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Author Topic: Does the microwave give off consistent energy levels?  (Read 1022 times)

Offline Duffield1

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Does the microwave give off consistent energy levels?
« on: 16 December, 2016, 07:24:22 AM »
Each morning, I make two bowls of Oats So Simple porridge, using exactly the same amount of milk in both and a full sachet of oats.  Each morning, I microwave each in an identical bowl for two minutes and thirty seconds.  Each morning, the porridge is almost boiling over in the first bowl when the timer bleeps, whilst the porridge second bowl is still about 1cm from the rim.

I thought this might be to do with slight inconsistencies in the oats, but it happens the same every morning.

Any suggestions on why?

Offline siasl

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Re: Does the microwave give off consistent energy levels?
« Reply #1 on: 16 December, 2016, 08:26:18 AM »
It could be to do with where you put the bowl in the microwave, even if on a rotating platter. Also, it could be to do with the distribution of milk & oats in the bowl.

For the first part of my answer, microwaves emit what's called "standing waves" which means the energy absorption pattern is not evenly distributed in space inside the oven compartment. Putting the bowl in a different place in relation to this will result in a different heating pattern.

Secondly, the heating power of the microwave is all to do with the absorption spectrum of the materials being heated. IIRC, microwaves operate at frequencies associated with high absorption by water, which results in most liquids heating quite nicely, but you'll find dry powders not heating much at all. Oats are quite dry. So if the bowls are mixed differently, this may account for that behaviour. Perhaps one is left to "steep" for longer, resulting in damper oats, perhaps making for a more spread out heat distribution, which means no boiling over?

The last possibly alternative is user idiocy :) Are you microwaving them both on the same setting?  whisl

Offline crabfoot

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Re: Does the microwave give off consistent energy levels?
« Reply #2 on: 22 January, 2017, 12:05:31 AM »
The heart of a microwave is a valve - a magnetron (in old ones) or a klystron - which generates the microwaves.

After running for a while the circuit can go into a state of saturation, at which point the microwave emission is reduced or stops entirely.
The first domestic microwaves' klystrons became saturated in a few minutes, but modern klystrons do not suffer from the same problems.

However, the klystron can wear out in a microwave, and from what you say it looks like yours is on the way out.  You are describing the saturation effect of a tired klystron.

You cannot legally replace the klystron - it is a simple task, but after it has been done the microwave has to be tested for leaks in the shielding, to ensure it is not cooking anything outside the microwave. 

In the 1970s the manager of a fish and chip shop I frequented cooked his kidneys into failure by using a badly shielded microwave for warming up meat pies. The door didn't close properly, and the leak was at kidney height.  He was only 36.

Generally, it is not worth fixing a microwave because of the testing needed. Buy a new one.
« Last Edit: 22 January, 2017, 12:15:23 AM by crabfoot »