So, just how powerful is that Microwave??

[kb0nes]

I have several different microwave ovens I use on a regular basis, both vintage and modern. I have noticed marked performance differences between them. So I wanted to attempt to measure the actual output power of each oven.

Its actually a simple procedure to just measure the heating of a known quantity of water in a defined amount of time. Years ago a calorimetric power test like this was done to measure transmitter power on UHF and higher frequencies where availability of accurate meters was limited.

The math to derive wattage is actually pretty simple, here is the formula:

Power(watts) = Vol of H2O(ml) x Temp Rise(C) x 4.187 / Time(sec)

For the curious the 4.187 constant is the conversion from calories to watt seconds. One calorie is needed to raise a ml of water 1 degree Celsius.

Here is a photo if the 1984 Whirlpool microwave used for this test. The next couple posts will show the simple process.

 

[kb0nes]

Known Volume of Water

First off find a microwave safe container. I used an old 1 gallon milk jug since it has very little thermal mass to induce error in the results.

1 milliliter of water happens to weigh 1 gram (coincidence eh?) so I just used a postal scale. The jug weighed 66 grams which I normalized out with the Tare function.

Photo shows the jug with 1,000 grams of water. If you don't have a scale you can measure 1000ml or 33.8oz.

 

[kb0nes]

Starting Temp

Using a thermometer record the starting temperature of the water. Ideally the water should be in the 20 deg C range to prevent it from getting too hot when its heated. If we get close to the boiling point then the temperature change goes non-linear.

In my case I had to lay the jug on its side to fit in the oven cavity.

Starting temperature is 25.3C

For this test I chose to run the oven for a total of 200 seconds (3:20) total time. The greater the volume of water and the longer the run time the more the variables are normalized. Things like thermal mass of the jug and warmup time of the Magnetron etc.

 

[kb0nes]

Ending Temp

was 53.5C for a temperature rise of 28.2C for this oven over the 200 second run. Its important to stir or agitate the water a bit before taking the final reading, warm spots can stagnate skewing the results.[this post was last edited: 7/9/2014-19:04]

 

[tomturbomatic]

This is a report in progress.

 

[kb0nes]

So plugging all that into the formula:

(1000ml x 28.2C x 4.187) / 200 = 590 watts which seems a bit low but is believable.

Out of the three microwaves I use this one seems to be the slowest. The Panasonic Inverter model I repaired a couple years ago is WAY faster.

If input power was measured while doing this test then efficiency (or lack thereof) could be easily measured.

This test could be easily duplicated and results should be pretty repeatable. It would be interesting to hear how some of those really vintage microwave ovens stack up against the more modern units too. If anyone else bothers to try this at home please post your results!

 

[pcasa]

radarange

I love all my Amana Radaranges - but they suck at popping popcorn

 

[xraytech]

Paula,

I think microwave popcorn can be harmful to your microwave oven.

Besides popcorn tastes so much better stovetop or from a vintage popper

 

[jeffg]

What's strange imo is that there doesn't seem to be much relation between output wattage and actual cooking performance. E.g. a 1980 Panasonic was our daily driver for 30+ years (until it finally developed door latch problems). and according to specs it's only a 700W unit. But frozen food cooked according to package directions comes out perfect, while the 1200W inverter Panasonic unit we replaced it with produces frozen food badly overcooked at the edges, and/or underdone in the middle. Even when power output is reduced to what should be the same or similar as the 700W. The two units are very similar in design (size, turntable etc) so we're at a loss to explain it.

 

[kb0nes]

Jeff,

With actual food in the cavity energy distribution is likely to be every bit as important as output power. Especially frozen food which is difficult not to over cook the outsides and thin sections while still leaving the middle frozen.

Heating water is easy since the the convection in the fluid minimizes temperature variations. Also with that much water in the cavity the loading is very uniform which may not be true with a piece of frozen food in there.

The way older microwaves controlled their output by pulsing the magnetron is actually beneficial for defosting since it allowed dwell time for heat to conduct through the food. Modern inverter units that modulate actual RF levels may have more uniformity issues defrosting frozen foods.

I was only trying to test ultimate power, trying to test actually cook-ability is an entirely different and deep can of worms! I'll let someone else sort that out...

 

[jeffg]

"The way older microwaves controlled their output by pulsing the magnetron is actually beneficial for defosting since it allowed dwell time for heat to conduct through the food. Modern inverter units that modulate actual RF levels may have more uniformity issues defrosting frozen foods."

Thanks for the info. It explains a lot of online user reviews for microwaves.

 

[earthling177]

Jeff, I don't know your old microwave, but a lot of older nukers had a stirrer fan in lieu of a carrousel tray to distribute the microwaves. Some of the first nukers to have the rotating tray (to improve cooking uniformity) also kept the stirrer fan and those machines cooked very evenly.

When the bean counters removed the stirrer fan to save a buck or two, it became obvious to me that just the rotating tray doesn't cut it, but most people don't clue into it because they just use the nukers to rewarm food and make pop corn, while people who actually want to use the machine to actually defrost and/or cook are few and far between.

 

[jerrod6]

Can't we just look at the label on it?

 

[whirlcool]

I believe our old Litton was 750W and it was a pulser. You could see the fan on top rotating throught the translucent shield in the top of the oven cavity. It was fast for it's time.

Now our 2008 GE microwave (1100W) is much faster, but it's a pulser too. You can see and hear the pulsing with the light dimming/brighter. We really like this oven. It's an over the range oven and even though it's been in daily use for six years the thing still looks like new, it sparkles! What I like best about it is that the turntable platter and the little ring it rides on can all go in the dishwasher for cleaning! And just boil some water with vinegar and lemon juice and the thing cleans up just by wiping it out.

 

[kb0nes]

Can't we just look at the label on it?

Sure we can... But it only tells you the average power consumed, it may or may not correlate well with the RF power into the cavity. Efficiency levels vary with differing circuitry and there is always the possibility that something has failed in a given unit.

I'm especially hoping that we can see some tests run on the earliest vintage microwaves, like some of the 900Mhz microwave/electric resistance combo ovens. I'd love to get an idea on the output of those beasts.

 

[wayupnorth]

My 1979 Whirlpool looks similar. It is a Mark Series, has 3 cooking cycles, probe etc., that I never use along with the huge harcover cookbook that came with it. It says output 350 watts. I use the minute timer, clock keeps perfect time, set it at 1 minute start. If its not warm enough, repeat. Thats what I have done for 35 years.

 

[pcasa]

the "pulsating"

.. just a funny to share - before I changed out the power panel in my 1955 house and upgraded the electrical service from the pole - my entire house would "pulsate" when I ran the Radarange on defrost - you could stand outside and watch the house lights slightly dim - then bright again rhythmically - hahahahahahahhahahahahaha

 

[kb0nes]

Just repeated the test on my home over the range KitchenAid microwave.

Results of two runs averaged 693 watts calculated.

Model tag says 700 watt output. Assuming the appliance is in good shape that validates the test. I would have estimated this oven to be about 20% better then the first one I tested in this thread.

 

[gansky1]

Very interesting thread, Phil, as always!

I will give this test a go in the Hotpoint micro-range over the weekend when I have some time.

 

[tomturbomatic]

Only pop corn in an Amana with the Amana cornpopper. It has metal in it to capture the power and provide heat for the corn. Most popcorn does not have enough moisture to capture the microwaves so you risk overheating the magnetron if you try to pop corn without the popper.

 

[kb0nes]

Tom,

I remember a service call I made with Jeff in the early 80's where a microwave was dead. Inspection upon dis-assembly showed a hole melted through the glass envelope on the magnetron. The hole was at a hot spot due to low cooling airflow being sheltered behind some support structure.

We noticed one of the old cone shaped plastic microwave popcorn poppers in the dish drainer. We asked about it and they said they had popped a couple batches then it just quit! Back then I don't think those poppers had anything to absorb energy in the base.

At least popping pre-packaged microwave popcorn is generally better for the oven since there is usually a fair amount of fat/oil in the package. The low fat products though surely don't provide an adequate load for the oven. Most ovens state minimum loading of 50ml water or the equivalent.

 

[philr]

I did an experiment with two of my microwave ovens. I didn't calculate how powerful they were but I just wanted to see if there was a difference between both. 

 

I heated two cups of water siultaneously in both my 1969 Amana RR-2 and my 1978 Frigidaire RCM-10 for 3 minutes. I didn't have identical plastic containers on hand so I used regular bowls which will certainly affect the result (as they did absorb quite a lot of heat from the water).

Phil, if you're able to calculate the approximate wattage from the quantities and temp rise shown, I'd like that! I'm a bit confused by your formula (I'm very poor in Maths!). Sorry I didn't think about switching the thermometer to metric values.

 

The pictures show the temperature readings at the beginning of the experiment and both microwave ovens set on 3 minutes and full power (not that I had much choice with the RR-2!).

 

The video shows the last 20 seconds of heating and the not very scientific results!

 

 

[kb0nes]

Phil,

I'd be glad to help with the math part.

Converting the 2 cups to metric gives us 500ml

The Temp rise is 43.7C for the RR2 and 48.6C for the RCM-10

Throwing those numbers into the formula and using 180 seconds for the run time we get (drumroll)

The Amana RR-2 is 508 watts and the Frigidaire RCM-10 is 565 watts.

Depending on the precision of the volume of water measurement, the accuracy of the infrared thermometer and thermal loss into the bowls these numbers may have a little variance. But they are surely in the ball park for what one would expect!

Thanks for playing along Phil! Next microwave experiment may involve a marshmallow, chocolate and graham crackers

 

[kb0nes]

We have a new leader!

Here is a ~ 10 year old Panasonic inverter microwave that I just tested. This thing is hell for strong:

929 watts measured!!

It's the first appliance to wig out my Kill-A-Watt too drawing almost 19 amps(!!) from the wall. Well more then a 120v wall outlet, even a 20 amp, should endure with any duty cycle.

 

[philr]

Thanks Phil for the maths! That's about what I expected. I think the Amana is rated at 675 watts and the Frigidaire at 750. 

 

I need to do the same experiment with plastic containers like you did to see if there's any difference! 

 

 

 

[kb0nes]

Here is the test results of the Panasonic microwave I serviced a couple years ago. This is the one at Annette's parents house that had a catastrophic inverter failure (big cloud of vaporized transistor smoke!). This oven has the old style inverter design from roughly 15 years ago.

The inverter repair thread is here:

http://www.automaticwasher.org/cgi-bin/TD/TD-VIEWTHREAD.cgi?43694

In any case this oven tested at 740 watts, not nearly as strong as the newer model I tested up thread. That oven has the new style inverter design. I have a feeling I will need to pull it apart next weekend and measure the B+ voltage to the magnetron. The older design was 3.6Kv into the tube, I wonder if the newer oven is higher, hmmm

 

[sudsmaster]

I have both a Panasonic Genius Inverter microwave, circa about 2006, and an earlier, circa 1999, Kenmore Elite microwave.

First, both ovens have a magnetron. You can't have a microwave without one. The difference is in the power transformer. Older designs us a heavy magnetic power transformer that can only output one power level, and achieves lower power settings by pulsing, or turning the power on and off. The inverter power transformer can output multiple power levels without having to pulse 100% power.

The Kenmore has both a rotating turntable and a big six inch stirrer on the side of the chamber.

Each one has its own good points and lesser points.

The Panasonic, I think, is better for low power, slow heating of foods. However it would be wrong to say that it doesn't pulse. I recall reading somewhere that the inverter really only has about three power levels: 100%, 60%, and 30%. The power levels in between that are achieved by pulsing the next higher level. It's a more gentle pulsing than the regular microwave, not a full off/on cycle, but detectable in a quiet room nonetheless. The turbo defrost function is interesting. Although the control panel says it's at 30% power, it seems to me that it's a lot higher than that, because setting the control for 30% for the same amount of time results in much more gentle heating that the turbo defrost can do. I often use the turbo defrost to heat up small amounts, such as a pizza slice. It works well for that.

A drawback to the Panasonic is that it seems to heat foods from the bottom. Quite often, such as when heating a frozen pre-cooked burger patty, I find it advantageous to heat for half the time, flip the patty over, and finish the heating. Same for burritos or chimichangas.

The Kenmore, which has a magnetic transformer, really excels at microwave popcorn. The automatic setting if very good, never results in burned kernels, and pops all but a few of them. I find it's also very good for reheating beverages. The Kenmore seems to heat more evenly than the Panasonic, without causing hot spots on the bottom of foods. Still, I find myself using the Panasonic more often because it's in a more convenient location and I find the Panasonic controls to be easier to use than the Kenmore (which has a silly rotary knob to make selections and set times). The Kenmore seems to have better build quality, and I prefer the look of its magnetron cover, which is a large translucent amber plastic dish behind which one can see the rotating wave stirrer. The Panasonic has the more typical funky and harder to clean gray mica based magnetron cover.

 

[kb0nes]

Rich,

Indeed there are a lot of variables when it comes down to "cookability" of the appliance. Cooking performance goes far beyond just how the power level is adjusted, duty-cycle vs variable RF output. Cavity size and layout, stirrer or turntable (or both?) make a bigger difference in the way the oven actually performs when used to cook food.

In this testing I am only looking at ultimate RF power, not considering distribution at all. When heating liquids nearly 100% of the RF will end up absorbed into the water eventually after bouncing around a bit. Stirring the water before taking the final temperature measurement averages any hot spots that might exist in the container.

I will be curious to see if the two Panasonic microwaves I tested have the same magenetron model. If the tube is the same then it all comes down the B+ voltage that they hit the tube with.

The Panasonic inverter is a semi-continuously variable output voltage design, there are some Hams out there that are experimenting with using them to generate HV for tube radio transmitters. For grins I may have to do some measured testing to see how the power varies. If nothing else it would be interesting to see a curve of generated RF from the magnetron vs B+ voltage. I'm not sure how linear the transfer curve is. This could be calculated from the device performance curves but its fun to see if theory aligns with practice too.

Now I just wish I had a spectrum analyzer that would go to 2.5Ghz, mine stops at 1.2G