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Amblydoper
12-15-2008, 08:23 PM
I work in a restaurant kitchen, and we have a cheap, $7 microwave that we use for warming up pastries, as well as "oops" situations ("Chef Mike" is faster then any line cook). Several of my coworkers claim that this POS microwave will continue to "cook" with the door open, if the timer hasn't stopped yet. My Boss claims he can feel his legs tingling (its placed on a low shelf). I called bullshit, and performed a scientific experiment.

I filled a plastic container with tap water, measured the temperature (61˚F) and divided it equally into two identical plastic containers. I placed one on a shelf, a couple meters away from the microwave, and the other inside the microwave. I set the timer, but left the door partially open. After 4 minutes, I measured the temperatures of the water, both 62˚F. From this, I concluded that the microwave was NOT cooking while the door was open.

My coworkers scoffed at my results.

So, what do you dopers think? Was my experiment flawed, or are my coworkers paranoid?

What would be the risks involved with such a malfunctioning microwave?

Q.E.D.
12-15-2008, 08:32 PM
Highly unlikely the oven was running with the door open. There are not one, but three, independent interlock switches on the door latches which would require multiple, atypical failures in order to allow such a condition. Two of the interlocks prevent the primary of the HV transformer from energizing if the door isn't closed AND latched. The third interlock is a fail-safe which will blow the primary fuse if the oven is started while the door is open. However, the light, fan and/or turntable may operate, giving the impression that the oven is running, when it really isn't.

Duckster
12-15-2008, 08:33 PM
Run the test this way:


Fill three cups of water from the tap.
Measure their temperature and record it.
Put Cup A into the microwave, close the door, and set the timer for five minutes.

When the time is up, measure the water temperature from Cup A and record it.

Now take Cup B and place it in the microwave. Leave the door open.
Take Cup C and place it one foot away from the open door.
Turn on the timer for five minutes.

When the time is up, remeasure Cups A, B And C.

Cup A should have cooled but it should not be near its initial temperature.
Cup B might be a bit warmer than its initial temperature, but only because of the latent heat created by Cup A inside the microwave.
Cup C should be at the same temperature as its first measurement, or close to it.

CookingWithGas
12-15-2008, 08:37 PM
I would say your experiment was well designed. It tested a specific hypothesis, and used a control. There is no way that a functioning microwave would fail to boil a cup of water in 4 minutes.

If a microwave is running, you generally hear the fan running (I don't know if the magnetron itself makes any noise), so there's usually no confusion about whether it's on. However, it would be odd if the timer kept running after the door was open. If that's the case the microwave is probably broken.

The bit about the legs tingling could be psychological. I don't know what it would feel like to be cooked by a microwave but I would think heat and burns would be the result, rather than tingling, which would be more like a mild electric shock.

Q.E.D.
12-15-2008, 08:40 PM
There is no way that a functioning microwave would fail to boil a cup of water in 4 minutes.

With the door open (assuming the magnetron was actually running), it probably couldn't bring a cup of water to boil at all, no matter how long you ran it--most of the radiation would escape before it was absorbed by the water.

engineer_comp_geek
12-16-2008, 12:41 AM
If it is one of those el-cheapo rotary dial timers then the timer very well could keep running even if the door is opened. I've seen some microwaves keep the fan running too even if the magnetron shuts off, and since people hear the fan going they think the microwave is still "on".

You could buy a cheap microwave leakage detector and settle this once and for all very easily. An open running microwave is a health hazard and violates a few safety laws. The biggest danger is an RF burn, which is a deep tissue burn and hurts like a sonofabitch.

clayton_e
12-16-2008, 05:55 AM
Rather than measuring temps of large amounts of water why not just put a thermometer in a shot glass full of water (a microwave, btw, resonates at the same frequency that water vibrates, creating friction which creates heat... so why bother heating up so much of it? someone correct me if my understanding of this is wrong).


If it goes up even one degree after several minutes, for this situation let's forget about boiling if all that microwave energy is escaping, a shot glass full of water should heat up some.

And don't kneel right in front of it watching it. I don't recommend it... just in case.

beowulff
12-16-2008, 09:09 AM
Just wave a neon light or fluorescent tube in front of it. If it lights up, it's running.

mks57
12-16-2008, 10:19 AM
(a microwave, btw, resonates at the same frequency that water vibrates, creating friction which creates heat... so why bother heating up so much of it? someone correct me if my understanding of this is wrong).

Um, no. The frequency is irrelevant, and it isn't remotely close to any absorption peaks in the water molecule. The water molecule behaves like a tiny magnet. The EM field from the oven tries to twist them back and forth, which generates heat via friction.

troub
12-16-2008, 10:33 AM
We have one in the lounge at work where the fan starts running (for what reason I don't know) as soon as you open the door. This leads to confusion, but the microwave is not "on" at that time. So yeah, on some microwaves, the fan runs even when they're not heating.

clayton_e
12-16-2008, 11:42 AM
Um, no. The frequency is irrelevant, and it isn't remotely close to any absorption peaks in the water molecule. The water molecule behaves like a tiny magnet. The EM field from the oven tries to twist them back and forth, which generates heat via friction.

Ah, thanks for correcting me, I looked it up myself just now. Regardless of how a microwave functions my idea for an experiment should still work.