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I would like to leave a microwave running for several minutes (2-5, the longer the better) as part of a research project (I'm measuring the microwave emissions).

Is it safe to put a bowl of water in the microwave to prevent the magnetron from overheating? If not is there something else I could put in.

To answer the obvious question, I do not want to put food in the microwave because I intend to be running the microwave frequently, and it is not sustainable to buy a frozen pizza every time I want to run the microwave.

user2562609
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  • I'd suggest using a plastic bowl. Ceramics and glass seem to work as heatsinks, that's why the plate sometimes feels hotter than the food. – Oskar Skog Apr 13 '17 at 07:42

1 Answers1

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A bowl of water is the standard load for testing microwave ovens. Use a microwave-safe material for the bowl. Simple plastics like polyethylene or polypropylene are OK, not melamine which will overheat. Glass is OK, as are most plain ceramics, but some decorative ceramic glazes can overheat.

It's not so much that the absorption of microwave energy by the water stops the magnetron overheating (it's the noisy fan that does that), but that it prevents reflected energy causing high voltages at the magnetron output which could destroy it.

Use a large enough bowl so the water doesn't boil, some microwave ovens (like mine) do a rubbish job of ventilating steam from the cavity, leading to a lot of condensation. Condensation in the door seal may well alter your microwave leakage measurements.

As a rule of thumb, it takes more than 5 times as long to boil a quantity of water away, as it does to heat that quantity from room temperature to boiling. That will give you an idea of the safety margin for heating time. If the bowl boils dry, then you've lost the protection of the water.

As MrPhooky and Robherc have pointed out, it's possible to superheat water in a microwave under sufficiently clean conditions. If there are no nucleii to initiate bubbles of steam, the water could exceed 100C. If you then disturb the bowl, the water could flash boil, dumping all of its excess heat out as steam, very dangerous. This happens rarely, but once in your face is once too often. Mitigations are to a) make sure the heating time and water quantity take you nowhere near to boiling or b) use boiling stones or some other source of boiling nucleii in the water (a bit of eggshell, or a carrot top). Tap water is usually impure enough to be OK, but don't bank on it. Robherc linked this video which shows flash boiling.

Neil_UK
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  • What about heating steam and an empty bowl? – Oskar Skog Apr 13 '17 at 07:45
  • They must make sure the bowl is not perfectly clean as well, otherwise - boom. – MrPhooky Apr 13 '17 at 08:04
  • Actually, using a perfectly clean bowl & deionized/distilled water will give you some "free bonus" time before the water begins to boil. Without any deposits in the bowl, or concentrations of ions in the water, the water can easily be "superheated" in the microwave without initiating boiling due to a lack of anythign for the first "bubble" of steam to form around. This can safely be done, but it is imperative* that the water be allowed to cool below 100*C before* disturbing it...otherwise, this could cost you a bit of a hospital bill! – Robherc KV5ROB Apr 13 '17 at 08:31
  • +1 for the safety advice alone, but there's lots of great info in this answer. I remember PSA's from the 1980s warning about the danger of heating water in a coffee cup using a microwave. Simply dropping a teaspoon into the cup full of hot water could cause the whole cupful of water to burst out and badly scalding the person. Scary stuff! –  Apr 13 '17 at 09:27