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Steve Shaw wrote:
> Dave, You are without doubt a modern-day Aristotle.
> >> How about initial condensation on the bottom of the pan when you
> fill it with cold water, or else a tiny pin-hole somewhere, to give
> little water droplets and tiny gas explosions (steam) as the driving
> force? This would levitate the pan slightly so it would be mounted
> briefly on a virtually frictionless bearing, and then would be able
> rotate a bit to try to neutralize its undoubted out-of-balance state
> (not possible to exactly balance a lumpy old pot). Next droplet, next
> explosion, 10 per second. Or, with steam still the driving force, the
> frequency might be some natural vibration of the pot and contents, or
> of the pot and its handle (try over- or under-filling it to change the
> frequency, and gluing up the handle).
> >> Try filling it the night before with water and allowing this to
> equilibrate to room temperature (control against external
> condensation), and then putting it on a layer of paper towel presoaked
> in copper sulphate solution and thoroughly re-dried (a check for
> minute leaks -- would turn locally blue -> pink)? -- just a thought.
> >> Sounds like very inefficient heat -> vapour conversion-- acquire a
> new pan with a flat bottom?
> Looking forward to your step-by-step repudiation of these ideas, as
> usual.
> Steve, Halifax
Hi Steve & All, Jan 27, 2007
At this time of year, with a wood fire going 24/7 and indoor air
correspondingly dry (what goes up must come in as dry cold air), one
would need a very cold pot to gather condensation and the trace that
might get collected on a very cold pot would not last many cycles on a
hot stove.
As a kid, we always had a teakettle on the stove and they all
eventually developed a pinhole leak so I am familiar with the psttf,
pause, psttf, pause of a leaky pot. This is a stainless steel pot and I
am sure that leaking is not involved.
[DIGRESSION RANT: For a leak test I think you meant Cobalous
chloride; blue when dry, pink when moist. Thanks to some MSDS
numb-skull, Colbaltous chloride, aka Cobalt chloride, is classed as a
dangerous material. In these MSDS mumbo-jumbos Common Salt also sounds
dangerous. Several years ago a 500g bottle of indicating Drierite
(contains 3% Cobalt chloride) cost me $39.00 + $15.00 Hazardous goods
surcharge + H.S.T. The 4-page MSDS has everything one does not need to
know and neither it nor the label tells how to regenerate Drierite
(200-225o C; 1-2 hrs). And they also fail to do the sensible; show on
the label that the product contains 3% Cocl2. Before this dysfunctional
MSDS system was born, one could get 5 lb of Drierite for $10.25; sigh.
And one could buy a wide selection of quick chemical tests at reasonable
cost. With the advent of MSDS, most of these test kits were no longer
marketed or were sold at prohibitive cost; groan. END OF DIGRESSION RANT]
The amount of water seems to affect the feedback necessary for
sustained oscillation. In this 18.0 cm diam pot, oscillations faded
quickly with 350 and 550 ml, were sustained but weak at 450 ml and
sustained and strong but still not maximum at 425 ml. When I noticed
this vibration initially I must have by chance had about the right
amount of water. Part of the feedback is provided by the edge of the pot
hitting the stove, thus the tuck, tuck, tuck sound of an exhaust fan,
but something else must power even this.
The idea of a leaking pot brought to mind an image of skittering
spheres of water. In some context, that I don't recall now, water is
sprinkled on hot metal to see if it is hot enough. Water, for obvious
reasons, can not wet a hot surface, so the drops of water form small
spheres that bounce around and quickly shrink to nothing.
Heating of water in a pot with a rounded bottom is surprisingly
rapid when the source of heat is much larger than the pot, as opposed to
heating on a small electric ring. Rounding no doubt slows heating but
for humidification, a slow release of water vapor is better than a
vigorous boil that leads to condensation on windows.
The sides of the pot above the water level quickly heat up to a
temperature much higher than below water level.. Consequently, as one
would expect from the skittering spheres, the meniscus [as viewed by a
light above forehead] does not appear to move at all relative to the
pot, so surface tension may push the low side up. A thin layer of light
oil, that can wet even a hot metal surface should dampen the
oscillations if failure to wet the pot wall is involved.
Based on unequal lighting of the pot bottom, from an overhead light,
there appears to be a nearly stationary standing wave about half way
between pot middle & side.
One could use a humidifier pan that had a flat bottom but think of
the fun that would be missed !
Yt, DW, Kentville
>
> On 22-Jan-07, at 9:54 PM, David & Alison Webster wrote:
>
>> Dear All, Jan 22, 2007
>> I put a saucepan of water onto a piping hot wood stove tonight and
>> a few minutes later wondered why someone had left the bathroom fan
>> on. The 'fan' turned out to be sound produced by up and down
>> oscillation of the saucepan about 10 times a second and with an
>> amplitude, at the end of the handle, of about 3 mm.
>>
>> The pan has a slightly rounded bottom from having run out of water
>> several times and, presumably because the rounding is a bit
>> lop-sided, the pan when oscillating also graduallly turns clockwise
>> as viewed from above. To start this oscillation, one must set the pan
>> onto the stove with the handle up ~one cm from level and with no
>> sideways slop of water. Once the oscillation is started then it
>> continues until the pot handle rotates around enough to touch the
>> back of the stove or until the water starts to ping.
>>
>> Some 12 years ago I saw oscillation of this type involving a
>> larger pot, which of course also had a rounded bottom, a 7' long
>> steam box that was balanced on the pot, a period that (I think)
>> gradually increased to about 4 sec and amplitude that gradually
>> increased to about one foot [at which point I steadied the box & put
>> a prop under one end].
>> Having something barely balanced (handle & pot or box much longer
>> than pot diameter along with rounded pot bottoms) is probably a
>> necessary condition as is a relatively hot surface but I have yet to
>> fathom how heat drives the oscillation. Any ideas ?
>>
>> It does not appear to be related to surface tension effects
>> because adding a drop of detergent to an oscillating pot had no
>> effect. It can be difficult to duplicate and may be dependent upon
>> some subtle configuration of the rounding that changes with recent
>> heating and/or something related to rate of heat transfer (e.g. a
>> cold pot and a brisk fire). After starting it 10-15 times without a
>> hitch, after I first noticed it, I can not now get a sustained
>> oscillation.
>>
>> Yours truly, Dave Webster, Kentville
>>
>>
>>
>
>
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