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Re: microwave, RF heating



At 17:23 5/29/00 -0400, John Denker wrote:
At 03:41 PM 5/29/00 -0500, brian whatcott wrote:
I conclude that ice and water absorbed microwave energy at about the same
rate.

I'm surprised. Please retry the experiment placing the ice in
a) a styrofoam cup, with
b) holes punched in the bottom, so any liquid that forms can drain away.

When I did this with a smallish ice sample, it had all melted and drained
in one minute.

It's evident that in my conditions, the ice is collecting as much, perhaps
more energy than the water.

I reran the test with styrofoam cups, ice and water weighed to 0.1g
and start and end temperatures measured and evaporation loss (principally
affecting the hotter water) accounted for, in order to arrive at
this conclusion.

Some differences that may be significant:
the ice samples were rather unusually shaped: imagine a 20 cm diameter
cylinder that is 1 cm deep.

Image a slice taken from one edge of this cylinder, so the ice chips are
arcs, not more than 1 cm thick whose chord is perhaps 4 cm long.
What is more - consider that some of these chips were hollow shells
with thin walls (a peculiarity of this ice maker).

The ambient temperature at the ice maker is -15degC
It is evident that these chips are far removed from the 3 cm cubes you
may be familiar with. They occupy much more space than the same mass
of water. I expect their cross section, so to speak is higher.

I checked the melt conductivity in a cursory way - by dipping the pin
electrodes of two DVMs in the liquid successively.
Both ice melt and heated water gave a reading of about 400kohms between
pin electrodes held 3 cm apart and submerged 3 cm.

Possible reasons? The local rocks carry gypsum strata, so sulphate irons
would not be surprizing. The local water plant doses quite heavily
(in the customary American way) with chlorine as well as the usual
flocculants and pH balancers.


I worry that your fancy glasses might be absorbing some heat by themselves,
and I worry that once liquid starts to accumulate we get runaway melting.

The champagne glasses were in fact marked 'lead crystal' meaning lead doped
glass produced to enhance the 'sparkle' due to refraction. An empty glass
spun as a control did not get appreciably warm however.
I sensed the styrofoam cups were warmer (though because of their very
low thermal capacity this could well be due to the warm air blown over the
magnetron into the chamber, I imagine.)
My data imply the reverse of runaway heating with liquid accumulation
in the ice container.


I conclude that if John thinks the glasses were responsible for the
microwave absorption by ice chips here, he should in his memorable phrase,
'think again'.

My data indicate that microwave heating is directed 'outside in' and that
low effective density increases heat transfer i.e that self shielding is
significant.
brian whatcott <inet@intellisys.net>
Altus OK