Kettle doesn't switch off

[Skogsgurra]

We have a kettle with a thermostat that switches off when water boils. At least we thought so.

But, today, it didn't. It went on boiling and boiling until I touched the lever and stopped it. Hmm, I thought. Are we at a high altitude? Looked out the window, but the usual forest was still there. Then I had a look at the old barometer, 738 mm Hg. That is low.

Would that explain it? Anyone seen this before?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[msquared48]

That's only about 29.05" of Hg. It is low, but not that low.

Could the lever developing extra friction with use, inhibiting its release?

Mike McCann
MMC Engineering

 

[Skogsgurra]

Could be. We hadn't used it for a while. I'm afraid I ruined the stickiness when i helped it off.

But, considering that the switch off temperature has to be fairly close to 100 C, or the water wouldn't boil, I can imagine that a somewhat lower temperature has an influence.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Compositepro]

Yes, that could explain it, or it could be a slight change in the thermostat. The thermostat must switch-off at a temperature below boiling or it will never switch-off. Setting the thermostat too high will make the switch-off unreliable due to a number of factors. Boiling only occurs at the heater surface. Technically, a thermostat will not work to control temperature exactly at boiling. It can only control at some temperature less than boiling.

The location of the temperature sensor is also an important factor. In some cases it may be mounted in contact with the heater sheath. In this case its setting can be slightly above boiling and it will still work.

 

[MintJulep]

According to an unverified internet source:

Boiling point (degrees F) = 49.161 * Ln (in. Hg) + 44.932

So at 29.05" of Hg it's 210.5 F or 99.2 C.

 

[MikeHalloran]

High power mechanical thermostats have a finite life, and sometimes signal the end of it by failing closed.



Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

If I'm not mistaken, the new "wireless" kettles where the base is plugged in but the kettle is free from direct connection, use bimetal strips in the kettle portion that respond to heat from the STEAM, not from the water itself. I'm by far no steam expert but my father was, and I vaguely recall there is an issue with steam saturation and atmospheric pressures. So if the barometric pressure was so low as to inhibit saturated steam formation, it's possible that the heat didn't adequately transfer to the bimetal strips. Most of these kettles have another backup safety feature in the base that will cut power if all the water boils away completely.

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[VEBill]

I agree with jraef.

It's the steam that carries the heat that triggers the switch once the water has reached a good boil. The difference is the positioning of the sensor.

 

[patprimmer]

I also agree with jraef. It is steam generation that triggers the turn off. At least my kettle always maintains a rolling boil for a few seconds before it turns itself off.

Also steam escapes from a vent just before it turns off. I expect a blocked steam vent could cause a failure to activate the switch.

A system depending on temperature alone would be to prone to altitude and water quality.

Boiler scale may be blocking a passage not normally visible or accessible.

Regards
Pat
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[Skogsgurra]

I'll wait till the pressure (and heat) is on to see if it works better then.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[GregLocock]

You couldn't use a thermostat to sense boiling, to the standard required for tea making. If the thermostat was at 99 the water wouldn't be boiling, if it was at 101 it would never get there, until all the water had boiled off.

I must admit the lateral thinking to use a steam detector instead is very clever, but I suppose the old whistling kettle may have inspired that idea.



Cheers

Greg Locock


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[Skogsgurra]

There is a very simple explanation. And I do not think that it is about barometric pressure.

I tested it today, still 738 mm Hg, and it didn't shut off. Boiled for a good 60 seconds. I then did what the Missus missed to do yesterday and it shut off seven seconds after that.

The outcome verifies the steam thinking. One cup of freshly brewn tea for the first to tell what I did.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rowingengineer]

given I have no idea this is a guess, did you by any chance block teh vent?

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[MintJulep]

You started counting to 7?

 

[MikeyP]

Close the lid properly? If you leave the lid open then the steam pressure cannot build up at the sensor.

M

--
Dr Michael F Platten

 

[Skogsgurra]

Right, Dr Platten! You want cucumber sandwiches with your tea?

Closed the lid and it works the way it should.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[SomptingGuy]

That's what I was thinking too (the lid). Of course nobody will believe me ;-)

- Steve

 

[jraef]

On mine (Braun), opening the lid kills the power. They must have run into that problem already.

I like these things, but I do miss the whistle. I pre-boil my water for oatmeal (less heat in the kitchen that way) and when I used the old whistler, I knew it was ready. This thing just quietly switches off and if other noisy stuff is going on in the kitchen, I don't notice it. Then by the time I remember, the water has cooled off...

Still, my wife would ignore the whistle while doing other chores for long enough to fry the old kettles; three of them over the years. So all in all this is still better.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[patprimmer]

If we wanted to get real fancy, the steam activated switch could set off an alarm that went for up to say one minute with the heating maintained, then shut itself down if not attended to.

Personally, I prefer the old fashioned whistle option.

I was actually involved in what I believe was the first upright plastic electric kettle design in the word. I was a technical rep working to supply the plastic that did not degrade itself or taint the water and was clear enough to make the sight glass.

The kettle was the Sunbeam Express and it was a very creative design team involved. The Design department head was promoted to a job with Wilkison Sword in the USA and the designer most involved with the kettle is now working almost exclusively on product design for Chinese companies as Sunbeam closed down their local design department I think in the late 80s, early 90s.

Regards
Pat
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[jraef]

Ahhh... Sunbeam. I used to think the appliance company was the same as the car company, a common mistaken ID issue with essentially everyone I knew when I was into hot cars. We all liked test driving the Tiger, but nobody I know ever bought one. I'd bet that cross contamination of identity interfered with their success in the US. Who would want to buy a sports car from someone better known for waffle irons and mixers?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376