full of hot vapor.

We have a kettle that lost the metal screen over the pour spout. It won’t trip at all without the screen, but if we cover the pour spout opening with tin foil, it does. This leads me to suspect that the shut-off mechanism relies on the heated vapor being contained. So, no matter what temperature the liquid is when it boils (as long as it boils at the strip’s tripping temperature,) the strip will bend when it’s heated enough.
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- Peter on 13 February 2020 at 10:27
  
  That is exactly right. What triggers the switch, is not the temperature of the base, but the steam from the boiling water, reaching it through a steam tube. This ensures that it will always switch off when the water is boiling, regardless of boiling temperature (varying altitude). That’s also why it won’t shut off, if the lid is not properly closed, since the steam just goes out, instead of down the steam tube. 🙂
  Reply
  - Scottie on 13 February 2020 at 11:42
    
    No steam tube in my electric kettle…
    Reply
Elena on 20 March 2021 at 03:47

Hi! I’m doing an engineering project and am just wondering if there are standard metals that are used in bimetallic strips (eg aluminium and zinc have two different melting points) or if it is up to each producer to find the two metal that work best for them? Thanks!
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- Scottie on 20 March 2021 at 12:02
  
  I think they usually use steel and copper, or steel and brass. I guess there are probably other options, but probably it would need to be a combination of a strong metal and a weaker one such that the strip will bend due to the difference between them, but at the same time not break, crack, etc.
  Reply
j hamilton on 12 February 2022 at 13:30

Does the steam tube ever fill up with condensed water?.If so will it have to heat the water to stem temp?
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Chris Butler on 27 April 2022 at 15:46

Great video thanks!
Is there a way to purchase just the strip so I can replace my non working one?
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- Scottie on 27 April 2022 at 16:13
  
  Not that I know of. They’re usually pretty specific to each kettle.
  Reply