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Posted: Mon Oct 29, 2007 10:42 am
by MrAl
Hello again,

Ha ha, yeah. I have to add one other thing here too now.
The pot will have spagetti in it too. What happens is that
if i go away for 2 minutes when i come back the pot has
boiled over and the water gets all over the bottom of the
microwave oven and takes a while to clean up with rags.
I was hoping to find some way to avoid boiling over but
also avoid standing there and watching for some 5 minutes
because as you say it's hard to stand there for 5 minutes
with nothing happening.

I would try a steam whistle but where do i get one, or
alternately, can i build one with plastic or something?

The humidity sensor sounds reasonable. I might look into this
idea more to see what i can come up with.

Yes, that sounds good. The air comes out the back and it is
definitely warm and humid too as the stuff cooks inside. i
could easily mount something back there and it would be out
of the way too. This sounds like the first attempt i will

That's a good idea too for when i only boil water, but i realize
now that when i have the problem is after the spagetti is put
into the water and it boils over. The spagetti would get in
the way of anything floating on the surface so it would not
contact the sides too easily, but i guess a bell might work
if i could find one. Where do you get a glass bell?

i am thinking that my first attempt will be with a humidity sensor,
but im not sure if i want to go to a high voltage scheme. I'd like
to stick with low voltage if possible, like 12v (ac or dc) or less.

I'll have to see if i have an old sensor around. I think i threw
out most of my old old parts junk already though last spring cleanup
time. If possible i want the thing mounted outside the oven too.

I guess that could work too, if the beam gets through the front
glass door without having a problem. I sort of would like to
build something up instead of a purchase though ha ha.

Posted: Mon Oct 29, 2007 12:00 pm
by sparkle
MrAl wrote:...sparkle:
i am thinking that my first attempt will be with a humidity sensor,
but im not sure if i want to go to a high voltage scheme. I'd like
to stick with low voltage if possible, like 12v (ac or dc) or less...
My megohmmeter can apply from 1 to 999VDC, I had it cranked all the way up to sweep any ions as fast as possible to minimize lag. Trying it now dialed down, it detects moisture breathed onto new currency quickly just fine with the 1 volt setting. AC would be even better to avoid electrode polarization/plating effects long term. But I haven't seen any sensors rated high in sensitivity for water while rejecting spagetti sauce aerosol buildup, let alone all other cooking grease vapors...I wonder if the humidity sensors built into some ovens can be bought reasonably priced as replacement parts? :grin:

Posted: Mon Oct 29, 2007 1:04 pm
by MrAl
Hi again sparkle,

Yes that's a good idea too.
I think i will try the thermistor outside the oven first though cause that
will be easier and i already have software to use thermistors to measure
temperature so i could set this up within fifteen minutes.
There may be a good enough correlation between the air temp coming
out and the time just before it starts to boil, then i can watch it for
hopefully less than 60 seconds to see it start to boil.

If that doesnt work i'll have to experiment with humidity sensors.
I found one here

but it sure isnt that cheap!

Posted: Mon Oct 29, 2007 1:30 pm
by CeaSaR
Ok, my 2 cents.

Years ago, the little woman and I found a small device that pretty much
curbed the major boil over that plagues "spaghetti cooking/boiling". It is a
small double concave glass disk that is placed in the water before boiling
that traps some of the bubbles during boiling, rises to the top and breaks
the surface tension of the water/starch mix, thereby preventing
nasty "splortches". I've never used it in the microwave, but it should work
just the same.

Now for the electronic part: the glass disk makes small banging noises
when it is working properly during boiling. Use a microphone to sample
the microwave oven's sound, compare it to it's normal sampled sound
(stored in the hardware) and sound an alert when it detects multiple
differences in the samples over a pre-determined time period, say 5 to 10

As for where you can find the disk, you might try a place like Lechters or
some other place that sells mostly kitchen stuff. It's been so long ago that
I don't remember where we bought it.


Posted: Mon Oct 29, 2007 4:39 pm
by haklesup
Ceasar, I have never seen such a disk. It sounds like a lens made perhaps of Pyrex. Interesting approach to coping with the properties of sauce. I suspect such a product to be susceptible to chipping thus making it unpopular. Perhaps a ceramic or high temp plastic.

MrAl, The temp probe that I had with an older oven (still looking for it as I am sure I would have saved it) looked like a metal rod with a point on the end. It had a plastic handle at the hilt and a cable was shielded with a braided metal jacket. It terminated in a 1/4" mono jack (think headphones) into the wall of the oven. As you may know, not all metal is excluded from a microwave oven. Mostly you need to avoid planar surfaces that eddies can be established in and loops of metal. Thin linear and somewhat curved metal objects can be in the oven. The whole wall of the oven is a suitable grounding spot to attach a shield to protect a two wire cable to the probe.

I have a similar probe that came with a more recent remote BBQ meat thermometer that would also work IMO.

I looked in Digikey and they have a few humidity sensors in the under $20 range that would work when placed in the exhaust air stream of the oven.

Microvaves may cook from the inside out but that spot inside the food is usually not the middle. More often than not it is near the surface and most of the heat is conducted into the food just like in conventional cooking. The less water you have, the deeper the microwaves can penetrate. (bread is a worse case because as temp rises the water moves leading to uneven heating and moisture distribution of the cooked food)

For sauces and spagetti, it is usually better to cook at lower power like 50% (or lower). Since the sauce is very viscous, it does not move the heat around well by convection and due to the high density and nonhomogeneous nature of that food, conduction is also hampered. As a result, the energy from the microwaves is absorbed by the water nearest to the surface and the heat fails to dissipate deeper into the food fast enough. heat builds in the outside layer quickly leading to boiling at the edges and a cold center. Lower power allows conduction of the heat to the center to catch up. For very viscous sauces only stirring makes up for the lost convection.

Assuming you had adequate mixing or you were already cooking at low power then you really are looking for the boiling point of the whole pot (not the point at which it begins to spatter and needs stirring). Humidity sensing will be most reliable. Temp sensing may be fooled by uneven heating of the sauce.

I might consider placing the temp probe in a rigid configuration protruding down from the top and off center. This way it both senses and stirs the pot, increasing convection.

Posted: Mon Oct 29, 2007 8:01 pm
by Externet
Hello Mr. Al.
Seen those crystal bells in novelty stores, about 2", with their 'tongue' hanging with thread. Don't know the store name, they carry tons of crystal and porcelain adorns and gadgets.

Posted: Mon Oct 29, 2007 8:35 pm
by CeaSaR
Ceasar, I have never seen such a disk. It sounds like a lens made
perhaps of Pyrex. Interesting approach to coping with the properties of
sauce. I suspect such a product to be susceptible to chipping thus making
it unpopular. Perhaps a ceramic or high temp plastic.
It was some sort of glass, probaly pyrex type, with a "rim" (for lack of
better terms) on both sides and the center indent was slightly convex.
Never had a problem with chipping, but when it got lost in the sink under
the pots n pans.... THAT'S when it broke. Never did look for another one.

It worked on the same principle as the little baking powder submarines or
even the air powered aquarium toys where the bouyancy of the air/gas
bubble causes something to rise until the air is released.


Posted: Mon Oct 29, 2007 8:52 pm
by Dimbulb
My reply is to the original post.
an optical sensor that detects the presents of rising bubbles.

Posted: Tue Oct 30, 2007 10:39 am
by MrAl
Hello again,

I noticed something else too now. The boiling that occurs is
not like pure water boiling, in fact, with the spagetti in there
it acts more like wet lava. The bubbles sort of ooze out rather
than boil over violently, and this carries water with it and out
all over the turntable of the microwave and eventually on the bottom
of the oven too. This means im not so sure how something would
work that depends on violent boiling to make a sound. I am also
not too keen on letting glass smack up against glass to make the
sound. On the other hand, if i knew where to get one i would
still probably try it after i try the temperature probe (cause i
have everything for that here). I dont think we have that store
around here.

Ok it sounds like i'll have to build something up and test it out.
I guess if it arcs over it's no good :smile:
$20 doesnt sound too bad i guess, perhaps that's the easiest way to
go. I have software i can use to read the thing too already with
little modification.
Just to note, i dont have a problem with the sauce as i either put that
on very low setting (power 2 which is like 20 percent) or i just heat it
for 22 seconds and stir, another 22 seconds and stir, etc., until hot.
It's the spagetti in the water that causes the biggest pain because now
and then i get tired of watching it and go over to the computer to finish
something else up and when i come back there is water all over the oven
again ha ha. I want to avoid this without too much worry that it will
boil over and without standing there forever watching it.
Im not sure i can mount the probe as you say however, because i must
keep the top on the pot in order to get it to boil within a reasonable
amount of time. Without the top it might not boil for a very long time
or not at all (i have tried this). I like the idea of having it stir
as well as measure, but unfortunately i cant do it that way because of the
great heat loss with an open top. I also dont want to go through trying
to drill a small hole in the center of the pot so the probe can be inserted
without bothering anything while the pot rotates, so i guess my options are
limited to either probing outside the mic entirely (preferred) or inside
the oven close to the pot. I did notice a little steam comes out between
the pot side and top when it gets hot, so maybe mount the probe close to
the top edge of the pot so it senses steam there.

As i was saying above, im not sure now if the bell would work or not
but i guess it would be something to try if i knew for sure where to
get one and they are not like 20 bucks each ha ha.

Yes, i was thinking something like that too, as i think the slow bubbles
would deflect the light and reduce the total light reaching a sensor as
the water started to boil. I guess it's a matter of mounting everything
and testing it out. The nice thing about the humidity sensor is that i
can probably mount it outside the oven. They do have a bit of a slow
time response however, so i would have to see if that bothers the measurement
or not too.

Posted: Tue Oct 30, 2007 1:21 pm
by haklesup
MrAl, If you are specifically trying to cope with the troubles of boiling just the pasta noodles, I think I am getting you now. Even on a conventional stove top, you really can't use a lid on full boil or the starch will foam up and run over the sides. With a lid off, foaming is less of a problem but you still have some splatter from boiling bubbles bursting. On a conventional stove, we just turn it down until the boil is low and you can either leave the lid off or put it back on.

In a microwave, most brands use on/off for the power level so it is hard to maintian just the right boil in that kind of oven. (frankly I have always cooked pasta in a pot on a stove). Panasonic is the only brand I know that actually has a switching inverter driving the magnitron and thus has true continuous power adjustability.

Try using a paper plate with a dish towel on top instead of the lid and cook at 80%. This should lower the vapor pressure, the temprature and give a place for slight boil overs to be sopped up.

If I had to guess, starch foaming has to do with the rate of evaporation from the top surface of the foam bubbles. With the lid on, the humidity remains at 100% and the bubbles persist and build up into a foam. But with the lid off, the bubble film evaporates quickly enough for them to burst without building a thick foam which can lift the lid.

Most Pasta can be cooked by just bringing the water to a brief boil then maintaining a simmer for 15 minutes or so (see hamburger helper directions). Another tactic might be 100% for 4 minutes followed by 30% for 14 more minutes. With lid or no lid.

Having a lid or not will not change the time it takes to heat the volume to the boiling point. It will however make the starch foaming worse and make the boiling more apparent.

Posted: Tue Oct 30, 2007 2:57 pm
by CeaSaR

You are spot on about the starch in the water causing foam. That is what
really causes the big mess! A small trick I have found is to blow on the
boiling pot to reduce the bubbles/foam. Try it next time you are cooking
on the stove. Perhaps a small fan can be placed at a stragegic point to
blow over top of the pot to break up the surface tension, thereby
preventing boilover.

Surely there must be a way to convert some of the
microwave energy back into electricity to run a small DC motor and
plastic fan blade, maybe even mold it into a corner or commandeer a
small portion of the vent screen. If you could do it through the vent
screen, everything would be safely outside the cooking area. You could
even recirculate some of the air from the vent so as not to lose too much
heat. Hmmm, sounds a bit like a convection oven...


BTW Al, Howdy neighbor! PA and NJ are only brothers separated by a little water.

Posted: Tue Oct 30, 2007 3:30 pm
by MrAl
Hi again,

Yes my oven has the power settings too, from 1 to 9 i think, and i have
used them from time to time for other things. I dont have the inverter
type oven but i have priced them, so now you know why i dont have one :smile:
So you are saying rather than go with a sensor/control circuit to try
some different cooking techniques altogether? Well, that sounds reasonable.
I have tried cooking for something like 5 mins and then 11 mins at power 3,
but i prefer to do it this way:

Bring water to boil
Add salt, stir
Add spagetti, stir (this cools the water down)
Put on high for some minutes until boil
Put on power 2 or 3 for about 10 mins or so.

Somehow the spagetti seems to come out good this way.

But knowing what control freaks we are that we want to control the
world with our computer and a couple 99 cent sensors you still want me to
try other techiques ha ha? Well ok, i guess i will do that first.
I can also monitor the temperature coming out of the oven with
one of my indoor outdoor store bought thermometers (WalMart special).

Im not sure i would want to allow the pot to boil over and catch the
water/mess with a towel however, as the secondary effect is that the
amount of hot water in the pot is reduced which makes the spagetti
come out all sticky from the residue.
As i was saying before though, i cant leave the top off because it
will take way too long to boil in my particular oven and i have tried
this many times before with various size and types of pots. With the
pot i use now (big enough for the amount i need to make) it really does
need the top on it, although i could try leaving it partly on or sticking
a popsicle stick under the lid to leave some small gap to vent a little.

When you say that having a lid or not does not change the time to boil,
well that just isnt true for some reason with my oven and my pot. i dont
know what pot / oven you are using though.

BTW i dont have a problem with the water itself boiling, just after
the spagetti is put in it foams up and gets all over if not stopped
in time, and that time between not foaming and heavy foaming is
really really short so it's hard to catch without some sort of
monitor, which is what i was seeking to find.

I dont know if i could build something like that. It sounds harder
to do then the humidity sensor.

Posted: Tue Oct 30, 2007 4:48 pm
by haklesup
Caesar, Blowing on the foam increases the evaporation on the surface of the bubbles thus causing them to break sooner. Its a similar effect to beer and soda foam with a slightly different evaporation time coefficient.

MrAl, as long as you insist on using the lid, you will get foaming per my discussion above. The paper plate was really a compromise between having one and not. A solid lid also slightly raises the vapor pressure above ambient which I think also contributes to the foam formation (though I still think evaporation rate dominates)

Though I have not boiled much water with and without lids (in a microwave), I cannot say for sure that it has no effect but I wouldn't expect it to be very great. (excluding the foaming effects of the starch). I do think though that noodles cook at about the same rate in vigorously boiling water as they do in water kept just at boiling but not rolling (simmer). It is not necessary to maintain a boil for the whole time.

Getting away from cooking technique and back to a technical solution, you may want to back away from envoirnemntal sensors of temp and humidity and develop a foaming detector and attack the problem head on. Basically an adapted leak detector you might use in a wet basement.

I think a sensor comprised of two parallel electrodes (bare wires) held perhaps 2 mm apart would work. With no substrate between and spaced more than a typical condesation droplet is sized, it should not false trigger on steam or humidity (condensate dosen't conduct well anyway). With a little salt and starch in the water, the foam should be conductive enough to pass current to an op amp or transistor trigger circuit when it contacts both of the wires. The sensor can be affixed to the inside of the lid with a spacer (silicone RTV)

Pretty simple but you still need a pair of wires going to the lid. The sensor could be wired to trip a relay which is in series with the door interlock. This would stop the oven requiring a press of the start button to proceed. Wiring this relay in series with the magnitron somehow (or the relay on the control board that controls it) you can get auto shutdown when foaming is detected without need to manually restart.

I'm not entirely sure what stray signal the sensor would pick up from the microwaves. This could confound efforts to use the signal in a detector circuit. You may need to modify the concept with either a metal shield to prevent it from acting like an antenna or reducing the sensor contact area to a pinhead so that it is smaller than the wavelength and forms a poor antenna. The solution may be as simple as a capacitor to ground at the point where the cable exits the oven to shunt any AC to ground while preserving the DC sensor signal and a series resistor to prevent aquired charge from blowing the input.

One last comment, It should not be necessary to rotate a pot of mostly water. The rolling of the boil should provide plenty of convection in the water to spread the heat. Rotation is for foods with little or no convection internally which is anything solid.

BTW, my oven is 1500W so water boils easily with or without a lid.

The smaller countertop ovens from Panasonic (and maybe magic chef) with the inverter technology are only a little more than basic models. Its the over the oven models with higher power, sensor cook and other features that starts to get very expensive. I also read that early inverter ovens had some reliability problems that put a taint on their reputation. They had a high rate of failure when they just got out of warranty.

Posted: Tue Oct 30, 2007 4:51 pm
by CeaSaR
Well, at least it was electrically related! :grin:

Per previous suggestion: using a regular cloth towel in the microwave is
not a good idea. Cloth towels tend to get serious hot spots that can lead
to actual smoldering, even when damp. Not a pleasant smell. Paper
towels, on the other hand, tend to resist this phenomenon, especially
when they are damp to mildly wet.

So your major boil over problem is when you add the pasta? You say that
after boiling you add salt and stir, then add pasta. How vigorous was the
boiling? When you add the salt and stir, does the water have any reaction
to that?

If your "boil" wasn't vigorous, you are dealing with superheated water.
The water hasn't had a chance to release the energy built up in it during
it's journey toward boiling. This isn't scientific, just 23 + years of
observing said phenomenon. Next time you cook spaghetti, try using a
different type of spoon to stir the water, say, wood if you use stainless or
vice versa. If that does nothing, try dropping in just a few pieces of pasta
to act as a catalyst to start the boiling. Only using a few pieces reduces
the amount of surface area and starch released during this critical time.
Once the boil has gotten past the first violent period and settled down,
then you can add the rest of the pasta without fear of the dreaded "foam".

I know, not an electronic solution, but why reinvent the wheel?


Posted: Tue Oct 30, 2007 5:11 pm
by CeaSaR
BTW, to dispell an old wives tale, microwaves do not cook from the inside
out. The converse is the true reality. I won't go into it here. For further
information see: ... es+cook%3F
and the first listed was
with many more below that.