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.
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 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.
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.
Run 3: 12cm from the back Run 4: 16cm from the backThanks to Bec for making this GIF.
Run 6: 20cm from the back
Run 8: 27cm from the back
Run 10: diagonal
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.
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)