is there a best place to drill into a microwave oven? 1

[ProfK]

I wish to insert ~1/2 inch OD glass and/or plastic tubing through a commercial microwave oven cabinet wall. The question is, might there be a particulaly safe location to do drill? I refer possibly to the corners of the cavity, a mathematically calculated position like 1/2 wavelength in from an edge, or in the direct center of a wall. I know the loading can shift the hot nodes but they might not ever form in certain locations; and that would be the best point for drilling. The microwave of choice is fed from the side and not the top, if that makes a difference.

 

[logbook]

If it were a section of rectangular waveguide I would go in at the middle of a long side. This has minimum effect as the currents are low in this region (assuming TE10 mode).

 

[Higgler]

The best location depends on the oven's power source location. Nowadays, most feeds are vertically polarized and mounted on the short wall. If you are putting any lossy thing inside the oven, then it will affect the fields, and there won't be much of a resonance whereby one location is perfect compared to other locations. Hence just use anywhere on the far side wall. Either the middle or the top corner.

It sounds like you want to heat liquid via the tube. If this liquid is water, then the 0.5 inch hole will allow RF to propagate out the hole when the water flows and you may create a problem.
Waveguide cutoff is 0.5 * 11. 803/ 2.45 Ghz = 2.41 inches, but with water dielectric = 81 flowing thru, the cutoff dimension becomes 2.41/sqrt (81)= 0.27 inches.

Hence, it may be necessary to use smaller below cutoff holes and combine them (if the dielectric of your liquid flowing thru the tube is high like water).
If you have some 2.45 Ghz phones available to detect the leakage, it may give you a hint of just how bad the leakage is. With a potential leakage problem, having the hole nearer the corner of the oven may be best. i.e. the corner furthest away from the power source feed.


So how large is the heating need, slight or max?

kch

 

[ProfK]

Thanks Higgler, some of the comments are very worthwhile. The heating need is to saturate a low loss material (powdered rocks) with the full output of the magnetron while pumping to a vacuum pump; hence the tubing. So, no, water liquid or other lossy material will not be passed through the wall (water vapor will). I am concerned that a field concentration may develop around any hole I make and possibly melt the glass or rubber. Another concern is the vacuum might even drop to the point a plasma might develop in a hot spot inside the flask. I should have a tray of the rock material on the bottom of the cavity to help couple stray energy while the sample in a glass flask is resting in the tray as it is being pumped upon. In a hot spot, I have melted a hole in a glass jar in air with 5 minutes of heating. Clearly, I can't use the turntable.
The waveguide aperature is long way horizontal and is below the midpoint of the short end wall; so your suggestion is to drill into the corner where the opposing wall and top wall of the cavity meet and to do so midway between the corners?
Furthermore, just to spice things up even more, the starting temperature will be -100 C and I am trying to see how well I can couple to the rock and the moisture within it.

 

[Higgler]

Sounds like a standard modern microwave.
"suggestion is to drill into the corner where the opposing wall and top wall of the cavity meet " Yes, go as far away from the waveguide aperture as possible, I think the top far corner is probably the coolest field location to make your entry hole.

I once used marshmallows to find the hottest spot in an oven, arraying marshmallows on the bottom of the oven and whichever one blows up the tallest shows the hottest spot. Don't fill the entire bottom, maybe 50% or less. It gives a hint of the power levels anyhow, not a perfect situation, but is interesting.

If you need to limit the power to your unit, place a water filled tupperware inside the microwave to absorb the power. Cold water might be best, since it'll take longer to boil.

You will still have leakage out the hole you cut. You may want to get some threaded pipe (2-6 inches long)and cut the hole the same size and thread a tiny bit of the pipe into your hole (0.1, 0.2 inches). If only a little pipe comes inside the oven up in the corner, you won't get arcing. That'll reduce the leakage out the hole due to cutoff waveguide losses and will also help support the plastic pipe and stop it from being cut by the sharp edged hole you make in the oven wall.

I'm curious what you're finding in this experiment? What gas is contained in a volcano?

kch

 

[ProfK]

Gas in volcano... SO2

Purpose is to use uwaves to extract water from soil on an unspecified unearthly body. Need vacuum and cryogenic temperatures to approximate the environment.

BTW, how might one employ a cell phone to find leaks? All I found was a claim that if one was put inside a closed uwave oven, it could still receive a call and ring.

 

[zeitghost]

2.4Ghz video sender receivers are good for detecting microwave oven leakage.

And cheap too...

 

[Higgler]

The microwave oven and 2.45 Ghz phones used in the home are at the same frequency.
This 2.45 Ghz phone isn't a cell phone I noted, but a home cordless phone. Home phones come in different frequencies, but the 2.45 Ghz ones seem to be the most popular.
The phones hear the interference from the microwave oven and it sounds like static noise in the phone earpiece.
You could tell if the oven leakage jumps alot by how far away you have to be when the phone picks up static.

kch

 

[ProfK]

Thanks H. I gathered as much when you suggested it. I thought maybe there was more to it. Regarding video sender, I get interference on mine when uwave is on too, but it would be hard to use that as a measure of signal strength.

 

[BrianR]

Simple microwave leakage detectors are available from electronic hobbyist suppliers very cheaply. They use a CB with an etched folded dipole antenna and a diode detector and give a relative reading. Useful for detecting that leakage is present. I think from memory that most use a hot carrier diode rather than a point contact diode like a 1N123, for its higher power handling capability.

Calibrated devices cost several hundred dollars.

 

[yachtsman]

Hotspot problem-----
My home microwave 12 yrs old suddenly developed a hotspot at the front center bottom. After running the unit for 3 minutes, the plastic lip seal had melted and scorched. I found that the "stirrer" in the top was not turning and concluded that the belt was broken, so replaced it, and then patched the damaged front seal with similar plastic molded with a hot air gun.

Now it still gets hot after 3 minutes.

Is there a problem causing this local heating, or is there a requirement for the repaired seal to be "tighter to the front door?

The unit is not replaceable since none of the appliance companies make a combination range/microwave any more.

I'd very much like to rescue the unit since it works fine.

 

[yachtsman]

Microwave Hotspot problem-----
My home microwave 12 yrs old suddenly developed a hotspot at the front center bottom. After running the unit for 3 minutes, the plastic lip seal had melted and scorched. I found that the "stirrer" in the top was not turning and concluded that the belt was broken, so replaced it, and then patched the damaged front seal with similar plastic molded with a hot air gun.

Now it still gets hot after 3 minutes.

Is there a problem causing this local heating, or is there a requirement for the repaired seal to be "tighter to the front door?

The unit is not replaceable since none of the appliance companies make a combination range/microwave any more.

I'd very much like to rescue the unit since it works fine.

 

[BrianR]

The dielectric loss of the new plastic might be higher than that of the original seal. If there is a scrap piece put it in the microwave and zap it and include a glass of water if running the empty oven will harm it.

 

[Higgler]

Sounds like an Amana Microwave oven with the mode stirring description.

The Rf sealing function on a microwave oven is not from the plastic seals. The sealing of RF inside the oven is from the overlap in the door to the housing being set to 1/4 wavelength (electrical length) at 2.45 Ghz to make a quarter wave open = a short circuit. That's 1/4 wave electrically, which includes the dielectric constant of the plastic on the door edge. Adding goop to replace the original seal needs to be the same dielectric constant as the original seal, or the 1/4 wave length changes and the seal becomes poor.
I'd bet you are leaking alot of RF outside your oven, I'd suggest throwing the unit away, or find another one and replace the door. Get a leak detector to check it.

kch