When should you replace a microwave?

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Offline thedoc

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When should you replace a microwave?
« on: 07/12/2015 19:50:02 »
Pamela Coates asked the Naked Scientists:
   When should you replace a kitchen microwave unit? Ours is about 14 years old, works fine and has a small rust spot on the plastic frame of the  interior window door. It's a Panasonic the Genius NN-S578WA.
P. S. we love listening to you on Radio National Friday morning here in Perth,  Western Australia.
What do you think?
« Last Edit: 07/12/2015 19:50:02 by _system »

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Offline evan_au

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Re: When should you replace a microwave?
« Reply #1 on: 08/12/2015 10:37:01 »
If the door seal is torn, or doesn't sit straight when you close the door, microwaves will leak out. This isn't good for anybody standing nearby, so that is a good reason to replace it. You can buy little detectors that can tell you if microwave energy is escaping around the door (but I don't know if I would trust the calibration very much).

If the fan is making scraping noises, or the rotisserie plate is broken, those are other good reasons to replace it.

Microwaves are so cheap today, that "I don't like the color any more" is often enough reason to replace them.

Otherwise, if it defrosts your dinner, it is working, so keep it!

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Offline alancalverd

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Re: When should you replace a microwave?
« Reply #2 on: 08/12/2015 14:45:20 »
Given the choice, I would scrap any "domestic" microwave with a rotating plate, and get a commercial one with all-stainless construction  and a "stirrer" (no moving parts visible). Much easier to clean, cooks quicker (usually 1 - 1.5 kW compared with a domestic 800W) and more evenly, and easier to use.
helping to stem the tide of ignorance

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Offline Bored chemist

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Re: When should you replace a microwave?
« Reply #3 on: 08/12/2015 21:59:44 »
"When should you replace a microwave?"
Either when it stops working or in the January sales.
Please disregard all previous signatures.

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Offline chris

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Re: When should you replace a microwave?
« Reply #4 on: 08/12/2015 23:08:06 »
"stirrer" (no moving parts visible)

How do these work? i.e. how do microwaves without turntables achieve even heating of food?
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline evan_au

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Re: When should you replace a microwave?
« Reply #5 on: 09/12/2015 08:45:24 »
Quote from: chris
how do microwaves without turntables achieve even heating of food?
There is a device like a fan blade that turns in front of the microwave source.

This deflects the microwaves in different directions around the inside of the microwave, hitting the food from all angles, wherever it is sitting in the microwave cavity.

So instead of having a static microwave beam and move the food, you have static food and you move the microwave beam. In a well-designed system, they will achieve similar results.

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Offline SorryDnoodle

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Re: When should you replace a microwave?
« Reply #6 on: 10/12/2015 08:18:17 »
Microwaves will leak out. This isn't good for anybody standing nearby, so that is a good reason to replace it.

What exactly would this do?

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Offline evan_au

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Re: When should you replace a microwave?
« Reply #7 on: 10/12/2015 10:13:26 »
Quote from: SorryDnoodle
What exactly would (a leaky door seal) do?

The main risk from low-level microwave radiation is from heating.

Most parts of your body have a good blood supply, and will carry away small amounts of excess heat, such as from a leaky door seal.

However, the front of the eye must be transparent, so it has a very poor blood supply; if it heats up it can cause the lens to turn opaque (like boiling an egg). This is called cataracts, a condition mostly seen in the elderly.

See: http://en.wikipedia.org/wiki/Microwave#Effects_on_health

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Offline wolfekeeper

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Re: When should you replace a microwave?
« Reply #8 on: 10/12/2015 18:22:22 »
Nah, microwave ovens essentially never leak any significant amount of microwaves.

The only things you need to be careful of; the hinges must be in good condition, and make sure the door seal is reasonably clean, and makes sure there's no big holes in the metal work. Oh yeah, and the door mechanism that cuts off the oven has to be working- if it doesn't turn off when you open the door- shut it again, and then unplug immediately!!

The physics is fairly benign, a hole smaller than the wavelength blocks the microwaves; you get an evanescent standing wave a few times the hole's diameter outside the oven but beyond that, there's no radiation. That's how you can see through the door, it's got lots of tiny holes in it, and the door has a thickness which is a few times their diameter.

You'd need a gap that is bigger than 9 cm to let the microwaves out.

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Offline evan_au

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Re: When should you replace a microwave?
« Reply #9 on: 10/12/2015 20:32:45 »
I agree that microwave leakage is normally very low.

Quote from: wolfekeeper
You'd need a gap that is bigger than 9 cm to let the microwaves out.
That is true, but it doesn't mean a 9x9cm gaping hole.

It can be a 1-dimensional gap, 9cm long, eg along the door seal. That's why broken or damaged door seals are a good reason to discard the microwave oven.

As you say, a dirty door seal is a reason to clean it.

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Offline wolfekeeper

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Re: When should you replace a microwave?
« Reply #10 on: 12/12/2015 03:08:26 »
I agree that microwave leakage is normally very low.

Quote from: wolfekeeper
You'd need a gap that is bigger than 9 cm to let the microwaves out.
That is true, but it doesn't mean a 9x9cm gaping hole.

It can be a 1-dimensional gap, 9cm long, eg along the door seal. That's why broken or damaged door seals are a good reason to discard the microwave oven.
Although in theory a thin 9cm long gap would certainly allow one of the polarizations to escape, actually, from the googling I've just done, it looks like there's a sneaky hidden 1/2 wave arrangement around the edge of microwave oven doors, that stops the microwaves escaping, and so a small gap has no significant effect.

Makes a lot of sense if you think about it, if it was as simple as not cleaning the seal properly, virtually all ovens would be leaking like a sieve!
« Last Edit: 12/12/2015 04:16:52 by wolfekeeper »

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Offline evan_au

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Re: When should you replace a microwave?
« Reply #11 on: 12/12/2015 05:33:15 »
Quote
a sneaky hidden 1/2 wave arrangement around the edge of microwave oven doors
That sounds clever - like a soap bubble of a certain thickness blocks light of a particular wavelength, the microwave door doesn't let through microwaves of the wavelength used by the oven.

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Offline wolfekeeper

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Re: When should you replace a microwave?
« Reply #12 on: 12/12/2015 18:06:02 »
Yeah, I only just found this out, and it explains an observation I made a while back- if you put a cell phone in a microwave oven (with it switched off), and shut the door, and ring it from another phone, it will often ring.

That didn't make sense, how could it be in what was obviously a sealed Faraday cage, and yet still receive the radio signal???

The answer is the door seal, the 1/2 wave trick only works correctly at 2.4 GHz, so the phone signal still enters!!!

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Offline syhprum

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Re: When should you replace a microwave?
« Reply #13 on: 12/12/2015 22:26:25 »
One of the possible reasons for replacing a microwave cooker is if the motor that rotates the plate fails.
It is difficult to find a correct motor listed on the internet so one has to guess from pictures of what motors are available, I was lucky but buying unsuitable ones and trying to fit them with the work involved could be a wasteful exercise.
Another reason is if the glass plate is broken but with any luck you might find one thrown away with a suitable plate.
I grew up during the war when everything repairable had to be fixed and the thought of scrapping anything with a minor fault horrifies me.
A better test for the Faraday cage efficiency would be Bluetooth speaker that operates on 2.4GHz 
« Last Edit: 12/12/2015 22:34:26 by syhprum »
syhprum

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Offline marthaapodaca83

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Re: When should you replace a microwave?
« Reply #14 on: 15/01/2016 06:16:00 »
Pamela Coates asked the Naked Scientists:
   When should you replace a kitchen microwave unit? Ours is about 14 years old, works fine and has a small rust spot on the plastic frame of the  interior window door. It's a Panasonic the Genius NN-S578WA.
P. S. we love listening to you on Radio National Friday morning here in Perth,  Western Australia.
What do you think?

Hey, I think the best time to replace any of the electrical appliance is when it stops working and thus you are sure that there is not any other solution which can make you keep working with it.

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Offline klgrl

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Re: When should you replace a microwave?
« Reply #15 on: 18/12/2016 09:41:34 »
I strongly believe that after 3-4yrs MAX your microwave should be replaced (hence don't spend a fortune) In my opinion it boils down to the amount of food bacteria that stays inside the little holes, cracks and fans and anywhere else that can harbour these micro germs/bugs. If you think about how much food in 3 years has been heated all those small particles of food hid without the naked eye seeing it actually may make you reconsider using a microwave ever again. As soon as a temperature is selected and you hit the start button your now waiting for food to consume has millions of past food bacteria settling on your soon to be eaten food. Just my opinion, after learning the reality and legality  of dating a microwave when purchased because it falls under a MOL health and safety code of practice...

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Offline Bored chemist

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Re: When should you replace a microwave?
« Reply #16 on: 18/12/2016 11:25:16 »
I strongly believe that after 3-4yrs MAX your microwave should be replaced (hence don't spend a fortune) In my opinion it boils down to the amount of food bacteria that stays inside the little holes, cracks and fans and anywhere else that can harbour these micro germs/bugs. If you think about how much food in 3 years has been heated all those small particles of food hid without the naked eye seeing it actually may make you reconsider using a microwave ever again. As soon as a temperature is selected and you hit the start button your now waiting for food to consume has millions of past food bacteria settling on your soon to be eaten food. Just my opinion, after learning the reality and legality  of dating a microwave when purchased because it falls under a MOL health and safety code of practice...
The only thing I use my microwave for is heating food to a temperature where almost any bacteria would be killed, and I do this sufficiently shortly before eating it that any bacteria which survived wouldn't have time to get established and multiply to dangerous levels.
Also, since the oven is ventilated, it's usually too dry for bacteria to grow well so essentially any bacteria that might get introduced to it are likely to be dead in a week.

However your proposed argument for renewing ovens every 3 years does seem to apply to kitchens.
Please disregard all previous signatures.

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Offline evan_au

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Re: When should you replace a microwave?
« Reply #17 on: 21/01/2017 03:00:34 »
Quote from:  klgrl
the amount of food bacteria that stays inside the little holes, cracks
When I heat food in a microwave, I cover it (eg with a plate) so the steam can escape, but food doesn't splatter all over the inside of the microwave.

...I once saw the aftermath of someone trying to boil an egg in the microwave (with intact shell). The pressure inside built up until the shell exploded, spraying goopy egg throughout the microwave. Do not try this at home!

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Offline chris

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Re: When should you replace a microwave?
« Reply #18 on: 22/01/2017 13:03:00 »
I think we did that as a kitchen science once!
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline vhfpmr

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Re: When should you replace a microwave?
« Reply #19 on: 24/01/2017 13:02:55 »
The pressure inside built up until the shell exploded, spraying goopy egg throughout the microwave. Do not try this at home!

I once made the mistake of thinking that the explosion is caused by pressure building up inside the shell.

I had boiled an egg in a pan of water, and then when I peeled it and cut it open I found it was undercooked, and not hard boiled as I wanted it. No problem I thought, I can finish it off with a few seconds in the microwave now that it's shelled, so in it went. After a few seconds I took it out, put it on the worktop, and then as I stuck a fork in it: BANG!

In an instant the kitchen was redecorated in scrambled egg, up the walls, all over the worktop, all down the front of me........
« Last Edit: 24/01/2017 13:05:32 by vhfpmr »

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Offline Colin2B

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Re: When should you replace a microwave?
« Reply #20 on: 24/01/2017 14:47:39 »
I once made the mistake of thinking that the explosion is caused by pressure building up inside the shell.
When the shell is intact it is usually a pressure buildup. What you have come across is a superheated area in the egg. You've probably noticed this when heating up liquids, especially thick ones that don't mix easily eg custard. The superheated zone is contained by the weight of the surrounding material until disturbed, perhaps by stirring, and there have been a number of reported scaldings. In your case the white was just strong enough to hold the pressure.


I don't envy you the cleanup job.
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

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Offline vhfpmr

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Re: When should you replace a microwave?
« Reply #21 on: 24/01/2017 17:56:35 »
With superheating, you get much the same effect without the need for anything to confine the pressure, a superheated glass of water will explode.

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Offline Colin2B

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Re: When should you replace a microwave?
« Reply #22 on: 24/01/2017 23:09:16 »
With superheating there are 2 confining pressures, the weight of the water and the surface tension around the bubbles. Although these are small they are enough to contain the micro bubbles until the temp is several degrees above boiling point, when slight disturbance can cause them to group and explode.
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.