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Repost: Microwave Unsafe or Unsafe Microwave June 22, 2012

Posted by mareserinitatis in electromagnetics, engineering, food/cooking, science.
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(Note: this is from the old blog, back when living in Minneapolis)

There’s nothing like a nice, hot cup of English Breakfast or Earl Grey in the morning…until you reach into the microwave and burn your hand on your mug.

I’ve noticed something very irritating. Since I moved into my new place, all of my dishes get hot and some of them have cracking glaze after use in the microwave. The most irritating thing, aside from the pain, is that I’ve noticed my favorite mug is expanding and shrinking. It expands when heated and then contracts as it cools. This, unfortunately, has caused my tea basket to get physically stuck in the mug, which never happened with my old microwave (thus eliminating the notion that heating of the basket causes it to expand and get stuck).

Traditionally, this means that my dishes are not “microwave safe”. In other words, there is something in the dishes that heats up when put in the microwave. That means that you can destroy the dishes and burn yourself.

It wouldn’t be that big an issue except that all of these dishes worked fine in my other microwave back in Fargo.

 

This has led me to look into what might be causing the problem. Hypothetically, if something is microwave safe in one microwave, it should be that way in all microwaves.

Hypothetically…

There are lots of places that give you the basics of how a microwave works. A brief overview is that it emits electromagnetic waves which cause water molecules in food to rotate. The frequency of most commercial microwaves for the home is around 2.45 GHz, which is apparently a good frequency to get water molecules to “flip”. Flipping, rotating, shaking are all ways that molecules move, and molecular movement translates into heat. So the microwave makes all these water molecules do their jig because it excites them at just the right tempo. If you try exciting them at a different frequency or tempo, the water molecules won’t respond as well.

It’s harder to find information about how microwaves create these fields. It turns out that they generate electromagnetic waves with something called a magnetron. (An excellent and quite detailed description of how they work can be found here. According to The Art of Electronics, magnetrons fall under the category of “exotic devices”. This is probably code for “uses an electromagnetic field in a non-obvious way” or maybe “doesn’t always use silicon to do its job”. Interestingly enough, these are the same devices used to create fields for radar, including the Doppler radar that is used to look at cloud cover and precipitation. (If you’re a Wunderground nerd, like me, you spend a lot of time looking at images generated by Doppler radar.)

Again, I’ll summarize. There is a cathode (something which generates electrons) running down the middle of a cylindrical chamber. The chamber is subdivided into resonant chambers. Resonant chambers are areas where electromagnetic energy creates a standing wave. (A good though not exact analogy from sound, which is also a wave, would be an organ pipe.) The electrons formed around the cathode form into groups which spin and sweep past the resonant chamber openings. Because moving charge creates an electromagnetic wave which becomes a standing wave in the resonant chambers. This wave then creates a current in a wire or “feed”, which conducts a current to a waveguide. A waveguide is basically a replacement for a wire. It conducts an electromagnetic field when the power is too high or you could easily lose too much power through a wire. (Wires can be awfully lossy.) All it looks like is a rectangular tube, but the size of the tube is important because this will determine the frequency of the waves it can carry. (Remember, we want to have things pretty sharply focused at 2.45 GHz.) This tube leads into the microwave chamber which is tada! a Faraday cage. This is something that will contain electromagnetic energy inside of it without letting it escape as well as keep electromagnetic energy from your surroundings out. In this case, we want the energy inside. Waves which don’t hit our food will hit the side of the chamber and bounce around until it hits the food.

That metal screen is part of the Faraday cage and is keeping your brains from being baked when you’re pressing your nose to the glass going, “When will it be done?!”

Many microwaves contain things that look like fans but are actually “mixers” or “stirrers”. They cause the waves to bounce more randomly and create a more even distribution of the waves for heating. When the waves hit your food, they can only penetrate to about an inch. How far the wave goes into the food is quantified by something called a “skin depth”. Because your food isn’t a good conductor (like copper) which has pretty much no penetration depth, you will often notice that things get hot on the outside but not on the inside, like often happens to me when I reheat lasagna.

Food is also not a pure dielectric (like air or styrofoam) where the wave passes through and can’t generate a current inside. Food which is more conductive (which will likely have more water) will tend to heat up better or faster (as well as internally distribute that heat better) than food that doesn’t. Conductive food will also tend to have more water. In this case, you may be heating up a fruit-filled pie. The pie filling has a lot of water and will heat up fast, but the crust doesn’t and doesn’t seem to get as warm. You bite in, expecting the filling to be the same temp as the crust but end up getting burned instead.

People who design fast food meals ought to consult with microwave engineers on optimal heating set up. 🙂

As I mentioned before, microwave safe dishes don’t contain anything that will heat up when exposed to microwaves. Dishes which aren’t microwave safe contain some molecules that will be able to rotate, twist or vibrate in some way similar to water, causing the dish to heat up.

Sometimes you have dishes which are “thermally conductive”…that is, they transfer heat well. While you’re heating up your food, the dish is pulling a lot of that heat away from the food and into itself, causing the dish to get hot.

However, that doesn’t seem to be my situation. My previous microwave was much a higher power and seemed to heat up the food fine without heating up the dishes. My current one seems to do nearly the opposite. And since these are the same dishes, I have to conclude that it is in fact the microwave with the issue.

My first guess is one that doesn’t seem plausible. I don’t think it has anything to do with the size/shape of the magnetron or waveguide. Those are fairly large objects that can be mass constructed well within tolerances. I could be wrong, but that’s my initial guess. This also minimizes the chance that there may be some sort of mismatch between the magnetron and the waveguide.

Looking at the remaining possibilities, I’ve come up with three.

The first is that my microwave is poorly designed in the sense that it doesn’t direct electromagnetic energy well. This may be part of the problem as it seems to heat the dishes in areas away from the food. I don’t think that this is the entire issue because, if designed poorly, the wave should just bounce around until it hits something with high water content. However, I can’t say it’s not doing this.

There are two other possibilities. It turns out that magnetron frequency can change both with the temperature and the current through the cathode. Although the cathode temperatures get pretty high, I doubt that it would be that huge a change from a prototype once it gets over the initial change.

The last option seems most likely to me: the cathode isn’t working exactly the way it’s supposed to (which can be characterized by something called a “pushing curve”). If the current from the cathode is too high or too low, this will change the way the electrons behave, which will alter the frequency of the wave being generated by the magnetron.

In doing some research on my microwave, it turns out to have a horrid reputation. They die a lot, like within a year. Unfortunately, they’re so cheap that it’s not worth it to send them in for repairs because you have to pay for shipping to and from. When microwaves die like this, a lot of times it can be due to power problems, and thus the design of the controlling electronics or the high voltage power system can come into play. (Did I mention that magnetrons require huge voltages to operate???)

It appears that perhaps this line of microwaves may not have the best electronics design, and for whatever reason, the power into the magnetron isn’t quite right. This is causing my dishes to heat and expand while not heating my food optimally.

I guess I’ll be using oven mitts to take everything out of there until it decides to kick the bucket.

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