Re: superheated water

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

 From your demo. below I suspect that the boiling from first heating "new" oil is
superheated water.

I disagree with some of the statements below:

John Denker wrote:

> At 09:48 AM 2/4/00 -0500, Michael Edmiston wrote a very nice note, of which
> the focus was:
> > But my main point remains... water being heated in a microwave
> > does not set up convection currents because it is heating from the top down.
> > This is what I see in three different microwave ovens I have tried.

I suggest one try cooking a bunch of potatoes in an oven without the carousel (as
appropriate).  If it doesn't have a carousel,. it likely has a false roof. Remove it and
disable the "fan", thereby you will have a resonant cavity with lotsa "fixed" nodes and
antinodes.  The potatoes will detect them nicely by being hard throughout most of their
volumes with very cooked spots.  This assumes one doesn't cook for more than a few minutes
- depends on the power of the oven.  From the spots, I think most will agree that the
microwaves don't cook just from the top.  If you still don't believe me put, RT. water in
a tumbler in the oven.  Aspirate a small amount of fountain ink (I used jet black -
permanent) into a hypo needle -- wait -- wait some more, then "inject" a blob of ink near
the bot. of the tumbler. watch for a few moments -- the blob will let you know you didn't
wait very long -- no matter, close the door and fire up!  In all my trials, long before
the water had any bubbles, it was uniformly inked except for a thin layer on the bot. of
the glass.  Warning don't get cataracts  -- it's like a Lava Lamp.

> > Microwaves do not penetrate as far into water and into food as many people
> > think they do.

I don't know how far people assume they usefully penetrate -- I suppose many have tried to
defrost > two inch fish fillets and discovered that a practical thickness is one inch.  Do
what I did.  Make a double Dewar with two sizes of expanded polystyrene cups.  Double them
for added insulation.  Support the smaller inside the larger using a non conducting (micro
wave non absorbing) spacer, e.g. plastic bottle top.  Cut the proud one down so that
neither is.  Fill both with water nearly to their tops and cap with three layers (I found
sig. transmission through one layer.)  of al. foil.  Wave it.  I did this experiment about
four times and in each case about 1/3 of the total energy was absorbed by the inner cup (
I corrected for their thermal masses).  The thickness of the outer "wall" of water was >
one cm & < two.  From this I conclude that the 1/2 thickness of water is v. ~ one cm.

Note some plastics are not "microwave safe."  Especially melamine.  I have some bowls and
plates that in some circumstances will get hotter than their food contents.

> Right.
>
> 1) It is also worth keeping in mind that given the typical oven
> construction (metal walls) there is guaranteed to be a node at the bottom.

Not any more.  The only oven I ever saw that didn't have a false bottom or carousel. was
my first and it's dated by the fact that it had a mechanical timer!  I had to explain to
the beginning student workers why I had an inverted glass baking dish in it.

> For this reason, I installed a 3cm high spacer (i.e. a flat-bottomed glass
> casserole dish turned upside down) on top of the turntable.  It resides
> there permanently.
>
> I recommend this for routine cooking, and I especially recommend it if
> you're doing experiments.
>
> =========
>
> 2) In the course of today's experiments, I stumbled across a spectacular
> demonstration of superheating.
>
> We have previously discussed the role of air-filled cracks and scratches as
> sites for inhomogeneous nucleation.  It was pointed out that one way to
> turn off this effect is to boil some water in your chosen vessel (to drive
> off the air), let it cool in situ (to dissolve the air out of the cracks)
> and then re-boil it to demonstrate superheating.
>
> Here's another approach.
>
> Warning:  This is _too_ spectacular.
>    Plan A) Don't try it.
>    Plan B) If you must try it, use the sort of blast protection, fire
> protection, and remote manipulation you would use for pyrotechnic synthesis.
>
> You can use _oil_ to turn off the effect of the cracks and scratches.  Put
> a small amount of salad oil in a tall flask.  Swirl it around.  Then add a
> smaller amount of water.  Clamp everything in place.  Heat it gently with a
> fireproof electric heater (not with a flame).  I was able to get the oil to
> 250 F while there were still macroscopic amounts of water in there.  When
> the water boiled, it made loud pop, pop-pop noises and launched geysers of
> hot oil several inches above the surface.
>
> Yikes!
>
> Many variations are possible.  Be careful.