How to test your microwave

[Ozz]

Title says it all...
If you are wondering if your Oatmeal heater is functioning, here is the test: Testing the oven - the water heating test
The precise number of degrees a known quantity of water increases in temperature for a known time and power level is a very accurate test of the actual useful microwave power. A couple of minutes with a cup of water and a thermometer will conclusively determine if your microwave oven is weak or you are just less patient (or the manufacturer of your frozen dinners has increased their weight - sure, fat chance of that!)
You can skip the heavy math below and jump right to the final result if you like. However, for those who are interested:


1 Calorie (C) will raise the temperature of 1 gram (g) of liquid water exactly 1 degree Centigrade (DegC) or 9/5 degree Fahrenheit (DegF).

1 Calorie is equal to 4.184 Joules (J) or 1 J = 0.239 C.

1 Watt (W) of power is 1 J/s or 1 kW is 1000 J/s.

1 cup is 8 fluid ounces (fl.oz.) which is 8 x 29.57 g/fl.oz. = 236.6 g. (For Avoirdupois ounces, use 28.35 g.)

1 minute equals 60 s (but you know this!).
Therefore, in one minute, a 1 kW microwave oven will raise the temperature of 1 cup of water by:

T(rise) = (60 s * 1000 J/s * 0.239C/J * (g * DegC)/C)/(236.6 g) = 60.6 °C.

Or, if your prefer Fahrenheit: T(rise) = 109.8 °F.
To account for estimated losses due to conduction, convection, and imperfect power transfer, I suggest using temperature rises of 57 DegC and 135 DegF.

Therefore, a very simple test is to place a measured cup of water in the microwave from the tap and measure its temperature before and after heating for exactly 1 minute on HIGH. Scale the expected temperature rise by the ratio of the microwave (not AC line) power of your oven compared to a 1 kW unit.

Or, from a Litton microwave handbook:


Heat one Liter (L) of water on HIGH for 1 minute.

Oven power = temperature rise in DegC multiplied by 70.
Use a plastic container rather than a glass one to minimize the needed energy loss to raise its temperature by conduction from the hot water. There will be some losses due to convection but this should not be that significant for these short tests. For the ultimate in accuracy (as these things go), put the water in a styrofoam cup, invert another styrofoam cup over it, and poke your thermometer through it.

(Note: if the water is boiling when it comes out - at 100 DegC or 212 DegF, then the test is invalid - use colder water or a shorter time.)

The intermediate power levels can be tested as well. The heating effect of a microwave oven is nearly linear. Thus, a cup of water should take nearly roughly twice as long to heat a specific number of degrees on 50% power or 3.3 times as long on 30% power as on full power. However, for low power tests, increasing the time to 2 minutes with 2 cups of water will result in more accurate measurements due to the long period pulse width power control use by microwave ovens which may have a cycle of up to 30 seconds.

Any significant discrepancy between your measurements and the specified microwave power levels - say more than 10 % on HIGH - may indicate a problem. (Due to conduction and convection losses as well as the time required to heat the filament of the magnetron for each on-cycle, the accuracies of the intermediate power level measurements may be slightly lower).

[exav8tr]

Jim. That is interesting, I think I found another way. If the popcorn turns black after the given amount of time on the bag, your microwave is working fine!!!!! If the popcorn doesn't burn you count the number of Old Maids, throw them away and eat what's left!!!!!!!

[capn chris]

Ozz,
You seem to have run out of projects or Sue has taken a trip. Either way, you have way too much time on your hands!!! Your post didn't specify the make and model of the recommended thermometer? Kidding!

[Ozz]

Ha! That's a good one Phil. I read a popcorn package the other day, I thought a bag of popcorn was a light calorie treat....NOT. The kind I have has around 500 calories! My beer only has around 100.
That means we can drink 5 beers and forgo the popcorn
Chris, It was a project I have been putting off for a month now, I ran across the info when I was troubleshooting my home microwave.
The darn thing is extremely noisy, I got a 1500 watt Kenmore Elite Microwave/Convection oven on 4-09-07, it really never was as good as the one in the Monte. The loud buzzing noise just kept getting louder.
I removed one end of the lead from the high voltage capacitor to the transformer, it stopped, so I replaced the lead and removed the lead to the magnetron, the buzzing stopped, indicating it is the magnetron with a small internal short. (If it still hummed with the transformer connected, it would have been the transformer, (a common problem)
My warranty is good for 5 years on the magnetron, so I will deposit the unit at Sears local repair center today.
I will remove the Half Time from the Monte and use it in the interim, we like it better anyway.
We are still working on the darn house, Sue and I installed new tile flooring in our master bathroom, then I dug 50 feet of trenches and a 4' sump hole (by hand) the past few days, running extra circuits for lighting, the fountain, and new power to the power pole in installed for Monte. A new outdoor sump pump piped to the street for my foundation water issues, hope that will finish fixing that problem.
It's always something!

[capn chris]

LOL!! "You're a better man than I Gunga Din!" What a guy?! Wish you were joining us at the Rally. Bet you're following Ike's progress? Hard to believe how many folks have NOT evacuated. Liked your comments re. popcorn vs. beer calories. Think I need a fresh one. Thanks and best wishes,

[firetrucker]

That's a nice piece of work, Ozz, but when you decreased the temperature rise (degrees Celsius) to make up for inefficiencies, you increased the degF instead of decreasing it (to 102.6). Takes me back, way back, to my basic physics classes.

Bob

[Ozz]

Bob, that was a cut-and-paste, I am but a simple fix-it-man.
I dropped off our microwave/convection oven, had to pay Sears $22.00 for a ?drop off fee? told them if it wasn't the defective Magnetron, (Which it is)deposit it in the dumpster.
I found 5 identical (except for they were 1200 Watt microwave ovens) mine is a 1000 Watt combo, which we never use.. sale units are no convection, for $79.00 each.
I paid $240.00 for my unit.
It will all depend on the honesty and skill of the technician that services my unit there.
Wish me luck.
Ozz