Microwave oven efficiency - what happens to energy not absorbed by food?

[RedKnight]

Hi folks,

 

Info on microwave ovens often talks about the direct efficiency of converting electrical power to microwaves. Ok, that's pretty straightforward; I am NOT asking about that.

 

I can't find much of anything that addresses the "efficiency" of the microwaves that are created, being converted to heat in food. Said another way, what happens to microwaves that "miss" the food; are they simply lost? Many illustrations showing magnetron emissions appearing to bounce around in the oven, and info says the walls are highly reflective... do they keep bouncing until they hit food, conferring very high efficiency?

 

Or is this even the proper way to think about it, since they make standing waves?

 

I realize a lot depends on exactly what is being heated. To make things simple and practical, the core issue driving my question is that I'd like to know how much I should care about, say, spreading a big mound of mashed potatoes flat before microwaving it. How much will it matter in terms of efficiency (which also equals less time and electricity)?

 

Is it extremely inefficient to microwave things that are not flat - like tall cups of water?

 

Thanks if you can help!

 

Mike

 

 

[Enthalpy]

Yes, waves are mainly standing in an oven. Anyway, with around 0.1m wavelength, few lengths fit in the oven, so "bouncing" isn't as sharp as light on a mirror.

 

The oven cavity is meant to couple efficiently the wave to the target, and a standing wave helps there, the long wavelength also; especially, thoughts like line-of-sight, familiar with light, don't apply and are not a limit.

 

Not all the power is coupled into the target, sure. The walls are designed to absorb little, and even the door joint as far as possible, which is less easy. The rest would naturally bounce back to the emitting magnetron, but this is not desired, as it would hamper proper operation in extreme cases, especially when the oven is empty.

 

To avoid this back bounce (or "reflected wave"), the feed from the magnetron to the oven cavity includes a special (and a bit tricky) component that absorbs power only from one direction, that is the once bounced back.

[Sensei]

Wifi network around turned on microwave oven is stopping working or working significantly slower.
On YouTube there are videos showing download speed with turned off and turned on oven.
So at least some microwave photons get out of oven and are interrupting transfer.

Wifi 2.4 GHz uses the same frequency range as customers ovens.

I don't think so we can generalize efficiency. It's probably depending on manufacturer, used technologies etc. I don't expect much from no name, cheap stuff.

Come to oven with laptop with f.e. http://www.speedtest.net running and you will see.

 

[swansont]

Wifi network around turned on microwave oven is stopping working or working significantly slower.

On YouTube there are videos showing download speed with turned off and turned on oven.

So at least some microwave photons get out of oven and are interrupting transfer.

Wifi 2.4 GHz uses the same frequency range as customers ovens.

 

 

Wifi routers are generally at around 0.1 Watt of power, so if you have 1000W of microwaves in your oven you would need less than 0.01% leakage to be seriously interfering with the wifi signal.

[RedKnight]

Thanks for the general info, folks. But I thought it was clear I was looking for a quantitative or decisive answer. (Or a decisive answer that there can't be a decisive answer.)

 

Can anyone answer the question quantitatively? An actual value for how much it matters to flatten mashed potatoes? Or use a tall glass instead of a bowl, for the same amount of water?

 

Just ballpark estimates. Does it matter enough to worry about?

 

In case I need to clarify: "maybe" is not what I'm looking for. "It depends" isn't either, unless followed up with quantitative answers.

 

Thanks!

 

Mike

[Sensei]

Do you have microwave oven?

Do you have wattmeter? Similar to this one:

You can make set of experiments by yourself.

 

Measure water mass f.e. 100 grams.

Measure temperature of water.

Place in oven.

Turn microwave for f.e. 10 seconds.

Measure temperature of water again.

Now you know delta temperature between start and end of experiment.

 

We know that 1 calorie is energy needed to increase temperature of 1 gram of water for 1 C.

1 calorie = ~4.1855 J or so (not constant, depending on many variables)

http://en.wikipedia.org/wiki/Calorie

From wattmeter you know energy usually given in kWh so you need to convert to joules.

So you can calculate total energy consumed by device, (including lost energy).

According to wikipedia microwave oven can be up to 64% efficient.

 

f.e. mass of water is 100 g, start temperature of water = 0 C,

end temperature of water after some time will be 80 C

there is needed approximately 100 g * 4.1855 * 80 = 33484 J of energy.

 

With 1100 W microwave oven, with I= ~4.8 A and U=230 V, and efficiency 64% we should expect such temperature of 100 g of water after ~48 seconds (if glass/container won't absorb energy and there will be no further loses (hard to calculate in memory without performing experiment in real) ).

Edited March 22, 2014 by Sensei

[Enthalpy]

The size of the target makes no important difference to couple with the field. But the field doesn't penetrate water deeply.

 

I didn't find the penetration depth quickly in my doc, but for brain's white matter at 2.45GHz, the relative permittivity is like 40 and the conductivity 1S/m. This would give a depth <1cm. It explains why thick food must cook for longer in an oven, even with microwaves.

 

Wifi routers are generally at around 0.1 Watt of power, so if you have 1000W of microwaves in your oven you would need less than 0.01% leakage to be seriously interfering with the wifi signal.

 

And because the over-genius designers of Wi-fi and Gprs put their transmission in a band where ISM (ovens and so on) are primary users, and transmissions basically forbidden but meanwhile tolerated, a transmission at 2.45GHz hampered by an oven cannot complain. But if some day the 1MW oven of your neighbour industry were hampered by your 100mW Wi-fi, you would be requested to shut it off.