Microwave effectivness vs. amount of water?

[oisiaa]

I was recently trying to dry a rubbermaid bowl in my microwave. I expected it to evaporate the small droplets in short order but I was surprised to find that it only slowly evaporated the water. So why does a 600 watt microwave not evaporate a couple of grams of water in seconds?

 

[AtlantaChemical]

Hello Oisiaa,

I believe you observed the phenomena, because the additive in the elastomer (your bowl made of) has dielectric loss factor higher then the water. Most of the microwaves are absorbed more by the rubber than the liquid. Therefore the latter is heated slightly as a result of the heat conduction from the heated bowl not directly from the microwave irradiation.

 

[oisiaa]

Thanks for the response. The bowl is marked as microwave safe, but I assume that this means that it only heats to a limited degree as opposed to others.

 

[AtlantaChemical]

Hi again,
"microwave safe" doesn't mean that dishes won't get hot under microwave irradiation. It means that the material is not inappropriate or dangerous for using in microwave oven. For example some plastics may leach plasticizers into food, or metal film may cause sparks and fire inside the cavity.
Probably it's going to be interesting for you to know about special microwave ceramic cookware developed by an American an Japanese scientists. Visit my web site and go to section news archive i you can read more about it. Thanks for the attention.

 

[Higgler]

I believe it's that only a small fraction of the microwave energy hits the small droplett of water, hence you don't get much heating in a small drop. i.e. even though 600 watts comes out of the radiator, only one watt or so hits each of your drops.

Consider the parallel of 600 watts of wind power hitting a large sign versus a small stone. RF energy does actually flow across the microwave oven and you need some mass to absorb it.

In a larger bowl of water, you truly only heat the very outer edge of the water. That large amount of water will absorb a fair amount of the 600 watts. The power loss in water at microwave oven frequencies is 10 dB per inch.

The microwave safe note does hint low loss.

kch

 

[AtlantaChemical]

Mr. Higgler thanks for you opinion.
As matter of fact the irradiation inside the microwave oven is not uniform. There are so named “standing waves”, formed between the inner walls of the resonance cavity.
Their electromagnetic intensity is extremely high, because they represent the output power of the magnetron (in this case 600 W) not just equally spread in the whole volume of the cavity, but concentrated in these “hot spots”. If any of them hit small droplet of water situated on a transparent to microwaves object - it is going to evaporated it immediately. But if the object has higher (or equal) loss factor then it will absorb the electromagnetic energy eagerly and the water will be not heated directly by MW (microwave) irradiatiation!
The heating occurred by multiple hits, not just of one MW transition.
Thanks for your attention

 

[Higgler]

I agree in part.

RF energy distributed over a large volume of a metal walled oven or waveguide can't concentrate itself into one small droplet of water by magic though. If it did, this eng-tips question wouldn't have arisen. The amount of energy absorption of an rf field is based on the volume of the material and the loss and density of said material inserted into any field, plus the number of times the rf energy passes through the material (applies to waveguide absorption especially at the lowest frequency of operation of the waveguide).

Picture the oven filled entirely with water. The energy and heating is due to the first pass through the water. No energy will be bouncing back and forth inside the oven. With only one tiny drop of water in an oven, a fair amount of energy bounce back and forth, but that one tiny drop of water won't absorb alot of energy. Pretend that one drop is a few angstroms thick. As the volume of the material gets very small, absorption and heating gets very small.

Trays in the oven nowadays are there to absorb a small amount of energy and reduce the repeated bouncing back of energy in a near empty oven to reduce peak energy fields which protects the magnetron and lowers the cavity Q, which lessens the likelihood of the doors and front screens from arcing.


kch

 

[biff44]

Funny you should mention it. I did a quick study once of microwave oven cold spots. I used thermal printer paper glued (elmers glue) to cardboard backers, and then ran the oven with the boards in different locations. You can see the results here, with the black being the hot spot, and the white being the cold spots:

http://www.flickr.com/photos/34086315@N03/3173715795/

http://www.flickr.com/photos/34086315@N03/3173715751/

http://www.flickr.com/photos/34086315@N03/3173715711/

 

[Higgler]

Interesting plots biff44.
I assume you only had the paper in there.
The backwall plots are nice interferometer lobes. Shows the energy isn't flowing straight back and front.

The cavity is multiple wavelengths in each direction, so a normal organized energy field (TE10 ish) isn't expected.

I expect the shape would change alot when you add your lossy dinner in the oven.

Thanks,

kch

 

[biff44]

Yep, that was basically with an empty oven with the boards in it. The thermal printer paper itself would not heat up in the oven, but when you glued it to the cardboard with elmers glue, the glue absorbed the heat and the paper indicated it.

I am sure if I put something lossy in there it would have de-Qed the pattern significantly. The application was an industrial oven with paper letters in it for sterilization. The problem was that there were plenty of spaces in the oven where there was little or no heating going on.

The boards, btw, are about the size of a smallish microwave oven, and about 1/4 inch thick.

 

[AtlantaChemica]

The size of a water droplets absolutely has nothing to do with microwave absorption and corresponding heat release. It is because of the nature of the process. Water molecules are tiny dipoles, which are forced to move, because of the interactions with the magnetic field of the microwave. The magnetic field changes its polarity 2.45 billion times per second and the water molecules are following it.
During this repositioning, they literally rub each other and that’s how the heat is produced.
Even amount of one nanogram (one billion part of the gram) water, contents 376 trillion molecules moving, rubbing each other and producing heat.