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Finally done the expt. I promised. Results:
Equip: Glass measuring cup nearly full of tap water (Most of Monterey county's water is well water -- aquifer filled by Salinas river, so very hard.), dial stem bi-metalic thermometer, and the oven.
First several heatings: on removal, boiling vigorously when I opened the door. By the time I could read the TM it was 100 C and not boiling.
After several "trials" on removal, boiling sl., but continuously, temp. 107 Probably sl. higher at equilibrium while being heated. In ~ 3 s. stopped boiling @ ~105 C. Cools more slowly to 100 (and below). If I rub the bottom of the cup with the end of the TM, it boils and the temp. drops more rapidly to 100 C., at that point it them stops boiling
bc
"E.C. Muehleisen" wrote:
If I understand correctly, the water that is boiling after microwave heating
is above 100 C. The energy supplied by a microwave (klystron) tube is
funneled into a rather narrow surface layer. That layer can become
superheated since the the power input exceeds the power output from the
layer into the remaining mass of water. The fact that water has a rather
high specific heat exacerbates the situation. Thus when the surface layers
are superheated, the thermometric measurement probably weighs the surface
more than the bulk. Was the thermometer plunged into the middle of the
water or barely immersed?
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