Cool Thing Of The Week — Microwave Hot Spots

Ok, so we were sitting around thinking about our microwave. You’ve probably come across hot and cold spots in your microwave before. But where are they located, and how can we figure it out? Well, we figured that a good way to do it would be to use the microwave to heat something, and then image it with a thermal camera.

The photo on the right shows the microwave, with the target removed to be run under cold water between runs.

Ingredients:

  • 1x Microwave Oven (duh)
  • 1x Ordinary HDPE breadboard, that you don’t mind cutting up a little
  • 1x Ordinary A4 piece of paper
  • 1x Ridiculously expensive thermographic imaging camera, borrowed (Seriously. This cost about twice as much as my car did.)

Mixing:

Thermal Camera:

Ok, so the first rule to bear in mind is this. Thermal cameras will lie to you. I’ll say it again. Thermal cameras will lie to you. There are two ways in particular that I’ll cover, although there are a few more. Professional thermographers have an accreditation process and lots of training before they sell their services. Let’s take the photo below. There are two ways in which the image of yours truly enjoying a frosty beverage is inaccurate:

The first is that the scale of the image is only set from 12.1*C to 35.8*C. So Anything outside that region will not be displayed correctly. This isn’t too bad, and anyone who works with graphs or scientific images will expect that. The second way that the image will lie to us is that the measurement of the object depends on the emissivity of the material it’s made from. If you try and image shiny metal objects it’ll mostly capture the infrared light that’s been reflected off it, instead of the light it gives out iself. The emissivity of aluminium is about 0.09. So that means it’s mostly reflecting the temperature of the room behind the camera, instead of the can itself. The emissivity of paper is 0.93. So that’s pretty good as a target for our imaging.

Microwave Stand:

We used a layer of wet paper to absorb the microwaves and get hot for us to image. In order to hold it upright we made a stand out of an old breadboard. According to this, HDPE is listed as ‘may be’ microwave safe (I’m not entirely reassured…). So we’re assuming it’s not absorbing the microwaves too much.

Bec holding the HDPE & paper target

Before each run we carefully ran the target under cold water for 30sec to a minute. This was necessary to make sure it was a uniform temperature across the whole plate. We then microwaved the plate for 10 seconds, and observed the temperature through the camera.

Results:

Arghhh… I hate wordpress images! It won’t display them nicely. Check out the results here, if you’d like to see details: https://sites.google.com/site/mechatronicsguy/coolthingoftheweek/microwave-hot-spots

Run 1: 2cm from the back Run 2: 7cm from the back Run 3: 12cm from the back Run 4: 16cm from the back Run 5: 16cm from the back Run 6: 20cm from the back Run 7: 23cm from the back Run 8: 27cm from the back Run 9: diagonal Run 10: diagonal

Replication:

So the question is naturally raised, how repeatable is it? Are the patterns random each time, or is it determined by the position in the oven? So we ran the same position twice. And between each run we fully removed the target and ran it under water to cool it down to a uniform temperature, replacing it back in the same measured position in the oven.

So, we’ve got a really nice, repeatable result for where the hot spots are in the microwave. Does this mean that we can now put in our hot pockets in the optimum spot, safe in the knowledge that it’s perfectly scientifically validated?

Well, sadly not quite. Putting an object in the microwave will actually change the standing wave patterns, since it’ll absorb and reflect microwaves. Still, it was interesting to gain an insight into what’s normally an automatic process.

Bonus: Makerbot Thermographic Porn!

At the same time we were doing the microwave experiment, Chris & Max were assembling our new heated build platform for the makerbot. So we thought we’d get some shots of it in action… I was quite suprised how hot the extruder controller got. You can see the max temp is 68*C. (Sorry for the shoddy scaling in the image, anything over 35.8*C is saturated. The maximum is reliable, though)