Metal racks in microwave ovens


One of the first things people learn about microwaves ovens is to never put any metal objects in it. So I was surprised to see a microwave oven with a metal rack in the center, like those you find in regular ovens. Naturally I tried to find out how the manufacturers had achieved this and under what conditions metal could be used.

But the explanations on the internet were all over the place and few of them really convincing. For example, this article says: “These are actually safe because the metal used is tuned to the metal in the cavity of the oven. The racks attach to plastic or rubber clips on the sides of the oven, and as long as the rack does not touch the walls or floor of the microwave, no arcing or warping will occur.”

It is true that microwave ovens work by sending out waves in the microwave region of the electromagnetic spectrum, with wavelengths in the range of 0.1 cm to 100cm. These waves are in the range of frequencies that cause water molecules in the food to vibrate and thus heat the food. But what does it mean to ‘tune’ the metal in the rack? And why does the lack of direct contact of the rack with the sides of the oven matter?

The Wikipedia article suggests that the problem with metal objects is that with any sharp edges that most objects have, that when they in an environment where currents flow, one can get very electric high electric fields at the edges and points that can cause arcing and sparks, and that as long as the metal object, like the rack, is thick and does not have sharp edges it might be safe to use in a microwave.

But people are definitely discouraged from experimenting themselves. Leave it to the professionals and oven makers because it could be dangerous.

Comments

  1. says

    Well, I’m not about to experiment, but I have a more basic question: what’s the point of having a rack? In most microwaves, you can place two dishes side by side, and I’m not terribly convinced a rack would allow for better cooking. I’m biased though, because my microwave is primarily used for heating stuff up. I stick with my oven/stove for cooking.

  2. Richard Simons says

    You can also use aluminum foil in the microwave to shield parts you don’t want to cook as well – or so it is said by various sites on the internet. I’ve never tried it myself.

  3. chigau (違う) says

    I have had diced onions throw sparks in a microwave oven
    on three different occasions
    I didn’t believe it the first two times

  4. Smokey says

    If you do it right, grapes can produce plasma in a microwave oven.

    I’m not linking because there’s apparently a chance of damage or injury.

  5. snuffcurry says

    Peas and carrots in my cat’s canned food (sans can, obv) generate sparks and arcs all the time. Pretty common for certain veg and hotdogs, as well.

  6. spitzmutt says

    My Kenmore microwave Model 99921, built in 1981, has a metal rack which can be positioned at two different heights. The interior box is 15” wide, 16” deep and 10” high. It does not have a turntable.

    I don’t recall using the rack in the past twenty years.

    This microwave is used many times each day for heating coffee and several times each week for cooking.

  7. Reginald Selkirk says

    But what does it mean to ‘tune’ the metal in the rack? And why does the lack of direct contact of the rack with the sides of the oven matter?

    Frequency of a Microwave Oven
    Standard microwave frequency = 2.45 x 10^9 Hz
    speed of light = 2.99 x 10^8 m/s

    > spatial frequency of 12.2 cm

    If you set up the klystron to generate a standing wave in one dimension, it will have a natural spatial frequency of 12.2 cm. If the rack is arranged along that dimension, all the slats in the rack will have the same potential if they are spaced equal to the wavelength. If they are all at the same potential, there will be no tendency to spark.

    If any of the slats were grounded to the chassis, that would disrupt their ability to float in the standing wave.

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