I use a microwave oven for a lot of crafting projects (heating, curing, and drying materials). This often needs to be very precise and controlled. I experiment with times and power levels. However, I run into situations like these kinds of unexpected results:

  • I heat a material for 15 seconds at 50% power level and discover that it needs substantially more time. I repeat the experiment, doubling the time to 30 seconds at 50%, and get exactly the same result.
  • I heat a material for 30 seconds at 20% and discover that it needs just a tiny bit more time. I repeat the experiment, increasing the time slightly to 35 seconds at 20%, and find that the material is overheated and destroyed.

This isn't flaky microwave oven performance; the results are perfectly reproducible.

What's causing such big discrepancies from the expected results?

Just for clarification, a microwave oven is really the right tool for the applications I use it for. These are generally "interactive" requirements to add small, controlled increments of heat. The issue is that when power level settings are used for short durations, the results can be radically different from what is "logical".

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    Are you sure that the microwave oven is the right tool for the job? If heating is needed, maybe a proper heating oven should be used. From my experience (and countless reports on the net), the microwave oven is not good enough even for heating food. Not to mention the destructive change of the molecules of the substance, which might lead to veery unexpected failures over time. You might be able to re-purpose the microwave oven to a normal heating oven - and the heat there can be controlled very accurately with minimal "effort". – virolino Dec 19 '19 at 6:13
  • @virolino, good points, and for a lot of applications, that's all true. I use a microwave oven mainly for cases where I want to work at "room temperature" and generate small amounts of heat in the material quickly and in a controlled, interactive way. A microwave is convenient for things like working with tiny batches of material, doing partial cures to a specific point, drying in small increments, etc. Some "bulk" materials, like "cold porcelain" are typically made in a microwave oven because the process is kind of interactive, and it is easy to heat in small increments. – fixer1234 Dec 19 '19 at 7:15
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    The two examples you provided in the question make me assume that the controlling of the power is not he only (the main) problem, but an important thing is the less-than-uniform distribution of the microwaves inside the oven. So instead of fiddling with the power, fiddle with the position of the material on the plate (lower, higher, closer to the center, further away from the center...) – virolino Dec 19 '19 at 9:04

Only a few microwave ovens control the microwave power level on essentially a continuous basis. That is, if you set the power level at 30%, the oven runs at 30% power output. Most are based on turning the microwave power on at 100% for a given portion of a fixed time increment, like 30 seconds or 1 minute. Then the microwave power shuts off for the remainder of the time increment, and the cycle repeats.

My microwave oven is based on a 30 second cycle, so the power level setting translates to 10% increments of 30 seconds. At 50% power level, there is microwave power at 100% for 15 seconds, none for the next 15 seconds, then the cycle repeats. If the time is set to less than the resulting power-on time, the oven stops when the time runs out.

So in my first example, with a 50% power level, the material gets 15 seconds of full power whether the timer is set for 15 seconds or 30 seconds. In the second example, with a 20% power level, the material gets 6 seconds of full power at the start of the first 30 seconds, then another 5 seconds of full power before it shuts off at 35 seconds (almost double the energy despite the small time increase).

To get precise amounts of microwave power using the power level setting, you need to base it on the cycle time of your oven. You can tell what the cycle is for your oven by listening to it cycle the power on and off and check the oven's clock to see the time between power-on cycles. If you need brief bursts of microwave power, you may need to use a very low power level (and the process may take a long time).

The way the power level actually works makes it not particularly useful for short-duration requirements. For those, I typically do what Chris H suggests in the comment -- do it manually at full power to get exactly the duration needed. The main benefit of the power level setting is when you need a lot of cycles of short duration, and doing it manually would be tedious. Understanding the cycle action lets you ensure that the duration at full power doesn't exceed a certain number of seconds at a time.

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    Alternatively you can set full power (so it's predictabl) and run in much shorter bursts manually. – Chris H Dec 19 '19 at 7:22

I suspect you're heating fairly small quantities. If so another way (in addition to Fixer1234's answer) to reduce the power reaching your material is to heat something else at the same time. This could be scrap material, or it could be a cup of water (with a loose lid if you want to minimise steam).

Very very roughly the energy should be absorbed in proportion to the mass (I'm making some big assumptions* here, but it's somewhere to start). So if you weigh your materials, and add a cup with the same weight of water, you should effectively get about 50% power. Overall I suggest combining this with reducing the power setting.

I've actually given up on melting chocolate in the microwave, unless it's mixed with butter, as it's more fiddly than heating it in a dish over a pan of boiling water. That may be suitable for you.

* One important assumption is that the microwave heats evenly. That's probably not true; most have hotspots, so position is important. Another is that water and your material absorb microwaves equally. If the material is designed to be microwaved it's probably a decent absorber, likely water based, so it will be close enough.

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    This is a good idea for applications where you want a safety margin to avoid overheating. For my own case, I was really looking more at predictable amounts of power. Short durations using the power level settings were producing unpredictable results. Once I figured out why, I started doing what you suggested in your comment, which was spot on. – fixer1234 Dec 19 '19 at 9:17
  • @fixer1234 (just realising it's your Q as well) the effect should be predictable for a predictable ratio, e.g. if you always heat the same amount of material, always use the same amount of water. Other options include an inverter microwave and one with a lower maximum power (though those tend to have the most basic controls) – Chris H Dec 19 '19 at 9:55
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    LOL, an inverter microwave (and then lack of one) is how I ended up with the problem. My previous unit was an inverter microwave, and I assumed they all work the same way. The replacement unit isn't and doesn't, which led to this question. :-) – fixer1234 Dec 19 '19 at 10:07
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    @fixer1234 Here's an example of how uneven the heating can be: evilmadscientist.com/2011/… – user3067860 Dec 19 '19 at 15:37

Neilmed quote The pic from the NeilMed Instructions shows something that is important to know about microwave ovens, “Previously heated microwaves will produce much more energy.”

I wanted to have an official citation for this fact. This is the disinfection process for a neti pot or neti rinse. In fact, I have experienced melting the plastic from using the microwave too soon.

Consider adding rest periods during your timing experiment.

Hope this is helpful. 🌊 🌙

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    This is an important point for using a microwave oven for crafting. I think NeilMed's writers might have made a technical error or over-simplification in explaining "why", but it's correct that unexpected things can happen when the inside of the oven is hot. For example, glass is considered "microwave safe". But if it is hot, it absorbs microwaves and gets hotter. I couldn't lay my hands on a YouTube video I saw of the glass turntable in a microwave oven glowing red hot after running the oven with the turntable already hot. (cont'd) – fixer1234 Dec 21 '19 at 5:45
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    But here is a similar video of melting a glass bottle by preheating it: youtube.com/watch?v=xwEQZw3KPWg. If the inside of the oven or the turntable is hot, especially if you run it without much material that normally absorbs microwaves, you can get weird stuff, like the turntable acting like a hotplate, or in general, results not consistent with what happens when the oven is cool. – fixer1234 Dec 21 '19 at 5:45
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    There's another small but significant effect that you'll notice if you heat a second identical item. Heating something in a cold microwave warms the turntable and walls by normal thermal transfer from the warm food (or craft material). This slows the heating. When you put the second item into a warm microwave, it gains some of the heat stored in the microwave body. I'm not sure whether it's significant in the context of the question, but in the context of this answer it is. I've time I've noticed it is sterilising jars for making jam. – Chris H Dec 21 '19 at 7:18

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