Steve Hay > wrote:
>Blair P. Houghton wrote:
>
>> ** lag in measurement due to lack of convection; I didn't
>> stir this; once the heat builds up it begins convecting
>> itself; the boiling happens well below 100C measurement
>> because (1) this isn't sea level and (2) the probe is
>> still delayed a bit compared to the heating element;
>> while the absolute times are off because of these delays,
>> the differentials average out
>
>Pretty interesting. I remember hearing about chinese bubble-counting
>techniques. Given a constant or known heat flux, one can count the rate
>of bubble formation to determine the temperature. It is interesting,
>usually when something starts to boil, the heat transfer rate starts to
>go up due to the mass transport of steam bubbles into the cooler bulk.
>That effect would probably be hard to measure.

You get an inversion layer (hot water at the bottom,
cooler above) and convective flow as soon as the element
starts to heat.

Bubble counting might be a reasonably calibrated
temperature guide, but only for a given vessel. The
availability of cavitation sites makes a huge difference
and they are, of course, random.

I've been boiling water in clean smooth vessels in
the microwave at work a lot, lately*, and it's kind of
fun seeing the bubbles appearing right in the middle
of the cup. The water must be superheating in there.
This is the sort of water that bubbles up dramatically
when you add anything to it like a spoon or a teabag.
I give it 10 seconds or so to equilibrate after I remove
it now, because I don't know what tea might do if it hits
a pocket of 230-degree water.

* - interesting tip: this nuker is clearly designed so that
the turntable returns the food to the front exactly every
10 seconds, probably on the presumption that almost all
cooking times will be a multiple of 10 seconds; clever
*******s. But now I'd rather heat the water an extra 5
seconds than save that 5 seconds and have to reach to the
back (it's a *big* nuker).

--Blair
"The next 200 years of tea had better
be more interesting."