Microwave oven leakage test

[Norbet]

Is there any legal requirement to have regular leakage tests done on microwave ovens in an office environment? I understand that portable appliance tests PAT need to be conducted at regular intervals depending on the type of equipment and the environment they're in, but do leakage tests need to be conducted? I appreciate that if any equipment is suspected of damage then it should be tested for safety before using again - however it's unclear in our case if the equipment is indeed damaged. We have a microwave oven that has some small paint chips inside the oven under the turntable. We're not sure if this constitutes damage or not (it appears to be bare metal beneath the paint and I always thought that was a bad thing for microwaves but I could be wrong). It still cooks fine and doesn't appear to spark or anything during use.

At a cost of around =A335 I suspect that it may be cheaper just to replace the oven itself rather than go down the qualified tester route

- but I could do with some advice on whether this is necessary for the situation. It'd be a shame to throw out a perfectly good oven if it's unlikely that small paint chips are likely to affect the safe operation of the equipment. Has anyone any experience of this? What would be the legal position? Could a small paint chip affect the safe operation of a microwave oven?

Your advice would be greatly appreciated. N

[Dr Ivan D. Reid]

On 9 May 2006 04:10:38 -0700, Norbet wrote in :

No. The paint is only a cosmetic covering over the metal cavity of the microwave oven. There should be no risk of sparking as the cavity walls are nulls in the standing-wave pattern. The only place you should normally be concerned with leakage of a microwave is at the door seal -- assuming no-one has been silly enough to interfere with the magnetron feeding the cavity. Dirt on the seal or damage could allow leakage but given that the wavelength is about (3x10^10/2.45x10^9)=12.2 cm, you need a fairly large gap to get any significant leakage -- consider the size of the holes in the door-screen which allow you to see into the oven but do not let any microwaves escape.

[The Todal]

Not as such

Invest four quid in a leakage detector from Maplins

keep your distance from the machine if it is in use, and don't have your workstation next to it.

[Norbet]

Thankyou kindly for your advice - much appreciated!

[Dave Doubleyew~]

Huuummm, interesting, Ivan; is it possible to pad the cavity out so altering its resonant frequency, and furthermore is it also possible to give some idea what kind of modulation transformer would be needed so that one could use the oven as a source on 23 cms for EME? ;-}

Dave Doubleyew. ( G3F**.)

[Guy King]

The message from "Norbet" contains these words:

White-inside microwaves often do this. It's not leaking deadly rays all over the place at you, don't worry.

As for bare metal - the internal casings of microwaves are made (usually) of metal. Indeed, if they weren't the microwave wouldn't work properly as it relies on the metal (or other conductive material, for the pedants) to bounce the radiation around till it meets the meat (or other succulent comestible, for the pedants). Many microwave instruction manuals even suggest you cover parts of somethings, like the tips of poultry limbs, with tin foil to reflect the radiation away.

It would only be an issue if it had rusted right through and left a hole, and (even then the hole would have to be over a certain size to allow the particular wavelength through, for the pedants).

[Dr Ivan D. Reid]

On Tue, 9 May 2006 15:29:56 +0100, Dave Doubleyew~ wrote in :

Well, you're starting to get beyond my area of expertise. As a licenced ham you're probably better equipped to judge (I was just 2nd operator on VK0KC in 1980...) I'm not sure it's necessarily a _resonant_ cavity, but by definition the nulls had better be at the walls. I assume it's something else that sets the actual 2.45 GHz operating frequency -- I've had much more experience with thyratrons than magnetrons. :-0!

[Scott]

Interesting, wouldn't the microwaves actually penetrate the screen with an intensity which falls off exponentially with distance? The quantum analogy here being a particle in a finite potential well. (A solid door screen being analogous to an infinite potential well). - Just a thought that crossed my mind.

[Dave Doubleyew~]

Now think on about the third, fifth and seventh harmonics of the fundamental frequency generated. Then consider the wavelength they produce including the half-waves and the minimum escape apertures from within the cavity for those frequencies. At this point gentlemen anytime we enter a kitchen with a microwave we should don lead aprons. Also note the old penny size sticker on the door of the oven with point 2 sized print that reads "If you can read this then you are sterile!" :-)

Dave Doubleyew. (G3F**)

[Peter]

There presumably would be if the unit has suffered significant damage, etc.

A jewellers shop owner suffered multiple false alarms. A technician discovered that a microwave in a fast food across the road had the door interlock defeated. When staff opened the door the waves triggered the jeweller's alarm.

[meow2222]

A paint chip has no effect on the safe and proper working of a nuke.

There is no reason nor need to leakage test modern microwave ovens. A leakage check consists of visually checking the cooking cavity hasnt rusted through, bent out of shape, or had some wally cut a hole in it, and that the door closes ok.

Microwaves do not have door seals, and have not for decades. If you have a 1970s one that does, get rid of it.

Microwave leakage detectors are junk products.

NT, qualified

[Dr Ivan D. Reid]

On Tue, 09 May 2006 20:37:32 +0100, Scott wrote in :

Probably, to first order. Now, put a 6.1 cm wavelength over a 1.5 mm hole, and exponentiate that! (I get 10E-18.)

[Andrew Gabriel]

A PAT test on a microwave should include an inspection for damage which could cause microwave leaks, but no leak detector is required (and the cheap ones you could buy were useless; they were far too sensitive). The inspection should check for correct operation of the door interlock, damage to the casing, door, and hinges, etc. Rusting of the internal cavity metal liner is also a cause for PAT failure, as it can result in leakage. This commonly seems to happen in the corners and under the turntable.

As others said, paint chips alone are not an issue. However, if the paint has come off because the underlying metal is rusting, that would be a PAT test failure.

In theory, if you are having it PAT inspected/tested as part of your routine office saftey procedures, this should get picked up. Unfortunately, most PAT testing which is performed is a complete waste of time and money as the electricians who do it have never been trained in doing so, and have no idea what they are supposed to be doing. (Also, many companies PAT test far too often.)

[meow2222]

Its when rust has compromised the integrity of the steel cooking cavity that it is a problem, ie severe rust. I know many failures are declared following surface rust, but its a position that would be hard to defend.

NT

[Cynic]

No, the frequency is a function of the magnetron design and has nothing to do with the microwave oven cavity (which is *not* resonant).

You would need to filter the power supply to provide clean DC to the magnetron. Microwave ovens use an unfiltered supply (providing rectified AC to the magnetron).

The anode of the magnetron in a microwave oven is connected to common ground, and the cathode (which is also the heater filament) is fed from the negative output of the HV rectifier.

AM and FM modulation of a magnetron is extremely problematic, and so the usual modulation used is pulse modulation. It should not be too difficult to arrange the HV to the magnetron to be switched by a PCM or PWM circuit. I have no idea how fast you can switch a magnetron. A bonus would be that you could increase the peak power considerably. Used in conjunction with a large parabolic dish, I should think that its signals could be readily detected on Earth after being bounced off the Moon.

I have done a limited amount of experimentation with a microwave oven magnetron. If you plan on operating it outside the oven, be *very* aware of the dangers of high voltages (the mains transformer in the oven provides about 5000V IIRC and is *very* lethal), and also the danger of high intensity microwaves. Apart from the possibility of causing physical damage to yourself, you can fry electronic equipment from quite a distance away (as I discovered during my brief bout of experimentation).

There are lots of fascinating things you can do with an intact microwave oven. Try putting an old CD or DVD in the oven for a light show. Next time a lightbulb burns out, put the dud bulb in the oven to see it light up again (also works with non-dud bulbs). Be prepared to clean broken glass from the oven as the bulbs will sometimes shatter. A lit birthday cake candle also looks good in a microwave as the radiation hits the ionised flame. Be prepared to spend time cleaning the oven before the wife finds out.