Re: microwave, RF heating

[John Denker <jsd@MONMOUTH.COM>]

At 11:00 PM 5/30/00 -0700, Bernard G. Cleyet & Nancy Ann Seese wrote:

> In two identical (well, from the same mold)  goblets
> equal masses of U.S.P. mineral oil
> (edible) and olive oil  108 g.
>
> temp rises (respectively) four C. degs. and 36
> (note the glasses absorb, as the bases of
> both were quite warm (~ 45 C.)

This situation is best avoided, for two reasons.  Remember, ovens put out
constant power (to a reasonable approximation).  So if you put in samples
none of which has a high absorption coefficient, the power still goes
_somewhere_.  The standing-wave voltage (i.e. the energy density) builds up
to very high levels, until it can attack whatever weak absorbers are around.
 1) If the energy goes into the stem or base of a typical glass, the
consequences can be shattering.  I've seen it done.
 2) Also in this case you don't learn nearly as much about the
sample(s).  In particular the results are hard to compare with other runs
where the voltage was different.

The right strategy is to regulate the "effective source impedance" by
always including a known amount of a good absorber.  That gives you
something closer to a constant-voltage (rather than constant-power)
situation, improving safety and making the results easier to interpret.