Tag: microwave ovens

I don’t want to sound like I know everything in the world or even like I know quite a lot. But you had a question regarding “If a microwave oven door were to open while it was still on, what would happen? Could it hurt you?- JP”

Well ..Having the thought process that I have, kinda how should I put it? …Stupid? or inventive or even in-between. Well, my microwave door did happen to come off. Magic Chef 900-watt microwave. Well, I did my best to try to fix it but the hinge on one side did not attach properly, therefore having a gap between the door and the appliance. Being me (stupid) I wondered if it would burn fast or would it gradually warm up. I slid my finger between…You probably dying to hear what happened… But it didn’t gradually warm up at all. It was instant heat! It didn’t scar me or anything like that, but sure scared the H*** out of me to find out it got so hot so quick. I didn’t get any blisters either. But it just burned like touching something hot on the tip of my finger being that is the only thing I put in. Well you know the old adage, “You learn from your mistakes”, stands true. lol – Anonymous

What a remarkable story! As much as I like to think I can predict what should happen in many cases, there is just nothing like a good experiment to bring some reality to the situation. Your microwave evidently sent a significant fraction of its 900 watts of microwave radiation through that crack between cooking chamber and door and roasted your finger instantly. This is a good cautionary tale for those who are careless or curious with potentially dangerous household gadgets. While I continue to think that serious injuries are unlikely even in a leaky microwave oven, you have shown that there are cases of real danger. Fortunately, you had time to snap you finger away. It’s like Class 3 lasers, which are now common in the form of laser pointers and supermarket checkout systems: they can damage your vision if you stare into them, but your blink reflex is fast enough to keep you from suffering injury. Thanks for the anecdote and I’m glad your finger recovered.

Is it possible to heat up the surface of a stealth aircraft by exposing it to strong microwaves? Also, I heard that local forces in the recent Balkans conflict used cellular phone technology to down the U.S. stealth aircraft. Is that possible? – JG

Stealth aircraft are designed to absorb most of the microwave radiation that hits them and to reflect whatever they don’t absorb away from the microwave source. That way, any radar system that tries to see the aircraft by way of its microwave reflection is unlikely to detect anything returning from the aircraft. In effect, the stealth aircraft is “black” to microwaves and to the extent that it has any glossiness to its surfaces, those surfaces are tipped at angles that don’t let radar units see that glossiness. Since most radar units emit bright bursts of microwaves and look for reflections, stealth aircraft are hard to detect with conventional radar. Just as you can’t see a black bat against the night sky by shining a flashlight at it, you can’t see a stealth aircraft against the night sky by shining microwaves at it.

Like any black object, the stealth aircraft will heat up when exposed to intense electromagnetic waves. But trying to cook a stealth aircraft with microwaves isn’t worth the trouble. If someone can figure out where it is enough to focus intense microwaves on it, they can surely find something better with which to damage it.

As for detecting the stealth aircraft with the help of cell phones, that brings up the issue of what is invisibility. Like a black bat against the night sky, it’s hard to see a stealth aircraft simply by shining microwaves at it. Those microwaves don’t come back to you so you see no difference between the dark sky and the dark plane. But if you put the stealth aircraft against the equivalent of a white background, it will become painfully easy to see. Cell phones provide the microwave equivalent of a white background. If you look for microwave emission near the ground from high in the sky, you’ll see microwaves coming at you from every cell phone and telephone tower. If you now fly a microwave absorbing aircraft across that microwave-rich background, you’ll see the dark image as it blocks out all these microwave sources. Whether or not this effect was used in the Balkans, I can’t say. But it does point out that invisibility is never perfect and that excellent camouflage in one situation may be terrible in another.

When you are defrosting and the magnetron is turning on and off, when it is off, are the microwaves still bouncing around or is the food just sitting there warming itself up? – LEA, PA

During the defrost cycle, the microwave oven periodically turns off its magnetron so that heat can diffuse through the food naturally, from hot spots to cold spots. These quiet periods allow frozen parts of the food to melt the same way an ice cube would melt if you threw it into hot water. While the magnetron is off, it isn’t emitting any microwaves and the food is just sitting there spreading its thermal energy around.

Many of the new cordless phones operate at 2.4GHz like a microwave oven. Are we microwaving our ears when we use them, or is the wattage so small it doesn’t affect us? – R

As far as anyone has been able to determine so far, the wattage is so small that this microwave radiation doesn’t affect us. Not all radiations are the same, and radio or microwave radiation is particularly nondestructive at low intensities. It can’t do direct chemical damage and at low wattage can’t cause significant RF (radio frequency) heating. At present, there is thus no plausible physical mechanism by which these phones can cause injury. I don’t think that one will ever be found, so you’re probably just fine.

I thought microwave ovens were sealed shut to keep the waves inside. Why then can you smell the food as it is being cooked? – E

The cooking chamber of a microwave oven has mesh-covered holes to permit air to enter and exit. The holes in the metal mesh are small enough that the microwaves themselves cannot pass through and are instead reflected back into the cooking chamber. However, those holes are large enough that air (or light in the case of the viewing window) can pass through easily. Sending air through the cooking chamber keeps the cooking chamber from turning into a conventional hot oven and it carries food smells out into the kitchen.

I am planning to do an experiment with a microwave oven and want to videotape it. I want to operate the microwave oven with the door open. Will I be safe if I’m 15 feet away? Will opening the door nullify the “chamber” effect that the oven normally has? – E

Don’t operate the oven open. You’re just asking for trouble. The oven will emit between 500 and 1100 watts of microwaves, depending on its rating, and you don’t need to be exposed to such intense microwaves. The chamber effect is important; without the sealed chamber, the microwaves pass through the food only about once before heading off into the kitchen and you. The food won’t cook well and you’ll be bathed in the glow from a kilowatt source of invisible “light.”

Imagine standing in front of a 10-kilowatt light bulb (which emits about 1 kilowatt of visible light and the rest is other forms of heat) and then imagine that you can’t see light at all and can only feel it when it is causing potential damage. Would you feel safe? Your video camera won’t enjoy the microwave exposure, either.

If you want to videotape your experiments without having to view them through the metal mesh on the door, you can consider drilling a small hole in the side of the cooking chamber. If you keep the hole’s diameter to a few millimeters, the microwaves will not leak out. Then put one of the tiny inexpensive video cameras that widely available a centimeter or so away from that hole. You should get a nice unobstructed view of the cooking process without risking life and limb.

In regards to your discussion of superheating water in a microwave oven, I’ve found that it occurs most often when (1) I reheat water that has been heated before and (2) I heat water that has sat in the cup overnight. Why does that seem to reduce the number of seed bubbles? – JS

Both processes allow dissolved gases to escape from the water so that they can’t serve as seed bubbles for boiling. When you heat water and then let it cool, the gases that came out of solution as small bubbles on the walls of the container escape into the air and are not available when you reheat the water. When you let the water sit out overnight, those same dissolved gases have time to escape into the air and this also reduces the number and size of the gas bubbles that form when you finally heat the water. Without those dissolved gases and the bubbles they form during heating it’s much harder for the steam bubbles to form when the water reaches boiling. The water can then superheat more easily.

You must be busy since last night’s broadcast (Superheated Water Produced in Microwave Ovens on ABC Primetime 3/15/2001). Very, very scary as we have certainly done exactly what was shown. I have 3 little girls who love to “cook” their own soups, heat their dad’s coffee water, etc. in the microwave. This report terrified me. I am grateful no harm has come to them. My question is if we strictly use microwaveable plastic bowls, ceramic mugs, or other heavy mixing type bowls and avoid the glass, is the potential for the explosion still there?

I’m afraid that there’s no easy answer to this question. You can use a microwave oven to superheat water in any container that doesn’t assist bubble formation. How a particular container behaves is hard for me to say without experimenting. I’d heat a small amount of water (1/2 cup or less) in the container and look at it through the oven’s window to see if the water boils nicely, with lots of steam bubbles streaming upward from many different points on the inner surface of the container. The more easily water boils in the container, the less likely it is to superheat when you cook it too long. (If you try this experiment, leave the potentially superheated water in the closed microwave oven to cool!)

Glass containers are clearly the most likely to superheat water because their surfaces are essentially perfect. Glasses have the characteristics of frozen liquids and a glass surface is as smooth as… well, glass. When you overheat water in a clean glass measuring cup, your chances of superheating it at least mildly are surprisingly high. The spontaneous bubbling that occurs when you add sugar, coffee powder, or a teabag to microwave-heated water is the result of such mild superheating. Fortunately, severe superheating is much less common because defects, dirt, or other impurities usually help the water boil before it becomes truly dangerous. That’s why most of us avoid serious injuries.

However, even non-transparent microwaveable containers often have glass surfaces. Ceramics are “glazed,” which means that they are coated with glass for both sealing and decoration. Many heavy mixing bowls are glass or glass-ceramics. As you can see, it’s hard to get away from trouble. I simply don’t know how plastic microwaveable containers behave when heating water; they may be safe or they may be dangerous.

If you’re looking for a way out of this hazard, here are my suggestions. First, learn to know how long a given amount of liquid must be heated in your microwave in order to reach boiling and don’t cook it that long. If you really need to boil water, be very careful with it after microwaving or boil it on a stovetop instead. My microwave oven has a “beverage” setting that senses how hot the water is getting. If the water isn’t hot enough when that setting finishes, I add another 30 seconds and then test again. I never cook the water longer than I need to. Cooking water too long on a stovetop means that some of it boils away, but doing the same in a microwave oven may mean that it becomes dangerously superheated. Your children can still “cook” soup in the microwave if they use the right amount of time. Children don’t like boiling hot soup anyway, so if you figure out how long it takes to heat their soup to eating temperature and have them cook their soup only that long, they’ll never encounter superheating. As for dad’s coffee water, same advice. If dad wants his coffee boiling hot, then he should probably make it himself. Boiling water is a hazard for children even without superheating.

Second, handle liquids that have been heated in a microwave oven with respect. Don’t remove a liquid the instant the oven stops and then hover over it with your face exposed. If the water was bubbling spasmodically or not at all despite heavy heating, it may be superheated and deserves particular respect. But even if you see no indications of superheating, it takes no real effort to be careful. If you cooked the water long enough for it to reach boiling temperature, let it rest for a minute per cup before removing it from the microwave. Never put your face or body over the container and keep the container at a safe distance when you add things to it for the first time: powdered coffee, sugar, a teabag, or a spoon.

Finally, it would be great if some entrepreneurs came up with ways to avoid superheating altogether. The makers of glass containers don’t seem to recognize the dangers of superheating in microwave ovens, despite the mounting evidence for the problem. Absent any efforts on their parts to make the containers intrinsically safer, it would be nice to have some items to help the water boil: reusable or disposable inserts that you could leave in the water as it cooked or an edible powder that you could add to the water before cooking. Chemists have used boiling chips to prevent superheating for decades and making sanitary, nontoxic boiling sticks for microwaves shouldn’t be difficult. Similarly, it should be easy to find edible particles that would help the water boil. Activated carbon is one possibility.

Last night’s report wasn’t meant to scare you away from using your microwave oven or keep you from heating water in it. It was intended to show you that there is a potential hazard that you can avoid if you’re informed about it. Microwave ovens are wonderful devices and they prepare food safely and efficiently as long as you use them properly. “Using them properly” means not heating liquids too long in smooth-walled containers.

I saw the story on Primetime tonight (Superheated Water Produced in Microwave Ovens on ABC Primetime 3/15/2001), and at weird timing. Just yesterday, a co-worker and I were standing around the kitchen area talking, while she warmed up some coffee. All of a sudden, there was a loud POP, which startled both of us. Not knowing exactly what had happened, we stopped the microwave and opened the door, only to find the contents of the mug (coffee) everywhere on the inside of the cooking chamber, less a few drops at the bottom of the cup.

The story provided SOME insight into what exactly had happened, however, it was reported that the surface of the super-heated liquid had to be broken by something for an explosion to be triggered. In the explosion with the coffee, there were no other objects in the microwave other than the mug and the coffee it held. What then, caused the explosion if nothing was present to break the surface? – MM, Denver, CO

Superheated water doesn’t always wait until triggered before undergoing sudden boiling. All that’s needed to start an explosion is for something to introduce an initial “seed” bubble into the liquid. Sometimes the container already has everything necessary to form a seed bubble and it’s just a matter of getting the water hot enough to start that process. Many seed bubbles begin as trapped air in tiny crevices. As the water gets hotter, the size of any trapped air pocket grows and eventually it may be able to break free as a real seed bubble. When water is sufficiently superheated, just a single seed bubble is enough to start an explosion and empty the container completely. In your case, the coffee flash boiled spontaneously after something inside it nucleated the first bubble.

This sort of accident happens fairly often and we rarely think much about it as we sponge up the spilled liquid inside the microwave oven. But had your friend been unlucky enough to stop heating the coffee a second or two before that POP, she might have been injured while taking the coffee out of the oven. The moral of this story is to avoid overcooking any liquid in the microwave oven. If you must drink your coffee boiling hot, pay attention to it as it heats up so that it doesn’t cook too long and then let it sit for a minute after the oven turns off. If you don’t like your coffee boiling hot, then don’t heat it to boiling at all.

I left a spoon in my food and I put it in the microwave by accident. Is it dangerous to eat the food after it was put into the microwave with a metal object. Does it have any radiation? Could it cause cancer? – SK, Santa Monica, California

The spoon will have essentially no effect at all on the food. Metal left in the microwave oven during cooking will only cause trouble if (a) it is very thin or (b) it has sharp edges or points. The microwaves push electric charges back and forth in metal, so if the metal is too thin, it will heat up like the filament of a light bulb and may cause a fire. And if the metal has sharp edges or points, charges may accumulate on those sharp spots and then leap into space as a spark. But because your spoon was thick and had rounded edges, the charges that flowed through it during cooking didn’t have any bad effects on the spoon: no heating and no sparks.

As far as the food is concerned, the presence of the spoon redirected the microwaves somewhat, but probably without causing any noticeable changes in how the food cooked. There is certainly no residual radiation of any sort and the food is no more likely to cause cancer after being cooked with metal around than had there been no spoon with it. In general, leaving a spoon in a cup of coffee or bowl of oatmeal isn’t going to cause any trouble at all. I do it all the time. In fact, having a metal spoon in the liquid may reduce the likelihood of superheating the liquid, a dangerous phenomenon that occurs frequently in microwave cooking. Superheated liquids boil violently when you disturb them and can cause serious injuries as a result.