[wolfekeeper (OP)]

I've noticed that after you boil a kettle, they switch themselves off; which is good.

But quite a long while after that, maybe half an hour, there's suddenly a loud 'thunk' or 'click'.

Why?

[JP]

An electric kettle?  Mine doesn't do that.

I'd guess it's probably thermal contraction as it cools off, and something's not quite fit together properly so it builds up a bit of stress and then pops back into place.

Or kettle gnomes.

[wolfekeeper (OP)]

No, it's a mechanical switching sound; and I've noticed at least two different kettles do it (it just did it while I was typing); it's obviously doing something deliberate, it does it as the kettle cools down, and it's not just a random creaking noise, and it happens just once as it cools down.

[RD]

No, it's a mechanical switching sound ... it does it as the kettle cools down

You've answered your question : a bi-metallic thermostat is used to switch the kettle off when the water boils.

If the room is cold, (e.g. winter) the cooling clunk the noise may be more obvious as bi-metallic thermostat will flex/contract more in the cool direction.


Electrical equipment that utilizes heating and cooling systems often incorporate small pop-disc thermostats which open or close at a set temperature that cannot be changed by the end-user. The disc is formed from a bi-metallic sheet, with two layers of metal that expand at different rates, and is stamped into the final bowl shape. The switch contacts are in the center and around the circumference of the bowl.
At the transition temperature, the metallic expansion stresses on the bowl cause it to suddenly "pop" and invert itself. Depending on switch contact arrangement above and below the bowl, inversion may either open or close the electric circuit.
The inverted bowl is held under tension in the inverted shape, and it takes a certain amount of temperature drop for the bi-metallic stresses to build up sufficiently in the opposite direction, causing the bowl to "unpop" back to its original shape.

https://en.wikipedia.org/wiki/Tipping_point_%28physics%29#Set-point_thermostats
   

[wolfekeeper (OP)]

I suppose there's two sets of hysteresis then; the bimetallic strip and the switch.

The switch has hysteresis because it needs to be able to switch the contacts quickly to minimise arcing, but I'm not sure why the bimetallic strip does as well; clearly once it has flipped, it stays flipped, so it makes you wonder why that needs hysteresis as well. It doesn't seem to flip back until it gets down to ~30C or something which seems useless in the context of a kettle.

[RD]

The switch has hysteresis because it needs to be able to switch the contacts quickly to minimise arcing, but I'm not sure why the bimetallic strip does as well; clearly once it has flipped, it stays flipped, so it makes you wonder why that needs hysteresis as well. It doesn't seem to flip back until it gets down to ~30C or something which seems useless in the context of a kettle.

The kettle's electrical switch is spring loaded so will remain "open" once triggered by the bi-metallic pop-out.

I think the thing the user presses to switch on the kettle applies a force to the bi-metallic causing it to reset (pop back) if it is not too hot.

[wolfekeeper (OP)]

Well, that at least is a testable prediction; if I push the button when it's not plugged in, that should preclude it from popping.

I'll let you know...

[JP]

Interesting.  I was hoping for kettle gnomes.  :p

I'll have to listen to mine to see if it makes a noise, but I've never noticed it.

[wolfekeeper (OP)]

I think the thing the user presses to switch on the kettle applies a force to the bi-metallic causing it to reset (pop back) if it is not too hot.

Yup!

Mystery solved!

On mine you can force the tick to happen early, after a minute or two, if you empty the kettle and fill it with cold water, it cools the strip down; and you don't get the separate tick if you push the button down, so that is how it works.

[RD]

I'll have to listen to mine to see if it makes a noise, but I've never noticed it.

You may have a thunk-free up-market kettle with an electronic temperature sensor, rather than a cheaper electromechanical bi-metallic  thermostat, (if you can adjust the "off" temperature it has to be the electronic type : the electromechanical bi-metallic type is not adjustable).

[evan_au]

The switch has hysteresis because it needs to be able to switch the contacts quickly to minimise arcing, but I'm not sure why the bimetallic strip does as well

Hysteresis in the bimetallic strip is a side-effect of the inverted bowl construction - it must build up tension to flip in one direction, and then build up tension to flop in the other direction. This action releases the pent-up tension very rapidly, providing the mechanical impulse to trigger a mechanical switch.

I have seen a bimetallic strip in a fixed water heater which does not build up tension in this way, and does not exhibit hysteresis. However, it has a very gradual motion with changes in temperature, and it needs a very sensitive electrical switch (a mercury-wetted relay) to provide hysteresis, trigger the electrical circuit rapidly, and avoid arcing.

Since mercury is now recognised as an unnecessary health hazard, and is very sensitive to vibration (like someone picking up a kettle), a mechanical switch is much cheaper and more effective.

[wolfekeeper (OP)]

The heater is a different thing, with a kettle, the thermostat sees very little increase of temperature until the water boils, when suddenly lots of steam comes off, and goes past the themostat, which rapidly heats up and then inverts. It may only switch at 60 degrees or something; it doesn't have to detect 100C it just looks for the steam.

The water heater doesn't have the benefit of the steam, nothing boils, so it has to be much more sensitive to the temperature and the hysteresis is only in the switch contacts.

I think the mercury is there just because it's carrying a very high electrical load and needs to last a long while. I've got a thermostatically controlled electric frying pan that has a completely different design and uses no mercury at all.

[evan_au]

My clothes iron also goes "thunk".
Unlike the kettle, the iron has an adjustable temperature.