cheng@imsvax.UUCP (Steve Cheng) (01/21/85)
Usually we, micro-wave oven users are told not to put metal into the oven. Can
anyone give me the reason? Also inside the micro-wave oven, there is a rack
which is made of metal. Is it a special kind of metal or has it peculiar
shape which can bounce the micro-wave around? Thanks in advance!
--
Steve Cheng
UUCP: {umcp-cs!eneevax || seismo!rlgvax!elsie}!imsvax!chengnemo@rochester.UUCP (Wolfe) (01/23/85)
> Usually we, micro-wave oven users are told not to put metal into the oven. Can > anyone give me the reason? Also inside the micro-wave oven, there is a rack > which is made of metal. Is it a special kind of metal or has it peculiar > shape which can bounce the micro-wave around? Thanks in advance! > Steve Cheng Before we got a u-wave oven, I had always heard that one should NEVER NEVER NEVER put any metal in it. Ours came with a metal rack, and metal interior (with little ceramic pieces to hold the rack away from the sides), and instructions regarding metal in u-wave ovens. Metals reflect microwaves (kind of like they do with radar), so you want to make sure that there is plenty of food to absorb the microwaves in a place where they'll hit. Also you shouldn't allow metal to touch metal. We have a ceramic plate that fits on the bottom of the oven, and frequently use a sheet of Al foil to heat up nachos on. Don't put metal over the top of the food or it will bounce the waves back to the magnetron, burning it out (so sez the booklet). Nemo
eac@drutx.UUCP (CveticEA) (01/27/85)
It is my understanding that metal is much more excitable by microwaves than other materials, and will become hot VERY fast. For example, a friend of mine forgot to take a twist tie with a thin metal wire in it off of a plastic bag of bread. The plastic burst into flame when the twist tie rapidly became red hot. Another point--ceramics with a high metal content (such as iron) will absorb most of the microwaves instead of the food they contain, hence are not suitable for microwave cooking. You can test your dishes and bowls if you aren't sure of their content by filling them with water and m-waving them for 1 minute. If the bowl is as warm or warmer than the water, it probably shouldn't be used. Betsy Cvetic ihnp4!drutx!eac
boris@mit-athena.ARPA (Boris N Goldowsky) (01/27/85)
> Usually we, micro-wave oven users are told not to put metal into the oven. Can > anyone give me the reason? Also inside the micro-wave oven, there is a rack > which is made of metal. Is it a special kind of metal or has it peculiar > shape which can bounce the micro-wave around? Thanks in advance! > Steve Cheng I was told a story about someone putting an aluminum-foil covered saucepan in the oven and watching a current arc from one side of the oven to the foil and across to the other side. I'm not sure whether I should believe that story, but I haven't put metal in a microwave since! --boris
bmt@we53.UUCP ( B. M. Thomas ) (01/28/85)
Power reflection is the problem. Conductors reflect electromagnetic waves and can reflect it back to the magnetron, which will start to arc and/or overheat and/or draw large currents from its high-voltage supply. The reason that the rack can be in there is likely that it is (a) placed in such a location or (b) is of such a construction as not to reflect a significant amount of power back to the device. I am sure that there are carefully worded instructions on the use of this rack in the manual. For the same reasons, they tell you not to operate the oven empty. With nothing in the cavity to absorb the output of the magnetron, the waves will simply be reflected back by the metallic walls, with damaging results.
jeff@rtech.ARPA (Jeff Lichtman) (01/30/85)
> Usually we, micro-wave oven users are told not to put metal into the oven. Can > anyone give me the reason? Also inside the micro-wave oven, there is a rack > which is made of metal. Is it a special kind of metal or has it peculiar > shape which can bounce the micro-wave around? Thanks in advance! > > -- > > Steve Cheng > > UUCP: {umcp-cs!eneevax || seismo!rlgvax!elsie}!imsvax!cheng Any electrically conductive material will act as an antenna. Food is a poor conductor, so it doesn't place much of a load on the oven. The effect is like putting a high resistance across a light socket; the resistor will get hot, but you won't blow a fuse, because the resistor won't draw much current. Metals are very good conductors, so putting metal in a microwave oven is like shorting out a light socket; it can burn out the source (if the metal doesn't burn up first). I'm not sure how metal racks and other metal microwave accessories work. It's possible that the food shields the rack from the source of the microwaves. -- Jeff Lichtman at rtech (Relational Technology, Inc.) aka Swazoo Koolak
tjarks@ihlts.UUCP (Tim Tjarks) (01/31/85)
I attended a microwave cooking class the other night at Friedman's in Aurora, Illinois. The instructor used metal in the oven (such as a fork left in a measuring cup for stirring eggs). He said this is possible if there is more food than metal. He also cooked TV dinners by removing the foil cover and putting the dinner back into the box for cooking. No need to dirty another plate or use wax paper or plastic wrap to cover. He even cooked frozen vegetables without removing them from the box--just freezer to microwave to serving dish. Pretty useful if you don't have a lot of microwave-safe dishes to use. Ellen Tjarks -- Tim Tjarks AT&T Bell Laboratories ihnp4!ihlts!tjarks
seifert@mako.UUCP (Snoopy) (02/01/85)
In article <119@rtech.ARPA> jeff@rtech.ARPA (Jeff Lichtman) writes: > Food is a poor conductor, so it doesn't place much of a load on the > oven. The effect is like putting a high resistance across a light > socket; the resistor will get hot, but you won't blow a fuse, because > the resistor won't draw much current. > Metals are very good conductors, so putting metal in a microwave oven is like > shorting out a light socket; it can burn out the source (if the metal doesn't > burn up first). > > I'm not sure how metal racks and other metal microwave accessories work. It's > possible that the food shields the rack from the source of the microwaves. >-- >Jeff Lichtman at rtech (Relational Technology, Inc.) >aka Swazoo Koolak Perhaps you could enlighten me, when was Ohm's law repealed? I assume that rtech hired you for software or some such rather than your extensive knowledge <cough> of electrical and thermal systems. Metal reflects microwaves. Food absorbs microwaves. The microwave energy is transformed into heat energy. V=IR P=VI F=MA Whether the resistor gets hot or not depends on the heat dissipation verses the amount of power, not on whether the resistor is a high value or a low value. The current does affect the power (P=VI), but some resistors can dissipate a lot of power without getting hot, while others can't, hence the "power rating" of a resister. Further discussion in net.analog or some such place, please. (you too can compare capacitors with batteries!) I promise to someday post my pizza recipe. _____ |___| the Bavarian Beagle _|___|_ Snoopy \_____/ tektronix!mako!seifert \___/
chip@t4test.UUCP (Chip Rosenthal) (02/02/85)
Metal can be used in a microwave...if used smartly. The classic use is to shield portions of poultry from overcooking. For example, I've softened frozen juices in a microwave. (You know, the ones in cardboard cans with metal ends.) I removed one of the can's ends first, and all went well. Three caveats have been mentioned in other articles which are true: 1) Make sure there is load for the microwaves. If you cover something totally with foil there will be nothing to absorb the energy. 2) Keep metal away from the metal sides. The foil-covered leg of my last turkey got a little close to the side and started arcing. I've got the zap marks left behind to prove it. 3) Be careful what the metal contacts. It can get hot, and as Betsy (eac@drutx) pointed out can start fires. Betsy also pointed out a method for testing the microwavability of utensils... > You can test your dishes and bowls if you aren't sure of their content > by filling them with water and m-waving them for 1 minute. If the > bowl is as warm or warmer than the water, it probably shouldn't be used. I'd suggest one slight change. Put the water in a known-microwavable utensil (e.g. your glass measuring cup) and see if the UUT (utensil under test) gets warm at all. While we are on the subject of microwave utensils, I've always had a preference for glass. But recently I got a 5qt plastic microwave pot. At first I was worried about staining from tomato sauces and such, but it seems to come clean very well. I'm ready to switch from glass to plastic. Does anybody else have any obervations on glass and plastic microwave cookware. (How about wooden cookware :-) In closing, I leave you with the words of my former antenna's prof: "What is the impedance of a potatoe?" -- Chip Rosenthal, Intel/Santa Clara {cbosgd,idi,intelca,icalqa,kremvax,qubix,ucscc} ! {t4test,t12tst} ! {chip,news}
tfitzger@uokvax.UUCP (02/08/85)
From what I've been told, the metal will reflect the microwaves, which
is not a good thing. Blue sparks will start shooting out of your oven,
and the gun which generates the microwaves will be ruined. Your rack
is probably made out of porcelain, not metal. This won't reflect the
microwaves, but will absorb them instead. This is also how your food gets
cooked, by absorbing the microwaves and not reflecting them.
Tim Fitzgerald
{allegra,ihnp4}!convex!ctvax!uokvax!tfitzgereac@drutx.UUCP (CveticEA) (02/10/85)
Very few tupperware-like bowls are safe to use with sugary or oily foods such as spaghetti sauce. The reason for this is that the plastic can't handle the intense heat that will build up on the outer edges of the food. The plastic will blister and sometime even deform. (I have a rubbermade bowl to prove it! Microwaves don't penetrate straight to the center, they heat the outside inch or so (depending on the food) and the heat then is slowly spread to the center (hence turning and resting the food between blasts). If you are going to heat something up in non-mwave proven plastic, stir it frequently, or if it isn't stirrable, stop it often and let the food sit for a minute or so. Some plastics are made just for micro-waves and the manufacturer will say so. Unfortunately, some of the microwave ware like the line sported by tupperware, is very expensive. It actually costs less in some instances to by Corningware, which is much more versatile--can be used on the range top and in the oven as well as the microwave. (And it looks decent enough to be a serving piece.) Betsy Cvetic ihnp4!drutx!eac