Kettles with concealed elements - are they all so noisy?

The message from "R.P.McMurphy" contains these words:

Certainly is.

What I can't find is the explanation for why the sound of tapping a spoon on the bottom of a fresh poured drink rises in pitch as you continue to tap. I saw one a while ago, but I can't remember the details. Something about microbubbles changing the speed of sound in the liquid but that doesn't always seem to fit what's happening.

Reply to
Guy King
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The Russel Hobbs kettle I mention otherwheres in the thread has a vertical pole inside, which seems to effectively cause the water to tumble, however, it only works when the kettle has around half a litre of water in or more.

Reply to
Ian Stirling

I have seen a number of debates on this subject none that claim to be conclusive.

The one that interested me was why the pitch rose as you stirred a powder like drinking chocolate into hot milk or water. The most noticeable one I found was stirring honey into hot milk - the pitch rises as you stir, but the falls again if you pause and restart - probably as the honey does not emulsify in the liquid and some starts to settle out again.

My best guess for the cause is that as you hit the cup you cause it to ring. It will ring at its "natural" resonant frequency. Control theory tells us that as you add damping to a system you also cause the frequency it will resonate at to move away from the natural frequency. I guess that is what is happening here. As the powder dissolves into the liquid it alters its viscosity and hence the damping effect it has on the ring and the more pitch shift.

Any other explanations you can think of?

Reply to
John Rumm

The stirring causes the liquid level to rise at the edges and thus increase the damping of the cup. Let it settle and it is back as it was. Repeat at will.

Chris

Reply to
Chris J Dixon

"Anna Pinnion" typed

I bought a reasonable Russell Hobbs from John Lewis 30 years ago; it's still going strong...

(The concealed element kettle I bought in 1999 failed a while ago...)

Reply to
Helen Deborah Vecht

The message from John Rumm contains these words:

Change of speed of sound in the fluid. That should affect the resonance much as breathing helium affects the pitch of your voice.

Rotation of the fluid in the container. Particularly if the fluid is rotating faster in the middle the propgation may be more spiral than radial, so the path is longer giving a longer wavelength. As it slows down the path shortens and the pitch rises. I really liked this idea until I tried it. Milk and honey with nutmeg on top to show the flow and the pitch continued to rise significantly long after any rotation ceased. Shame 'cos it was quite plausible! It may still play a part in the early stages though. I need to try it with a stable fluid rather than a freshly hot one.

What would really help would be having a name for the phenomenon to make it easier to find other research.

Reply to
Guy King

I remember buying one of those too, around that time. It was for our small office which didn't have a coffee machine (so the kettle got a fair amount of use). Worked flawlessly in all the years we were there.

We now use cheapo electric kettles which we can buy in the local Tesco's for a fiver. They tend to fail fairly often (maybe once a year or so), but for that price who cares?

Andrew

Reply to
Andrew McKay

The message from Chris J Dixon contains these words:

Hmmm, in my experience it isn't repeatable. Only works with a freshly poured cup, and restirring doesn't let it happen again. And anyway the pitch continues to rise after rotation has ceased.

I've a feeling it's to do with microbubbles in the liquid that change the speed of sound until they've all risen to the top. Perhaps I'll try it with bicarb solution and add a drop of vinegar. If I can make it repeat each time a drop's stirred in that may show the way.

Reply to
Guy King

The message from Helen Deborah Vecht contains these words:

That one with the lights round the bottom that looked and sounded like a rocket taking off?

Reply to
Guy King

Guy King typed

Yup, that's the one. It started leaking and I wouldn't dare mix electrickery with water.

It was from a set of three gadgets I bought from Tesco; the toasted sandwich maker let off the magic blue smoke when I toasted the mains cord, then the kettle leaked. I still use the toaster, but its pop-up springs are rather weak.

Reply to
Helen Deborah Vecht

Guy King wrote

When you pour a hot liquid into a (relatively) cool cup, the cup will expand. If you're stirring it, it will be ringing like a bell and a bigger bell means a...

Oh. Oh, well.

Reply to
Brian L Johnson

Don't know about concealed element types, mine has the normal exposed type. Hard or soft water appears to make some difference.

The water here is hard and the kettle made a tremendous racket from soon after getting warm until boiling. As you say the radio could not be heard above it. Then I started using bottled water and that began to dissolve the scale, though none could be seen on the element itself except around where the element is fixed to the vessel. I used a proprietory descaler to remove all the scale.

Now, using the bottled water it doesn't make any noise at all until just off boiling, but even then its much less than before. I can hear the radio again. So I think the noise is made much worse by hardness in the water, making a very thin - invisible micro scale layer on the element, but why that should be make the noise worse...? Something to do with the breakdown of the temporary hardness of the water during boiling perhaps.

Roger R

Reply to
Roger R

OTOH IME a more recent sample of the same make started leaking after just over one year (DIY fixed) and the switch packed up after a further

6 months (gave up).

R-H are part of the Salton Group now, formerly known as Pifco...

Reply to
Andy Wade

That is one suggestion I have seen, along with a suggestion that the swirling causes the spinning liquid to reduce the amount of exposed cup height thus altering the resonant frequency of the tuned pipe air column above the liquid.

I am not convinced by these, because with powders that disolve completely into the liquid the change in pitch is permenent - letting things settle does not reverse the pitch change. Also the air column height is so short that it would represent a quarter wave resonator for a much higher pitch that you hear in real life.

Reply to
John Rumm

Yup - that could certainly be the case... you would probably expect to detect a pitch change with temperature as well then as the density of the liquid changes.

The other problem with rotation based theorys is that I tend to stir side to side rather than by inducing rotation. You still get the same effect.

Perhaps there is a PhD thesis waiting to be had here ;-)

Reply to
John Rumm

It may be that the scale adds roughness to the surface which acts as a seed site making it eaiser for cavitation to occur....

Reply to
John Rumm

With a scaled kettle, the element runs moderately above boiling - as the scale insulates to a degree. This means that if a steam bubble forms, and insulates more than the water, then the scale patch locally heats up to the underlying element temperature.

Then, when the bubble collapses, the water hits the patch of scale that's now above boiling. For a clean metal element, this doesn't happen, as the temperature is closer to boiling.

My theory anyway.

Reply to
Ian Stirling

So youre paying =A350 per decade instead of a fraction of that for something decent. Plus the inconvenience of extra trips.

NT

Reply to
meow2222

Offset against the fact that new kettles every year won't need PAT-ing.

Owain

Reply to
Owain

I really don't understand the point you are trying to make. 30 years ago =

when I bought that Russell Hobbs kettle there were no plasticky kettles=20 selling for a fiver. And from memory I recall the Russell Hobbs kettle=20 cost about =A325. In today's money that might be =A3120 or more. It was=20 bought on company expense anyway so why should I care?

Are you trying to suggest that because I couldn't actually obtain a=20 kettle at a low price back then I have incurred expenditure which could=20 have been avoided? I really can't see how that could be.

I may misunderstand where you're coming from - perhaps you could explain.=

Andrew

--=20 You probably don't want to reply to my published newsgroup email address because your email is guaranteed to be treated as spam. If you really do need to get hold of me use my contact form at

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Reply to
Andrew McKay

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