Microwave Water Heating Tests

Microwave ovens have become a fixture in kitchens, offering a convenient way to heat or reheat foods quickly and efficiently. Internet opinions on their expected lifespan range somewhere from seven to ten years. Recently, my reheated leftovers occasionally came out cooler than expected. Is my microwave failing?

As always, the first step is to find documentation. Looking at the manufacturer’s plate at the back, I find it is a Sharp R-309YK. A PDF manual for R-309Y model line is available for download from Sharp. (The “K” at the end designated the color, which is black in my case.) I had hoped the manual would have a “Troubleshooting” section, as appliance manuals sometimes do, but not this one. The identification plate also said the microwave was manufactured in December 2014. Since we’ve passed the seven-year anniversary, a failure would be unfortunate but not completely unreasonable.

Absent a troubleshooting section in the manual, I went online and found several tests for microwave effectiveness by heating water. In increasing order of credibility in my book, the results were:

Test #1: Wikihow = Fail

This test heats two cups of water on high for one minute and measures the temperature difference before and after. A healthy microwave is expected to raise the temperature by 35 to 60 degrees Fahrenheit. Using my food thermometer I measured the starting temperature at 64.9F, ended at 90.0F, for a rise of 25.1F. This is lower than the accepted range.

Test #2: GE Appliances = Pass

This test doubles the amount of water to one quart, and more than doubles the heating time to two and a half minutes. Despite proportionally longer heating time, this test had lower expectation on heating with a target range of 28 to 40 degrees Fahrenheit. My test started at 69F and ended at 34F, right in the middle of the target range.

Test #3: USDA = Pass

This test is a little different than the other two. The quantity of water is smaller: only one cup, but the heating procedure is different. Instead of measuring temperature rise over a fixed time duration, we are going from freezing to boiling temperatures and measuring the time elapsed. The water started showing small bubbles at two and a half minutes, and a full roiling boil at three minutes. Based on a lookup chart accompanying this test, this is consistent with a microwave in the range of 700 to 800 Watts. Lower than the advertised 1000 Watt but still within the usable range.

Result: Two Out of Three

My microwave passed two of three tests. Furthermore, since I place more credibility with USDA and GE than whoever authored the Wikihow article, I’m inclined to put more weight in those results. It appears that my microwave is functional, at least nominally. But then how might I explain the lower-than-expected heating I experienced?

Unknown Cycling

The best guess is a behavior difference I noticed during these tests. They are all heating water on high power setting, which means everything should be running at full power at all times. But during normal use, something is cycling on-and-off. I could hear a change in sound, and the interior light would flicker. The magnetron is expected to cycle on-and-off during a partial power reheat, but not when it is set to full power.

Looking online for potential explanations, I read the magnetron may turn off for a few seconds if it got too hot. This could happen, for example, when there’s not quite enough food in the microwave to absorb all the energy. If that was the case, however, I thought my food would be piping hot. My current hypothesis: something is triggering a self-protection mode during normal use, but not during these water heating tests. I’ll keep my eyes open for further clues on microwave behavior… and also keep my eyes open for discounts on 1000-Watt microwaves.

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