will it take off?

[patrick_g]

Spidy and Egon, Some (most) of the microwave builders retained the duty cycle proportioning they had on lower power machines when going to more powerful outputs and at say 1200 watts of cooking power you can overdo stuff before the off portioin of the cycle is reached.

I am not aware of any ovens that turn the power down, unfortunately. They all seem to run full power and duty cycle time average from 10% to 100% in 10% steps. The nicest thing under this scheme is a unit that uses finer steps, i.e. shorter "chunks" of time. Still you get 10-100% in 100% increments but each "on" cycle is smaller so the averaging is smoother and the overcooking is way less. Still less than perfect but better.

Can't guarantee you wont pop a bean though if you aren't using a rotating platform and even then sopmetimes...

Luckilly I like beans that have been popped like puffed rice.

Pat

 

[daTeacha]

Well phooey, to coin a phrase. I thought I had posted a somewhat abbreviated reply to the freezing water experiment, but it somehow got lost prior to actually showing up here. In the interim between that posting and now, you guys have pretty well covered all the things I said and moved on to microwaves, so we just need to wait for the followup experiments.

On microwaves, we have an odd thing going with ours. It seems that often when my wife puts things in there and starts it she gets humming, buzzing, and sparks. I've never had it happen. I think it may relate to the size of the item she puts in the oven, and asked her to note not only portion size but also height. She says it seems to happen when the item is physically short, not sticking up above the plate much. Any ideas about what's going on?

 

[rback33]

Well phooey, to coin a phrase. I thought I had posted a somewhat abbreviated reply to the freezing water experiment, but it somehow got lost prior to actually showing up here. In the interim between that posting and now, you guys have pretty well covered all the things I said and moved on to microwaves, so we just need to wait for the followup experiments.

On microwaves, we have an odd thing going with ours. It seems that often when my wife puts things in there and starts it she gets humming, buzzing, and sparks. I've never had it happen. I think it may relate to the size of the item she puts in the oven, and asked her to note not only portion size but also height. She says it seems to happen when the item is physically short, not sticking up above the plate much. Any ideas about what's going on?

Sounds exactly like what happens if you microwave metal.... don't ask how I know Does it stink like crazy afterwards? Maybe she using a new plate that has some special metal inlay she doesn't know about? I actually put a metal bowl in once... not just a fork or something...

 

[BTDT]

I know if I use cheap styrofoam plate it seems to crackle like rice crispies, don't have a clue why.

 

[patrick_g]

There was a ceramic cup with a metal ring thing in the little finger hole (not visible) but the metal sure reacted with the microwave energy. Suspected the heating couild have been caused by a metal reinforcement in the porcelain and confirmed with a magnet. Was a surprise to have the finger hole a lot hotter than the reheated tea.

Sparks in the microwave oven are almost always due to metal or a fair conductor. Dishes with metallic glazes or gold or silver stripes (typically around the outside edge or such. The microwave energy is just an electromagnetic field at microwave freqs, a simple radar freq transmitter where the food is placed in the "near field" of the transmitting "antenna" (feed horn.) Any fair conductor in the oven cavity, especially metal acts like an antenna and given the the potential for high VSWR (Voltage Standing Wave Ratio) the voltages can be quite high, enough to cause arcs.

So called "Browning Pans" have a layer of a medium conductive material such as vapor deposited metal "fog" on them which conducts enough to take in plenty of microwave energy and has high enough resistance to convert that energy into heat so the surface of the browning accessory gets really hot.

I had a microwave that had a metal rack in the middle. Very handy. The rack would fit into the oven and could be removed, turned around and fit that way too. Unfortunately the one way placed the rack too close to a "hot spot" in the microwave field and the rack would arc over to the back of the oven cavity, burning the plastic a bit and smelled pretty rank. Solution, turn it back around. 50/50 chance of taking it out of the dishwasher and putting it in the RIGHT way.

A test that may be interesting is to put an empty bowl or plate in the microwave oven and also at the same time a glass filled about 1/2 way with water and a similar empty glass. Run the microwave for a few min, stopping when you see signs that the water is really hot (no need to boil it.) Carefully remove the test subject (empty bowl, plate, or whatever) If the test subject is much warmer than the empty glass then the test subject isn't a very good candidate for using in the microwave since it is absorbing a significant quantiy of the microwaves. I have had some test subjects get too hot to comfortably handle and had to rule them as off limits for the microwave.

Pat

 

[daTeacha]

This particular micowave apparently has the magnetotron located in the side, behind a removable plate of some kind. The arcing seems to emanate from the surface of this plate, which has scorch marks on it that look like spots left by a mini welder.

I haven't asked her to check which dish she is using when it happens. The everyday eating plates are cheapo's made in China, so there could be metallic impurities in the clays used. Some of those plates do get pretty hot when heating food in them, but the sparking only seems to happen when the quantities are small or low. I don't think the rotating feature would impact the phenomenon, but this one does rotate the food as it runs.

 

[patrick_g]

Some of those plates do get pretty hot when heating food in them, but the sparking only seems to happen when the quantities are small or low.

That's MAGNETRON vice magnetotron.

If you try the glass of water and empty plate test and the plates get pretty hot by the time the water starts to boil then the plates are definitely not the best for the microwave.

The low food issue seems to be consistent with the plates that absorb too much microwave and are lilely to contain conductive elements that cause VSWR problems for the oven.

The microwave energy would have to penetrate the food in the plate to get to the plate. More food would mean less microwave energy getting to the plate.

Try the empty plate and half a glass of water test and report the results. If the plate gets hot at least in some places then it is a bad choice for using in a microwave and over time could damage the oven (and burn your fingers?)

On the other hand I have seen new microwaves for under $40 so maybe you'd rather use the cheap plates and take your chances with microwave longivity.

Pat

 

[daTeacha]

Those same plates get hotter under some foods than others, even when heated at the same time. Reheating rice gets the plate a lot hotter than the chicken on the same plate.

 

[patrick_g]

Da Teacha, Well of course they do. Different foods have different interactions with the microwave energy. Stated in science terms... the exticition coeficient of foods varies in a complex manner dependent on among other things the water and fat content of the foods. The depth of penetration of the microwave enery is a decreasing exponential in a homogenous medium.

A large ham with the outer fat rind trimmed away will not have microwaves penetrate deep into the center and the center will cook because the ourter layers are hot. With the thick ouiter fat rind left in place the fat will be heated tremendously and the microwaves will not penetrate it very much.

The statement that microwaves cook from the inside out is not always precisely true. It is true in some instances. If you put a marshmallow in a microwave it will be charred to carbon in parts of the center by the time you see any visible evidence of cooking on the outside. I discovered that roasting/toasting marshmallows in the microwave was too delicate of a control problem for the course power settings and duty cycle time intervals or my limited culinary skills.

Rice has little fat and lets microwave energy pass through a fair layer with not so much absorbtion as you'd get with fat meat. Chicken absorbs microwave enery more readily than rice but whether it gets hotter faster than rice is a calorimetry problem relating to several factors. IF the chicken and rice were "MAGICALLY" suspended inthe center of the oven the chicken would get a lot more energy than the rice.. The hot chicken can heat the plate it is sitting on but probably not aas fast as the microwave energy will heat a "BAD" plate under the rice which lets eneergy pass through.

Microwave ovens run at about 2.45 GigaHertz which is selected to interact with the polar water molecules. I don't recommend you try this but if you put a half a glass of gasoline in the microwave it would not heat well at all since its molecules are long carbon chains and are not polar and are not resonant at 2.45 GHz.

If you like microwave experiments you can put a burned out tungsten light bulb in the microwave and it will light up when the oven is turned on. At least it will light up prior to exploding. Please don't do this at home kids without trained adult supervision, ANSI approved safety glasses and face shield and viewed preferably over CCTV!

Pat

 

[daTeacha]

How serious is the explosion, really? I would guess the oven door would contain it. Then again, the internet video clips of the diet Coke bottles becoming bottle rockets after adding some Mentos were pretty impressive.

Around here, "Amish Bombs" are pretty popular with the mailbox destroying set. A little HCl from the grocery store, a little aluminum foil, an empty 2 liter, and you're all set.