Measuring the output of microwave oven

[NeoPTLD]

We have a five year old microwave at home and its spec is 1,000W output.

Without specialized instruments, it is not possible to directly measure the ERP of a microwave oven and I attempted to measure it with a simple calorimeter. I believe this should give a relatively accurate ERP if I know the microwave absorption coefficient of water.

These are my data. Could I change them for better accuracy?

Thermal mass: 1,500g of water and 107g of polyethylene container. Covered with Saran Wrap to minimize error from heat of vaporization and the air flow cooling the surface.

(1,500g x 4.186J/g°C) +(107g x 0.6176J/g°C)=thermal mass of the whole thing=
6345.08J/°C

Starting temp=16.3°C(after allowed to stabilize for two hours)
Temp after heating=42.9°C
Change in temperature=26.6°C
Duration of heating=200 seconds

Heat gain=thermal mass x delta T
6345.08J/C° x 26.6°C =1.69x10^5 Joules

1.69x10^5Joules/2.00x10^2 secs =845 Joules per second


Therefore 845W is turned into useful work.

The ERP must be (845W/coefficient of absorption)+conductive and radiant loss

Do the numbers look about right?

 

[Mday]

Originally posted by: NeoPTLD
We have a five year old microwave at home and its spec is 1,000W output.

no, that would be how much power it uses.

 

[NeoPTLD]

Originally posted by: Mday

Originally posted by: NeoPTLD
We have a five year old microwave at home and its spec is 1,000W output.

no, that would be how much power it uses.

No, it isn't.

It uses something like 1,500W.

The spec specifically says OUTPUT 1,000W.

 

[Amorphus]

Originally posted by: NeoPTLD

Originally posted by: Mday

Originally posted by: NeoPTLD
We have a five year old microwave at home and its spec is 1,000W output.

no, that would be how much power it uses.

No, it isn't.

It uses something like 1,500W.

The spec specifically says OUTPUT 1,000W.

rather inefficient, wouldn't you say? 1500->845

 

[RossGr]

I think your numbers look pretty good. I like your methods. I wonder what you would get if you placed, say, four containers uniformly inside the microwave. Perhaps there is some beam effects that a more distributed load would yield a higer wattage.

 

[NeoPTLD]

Originally posted by: RossGr
I think your numbers look pretty good. I like your methods. I wonder what you would get if you placed, say, four containers uniformly inside the microwave. Perhaps there is some beam effects that a more distributed load would yield a higer wattage.

It might yield a different answer, but I think it makes it more difficult and increase the error.

Sources of added errors are:

Smaller containers have a greater surface area per unit volume contributing to conductive loss.

Taking temperature one after another means more delay and more chance for them to cool. Since I don't have four thermometers, I'll have to let it reach equilibrium(a min or two) four times.

If I mix the contents into one container, I can do away with one thermometer, but the heat capacity of containers won't be factored in.

 

[silverpig]

Why don't you try melting a single ice cube? The ice is at whatever temperature your freezer is set to, so throw it in the microwave on a large plate very quickly, then turn on the microwave for 10 minutes. Watch until the last of the ice melts, then figure out how much energy it took and the time it took to do it.

Yeah, you'll be heating up the water too after it melts, but then you'll have a lower limit on the power.

 

[dkozloski]

Unless you have a perfect impedance match between the water and the magnetron, you will be measuring the output for the particular setup you have rather than the possible output. Any power not absorbed by the ice will be reflected back to the magnetron and dissipated by it's coooling fan. I think you will be able to get about any result you could want by moving the target around in the oven. In any case the highest number you get will be the most accurate.

 

[DrPizza]

Originally posted by: silverpig
Why don't you try melting a single ice cube? The ice is at whatever temperature your freezer is set to, so throw it in the microwave on a large plate very quickly, then turn on the microwave for 10 minutes. Watch until the last of the ice melts, then figure out how much energy it took and the time it took to do it.

Yeah, you'll be heating up the water too after it melts, but then you'll have a lower limit on the power.

The optimal frequency for water is different than the optimal frequency for ice. Thus, you'll find a lower output for the ice as more of the energy is reflected back and radiated (heat) from the microwave.

 

[Armitage]

I remember doing an experiment in H.S. measuring the temperature change of pure H2o vs. a sugar solution in a microwave for a set amount of time. The sugar solution got significantly hotter because the bigger molecules interacted more with the microwaves. So, you may be measuring the efficiency of water at absorbing microwaves rather then the output. Would be interesting to try the experiment on some different fluids of known specific heat. Not sure what to suggest ... maybe mineral oil?

 

[NeoPTLD]

Originally posted by: ergeorge
I remember doing an experiment in H.S. measuring the temperature change of pure H2o vs. a sugar solution in a microwave for a set amount of time. The sugar solution got significantly hotter because the bigger molecules interacted more with the microwaves. So, you may be measuring the efficiency of water at absorbing microwaves rather then the output. Would be interesting to try the experiment on some different fluids of known specific heat. Not sure what to suggest ... maybe mineral oil?

What is the specific heat capacity of the sugar water at whatever concentration?

Without knowing the exact heat capacity of the medium you're heating, there's no way to calculate wattage. Water is has among the highest heat capacity per unit weight of most substance and I'm pretty certain sugar water has a lower heat capacity and I'm not surprised the temperature got higher.

 

[Geniere]

Nice experiment, but I?m surprised the efficiency was as high as it was. I can only think of 2 things that may improve accuracy.

Per Rossgr, I think a container covering the greatest area would improve results.

Put an insulator under the container.

Regards

 

[DrPizza]

Originally posted by: Geniere
Nice experiment, but I?m surprised the efficiency was as high as it was. I can only think of 2 things that may improve accuracy.

Per Rossgr, I think a container covering the greatest ^Volumewould improve results.

Put an insulator under the container.

Regards

Exactly what I was thinking too.
I checked the Handbook of Chemistry and Physics and couldn't find the specific heat for any aqueous solutions with water...

But, How about this for fun: Heat capacity of mercury is around .033 cal/g
I haven't seen that one on the dangerous fun with microwaves page... Tell us what happens. (if you don't die from the mercury vapor induced cancer and other adverse health effects)

Come to think of it... I think I may just try to microwave mercury to see what happens... (<1 drop in a sealed glass container) - would it turn into a lightbulb? Would it absorb enough energy to melt the glass?
Anyone know what would happen? I'm guessing a mercury vapor lamp.

 

[Geniere]

I don?t think a droplet of mercury would do anything unless it was properly located on a max node of the RF. To find a node, line the bottom with several slices of cheese or chocolate bars. Observe when melt just begins; should take only a second or three. Mark the melted (nodal) points. Position your mercury droplet there. Because the wavelength is longer than the droplet diameter, I expect you?ll only get a heating effect without exciting arcs and sparks. To get it to light up, you?ll need (I think) to have gaseous mercury in an evacuated container of a length larger then the microwave wavelength, which would be equal to the distance between melted spots on the cheese or chocolate. I think a glow would appear at node points as ionization occurs.

I often played with mercury when I was a kid. Except for some mild dementia and nervous eyelid tics, I have no other noticeable problems.