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Re: microwave, RF heating



At 02:50 AM 5/29/00 -0700, Bernard G. Cleyet & Nancy Ann Seese wrote:
I don't think conductivity has anything to do with microwave absorption.
Try heating a pair of glasses, one with brine, the other deionized water.
I'll be surprised if the temps are more different than expected due to
variations in the microwave field with position

Take a look at the data
http://www.deas.harvard.edu/courses/es151/pages/gallery/images/water_spec.
http://www.deas.harvard.edu/courses/es151/pages/gallery/images/water_spec.html

a) This thread started with a discussion of cell phones. At the frequency
of my cell phone (840 MHz), Jackson says there should be an
order-of-magnitude difference between plain water and sea water.

b) At the standard microwave oven frequency (2.45 GHz), Jackson reports no
data for sea water. I haven't tried sea water or DI water, but when I
compare New Jersey tap water with a saturated salt solution, the salt water
heats up more slowly, which is somewhat paradoxical. Perhaps it is so
conductive that it shields the inner part of the sample.

c) I repeated the experiment using a more relevant, less concentrated
solution, namely the broth from low-fat soup (minestrone). The result was
similar: the salty soup heated up more slowly. Also, the soup (unlike the
saturated brine) showed obvious sensitivity to stirring, which further
supports the shielding hypothesis.

None of this data is consistent with the assertion that conductivity
doesn't have "anything" to do with it. In fact it seems to suggest that
ionic conductivity is very significant.

Sea water is about 0.6 molar. Physiological saline is less, about 0.15
molar. The soup is 0.17 molar, if you believe the label.

-- you have a "carousel" oven?

Yes.

Both water and fats, fatty acids, glycerides, etc. have O-H bonds whose
stretching frequency is close to that of the oven's magnetron.

Really? I thought bond-stretching modes were in the THz range (thousands
of cm^-1). Bending frequencies are lower, but still not much below a THz,
typically. You would need a very weak bond and/or a very heavy ligand
before you could have a resonance at oven frequencies. And given such a
ligand and such a frequency, I would think that solvent effects would wipe
out the resonance. If anybody has evidence to the contrary I'd like to see it.

See e.g.
http://www2.trincoll.edu/~sstickle/handouts/fall/irhandout.html