Oil in a microwave oven

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Born2bwire

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The problem with using full power on 100% duty cycle for defrosting is the fact that you'll end up cooking the outside and having raw middle (mmmm... convenience store burritos). Don't forget that ice absorbs much less heat from the microwaves than liquid water. As the microwave penetrates the target, it gets absorbed as it transmits but most of its energy will be absorbed at the surface and outer volumes. So the outside will melt first and then you will get a snowball effect as the melted water on the exterior will absorb even more heat than the icy inside.

It's a balancing act of heating the target but in a way that is as even as possible.
 
I just spent the last 15 minutes reading this thread instead of doing my homework. Fascinating thread.

I don't mean to hijack but I have a question thats always bothered me. Why use 2.45 Ghz? Why not 2.44 or 2.46? What is so special about 2.45 that its used exclusively in microwave ovens? Is it because its optimized to heat water when its at room temperature?
I just spent the last 15 minutes reading this thread instead of doing my homework. Fascinating thread.

I don't mean to hijack but I have a question thats always bothered me. Why use 2.45 Ghz? Why not 2.44 or 2.46? What is so special about 2.45 that its used exclusively in microwave ovens? Is it because its optimized to heat water when its at room temperature?
I'm not sure why that particular frequency was chosen - could have been convenience - that was the magnetron that was being used in 1945 when the chocolate melted in Percy Spencer pocket and led to the concept of heating foods whit microwaves. But, let's look at it from a larger perspective. The electromagnetic spectrum is made up of all sorts of radiation and the part we are interested in - the microwave frequency band (300 MHz to 300 GHz) is used largely for communications: radar, cell phones, TV, FM, etc. In order to prevent devices from interfering with each other (otherwise talking on your cell phone might knock a plane out of the sky), the governments of the world have divided and assigned the spectrum into bands for use. In our case, microwave ovens fall into the ISM (Industrial, Scientific & Medical) band - there is another industrial microwave frequency at 915 MHz (33 cm wavelength) that is used for industrial heating things like the bacon on McDonald's Breakfast Sandwich on huge tunnel microwave ovens - 60 to 100 feet long.

Back to 2450 MHz - there is a bandwidth around it, so magnetrons can operate safely at 2400 to 2500 MHz. As to "is it best for water" - not really, but it does a good enough job. Also, we heat lots of other stuff in the microwave, so it has to deal with all of these and it does that pretty well. Another important thing is the size and dimensions of the oven cavity - the metal box the food goes into. they should be multiples of the wavelength in air: 12.2 cm (4.8 inches) - I won't attempt to explain that - it's complicated and would take a real treatise. But that wavelength is practical for a countertop oven. Many years ago GE introduced an oven operating at 915 MHz, but its 33 cm (13") wavelength made it impractically large. It was withdrawn.

One final thing - it's expensive to produce a totally new magnetron at a new frequency - probably a million dollars or more; but the magnetrons used in your home are cheap - $ 8 to 10 because at least 20 million are made annually.
 
Two more for the guru -
Why does metal foil spark when put in the microwave.
Potential builds up at the sharp edges? If yes, how?
Yes, there is a potential because the electric field induces a current in the metal that disappears when you shut off the oven, thereby removing the electric field. The metal is not like a capacitor that can store the potential. The metal doesn't discharge easily in the oven because the air acts s an insulator. The breakdown voltage of air under standard pressure conditions is 10,000 volts/cm. If you put the metal near a wall (say a millimeter) it can discharge and actually damage the wall. Arcs by themselves are not dangerous, but can be very dangerous if they ignite paper, pit the oven wall, etc. because they represent temperatures of many thousands of degrees.
 

RonL

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Are there books or websites that can serve as good resource points in using microwave components outside the application of an oven or cooking food ?
I have cannibalized about 15 microwave units, not knowing all the facts of how to keep things in control and a safe enviornment for the system, has kept me from using anything to date.
One goal was to have just a few mililiters of water flashed to steam in a closed cycle system, but the heavy transformers defeated the project as first considered.
 
Are there books or websites that can serve as good resource points in using microwave components outside the application of an oven or cooking food ?
I have cannibalized about 15 microwave units, not knowing all the facts of how to keep things in control and a safe enviornment for the system, has kept me from using anything to date.
One goal was to have just a few mililiters of water flashed to steam in a closed cycle system, but the heavy transformers defeated the project as first considered.
Hi RonL - I understand your desire to experiment - I did something similar starting in 1961. Looking back, a lot of what I did was loony and dangerous, but I didn't know about the danger at the time. So I managed to invent lots of hings and be killed or injured. Now I know more and I avoid what you are suggesting. But I do have a good friend and colleague who is the most innovative guy I know in this area. But, he is an electrical engineer from UK who has an incredible # of patents all invented and owned by himself. He is forever tearing apart and rebuilding ovens.

Here's what I suggest:
http://www.gallawa.com/microtech/dataselect.html Jihn Gallawa has a CD that you'll find worthwhile.
http://www.gallawa.com/microtech/ Similar
Google and search: microwave oven: repair and parts

Goodluck - but be very careful - Magnetron voltages are lethal!
 

RonL

Gold Member
1,090
214
Hi RonL - I understand your desire to experiment - I did something similar starting in 1961. Looking back, a lot of what I did was loony and dangerous, but I didn't know about the danger at the time. So I managed to invent lots of hings and be killed or injured. Now I know more and I avoid what you are suggesting. But I do have a good friend and colleague who is the most innovative guy I know in this area. But, he is an electrical engineer from UK who has an incredible # of patents all invented and owned by himself. He is forever tearing apart and rebuilding ovens.

Here's what I suggest:
http://www.gallawa.com/microtech/dataselect.html Jihn Gallawa has a CD that you'll find worthwhile.
http://www.gallawa.com/microtech/ Similar
Google and search: microwave oven: repair and parts

Goodluck - but be very careful - Magnetron voltages are lethal!
Thanks

I have a CD on the way, I saw lots of stuff on the web, just hope there's more on the CD:uhh:.
Keeping the waves where they belong is most important, I'll leave any other questions until after I can look the CD over.

Ron
 

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