and how effective are they at trapping the emissions?
They usually contain a metallic mesh, which should be an effective reflector of microwave frequencies.
The reason they don’t let the microwaves through is that they’re lined with metal mesh, as you can see inside the glass. The metal reflects the microwaves back into the oven.
Here is some info:
As for the holes in the mesh, don’t worry about them:
You don’t even need the glass in the door. I suspect it’s there more to give a sense of security.
Eh, not exactly. On it’s own the mesh is fairly fragile and unsturdy, and it would be easy for someone to carelessly kcock into it and tear it or push it out. It isn’t glass, either, it’s plexiglass, or occassionally some other polycarbonate plastic.
The mesh in my microwave is a punched aluminium or steel sheet. It’s not fragile at all. But I’d much rather be able to wipe down a smooth plastic sheet than try and get icky stuff out of a mesh.
The ones I’ve seen have a mesh that’s more of a foil sandwiched between two sheets of plexiglass or similar plastic. On it’s own it couldn’t stand up to an angry housefly, let alone normal kitchen wear and tear. They must make things tougher Down Under.
It’s just an ordinary Panasonic.
Anyway the amount of energy that will get through a mesh is given by:
1 - e[sup]-2x/λ[/sup]
where
λ = wavelength
x = aperture size
My microwave has holes 1 mm (0.001 m) in diameter in the mesh.
The microwave operates as 2.45 GHz, so:
λ = 3 x 10[sup]8[/sup] / 2.45 x 10[sup]9[/sup] = 0.12 m.
Plugging the numbers in:
1 - e[sup]-(2 x 0.001 / 0.12)[/sup] = 0.017 = 1.7%.
So the mesh stops 98.3% of the incident microwave energy from escaping, but 1.7% gets out. A good reason not to stand staring into the microwave while it’s operating.
Uh, so as not to be alarmist, I think what Desmostylus means by “incident microwave energy” is the stuff that isn’t dissipated anywhere else, like your food. So that 1000w microwave isn’t necessarily going to shoot 17 watts of RF energy at you.
Also, Desmostylus, that’s a pretty nifty equation and I’d never have though to dig it up. But would there be any difference between a grounded mesh vs. an ungrounded one? I have no idea.
Doesn’t make any difference. Why would it? We’re talking about e-m waves in space (air, but close enough), the concept of “ground” doesn’t really apply.
The equation gives the proportion transmitted vs reflected. It also assumes negligible dissipation in the mesh, which doesn’t affect these calculations terribly much.
And “incident microwave energy” just means what it says - the microwave energy hitting the door.
Thanks for the equation, Demostylus. That would be why the #%(#!& microwave in the lunchroom wipes out my wireless ethernet connection. 
Note that the mesh exists only in the door for the purposes of preventing microwaves from going thru the glass. If there were no glass, the microwave would be shielded in unperforated metal like the rest of it.
BTW, the 2nd opening of concern is for venting, usually from the side of the chamber out thru the back, with nice turns in between. Usually only messed up by DIY repairs or when the duct gets coated with stuff.
The 3rd source of leakage is around the edge of the door. If the door isn’t closing nicely, you’ve got a problem.
I am (among other things) an FCC-licensed broadcast engineer (GROL grandfathered from FCROL) and have in my bag of tricks a field-strength meter that functions at microwave oven frequencies and is calibrated, traceable to NIST standards.
I can put that meter up to the glass on the door of most microwave ovens and the needle barely budges, but if I move it to the door, the needle pegs.
Demonstrations freely available, but you’ll have to come over to my house, as I don’t make house calls.
Thanks for that equation, Des, that’s really interesting. I don’t get where you got the one value from:
λ = 3 x 10[sup]8[/sup] / 2.45 x 10[sup]9[/sup] = 0.12 m.
where does 3 x 10[sup]8[/sup] come from? (apologies if I’ve missed something obvious)
3 x 10[sup]8[/sup] metres per second is the speed of light.
To write the thing out properly:
λ = c / f = 3 x 10[sup]8[/sup] ms[sup]-1[/sup] / 2.45 x 10[sup]9[/sup] s[sup]-1[/sup] = 0.12 m.
:smack: 