Re: microwave, RF heating

["Bernard G. Cleyet & Nancy Ann Seese" <georgeann@REDSHIFT.COM>]

Yup!  I agree. I'm completely off

First I thought ovens operated in the X band (10 G Hz).  Second, I must stop believing
everything I'm told; my informant about stretching was wrong (I'm not very good at
integrating knowledge - I've used IR spectro-photometers since 1960!).  However note
attachment; what is the cause of the resonances shown?  If it's not simple conduction (
I'm about to perform some xpts. that may settle that issue), then perhaps it's due to
O=O..

bc

more later.

P.s. thanks to the person who found the SH of fatty acids.  That saves my* getting olive
oil over everything.  May be I'll do it anyway.

* note proper use of the possessive

John Denker wrote:

> At 08:26 AM 5/29/00 -0400, Chuck Britton wrote:
> > We had a MicroWave/IR spectroscopist PhD working for us as a
> > long-term sub, a coupla years back.
> > He INSISTED that MicroWave heating is ENTIRELY a conductivity effect
> > and that there are NO significant resonances anywhere near the 2.4
> > GHz household MicroWave frequency.
>
> I'm not ready to concur with quite such a categorical statement.
>
> Pro:  The result of the experiment suggested in my previous email is that
> ice absorbs vastly less power from the microwave oven than liquid water
> does (other things being equal, i.e. when they are in the oven together).
>
> Con:  If you look again at Jackson's 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
> you see that the curve for sea water cruises along with a slope of 3dB per
> octave for many, many octaves;  it is probably safe to extrapolate this to
> oven frequency (2.45 GHz).  OTOH the curve for pure water has a slope of
> 6dB per octave 10^8.5 Hz (and maybe lower) to 10^10.5 Hz, which definitely
> covers oven frequency.  So if the sea water curve is due to conductivity,
> why is the pure water curve different?
>
> Pro:  Meat and soup are probably salty enough that if we ignore all
> contributions other than ionic conductivity, we will get very nearly the
> right answer.
>
> Con:  When heating plain water, for instance when preparing a cup of tea,
> the magnitude and frequency dependence are not what I would expect from the
> conductivity explanation.
>
> Bottom line:  We've seen lots of wrong explanations.  I'm not at all sure
> we've seen a completely right explanation.
>
> =========
>
> I'd like to see more data.  It would be nice to see absorption-vs-frequency
> curves as a function of salinity and/or as a function of temperature.  That
> might make a good student project for somebody.
>
> The conductivity of non-frozen tap water probably varies only modestly as a
> function of temperature.  That's because it contains something like 0.001
> molar sodium chloride, which is 100% ionized under all
> conditions.  Biological tissues are a hundredfold saltier than that.  For
> all these materials presumably mobility is the controlling factor.
>
> OTOH, if you go to the trouble of making really pure water, then the
> conductivity will be a tremendously strong function of temperature.  The
> water itself ionizes at the level of 10^-7 molar at 25C (pH = 7), ranging
> from pH=7.5 at 0C to pH=6.1 at 100C.  That's well over an order of
> magnitude in ion concentration, which has a correspondingly huge effect on
> conductivity.

Attachment: microwave abs. by O2 & HOH
Description: Unknown Document