Calibrating a Thermapen in Boiling Water

[btbyrd]

This is why I'm grateful to have an electric kettle to boil water -- it's faster than my stove and there's no risk of explosions.

[Smithy]

Rotuts, I think the murky appearance is due to the development of microscopic air bubbles just before they start expanding and you get the explosion. Air bubbles at the right size will be invisible to your eye but still provide light refraction. Get enough of them and you'll have 'murky' (i.e. translucent but not transparent) water. (I used to marvel at how fresh city tap water was opaque to ultrasound until it had a chance to outgas. It came out of the tap looking clear, but 1MHz sound couldn't get through even a few inches; then it became cloudy to the naked eye; when it cleared, those microbubbles were gone and both light and ultrasound passed cleanly through.)

There's a pretty good writeup of how and why superheated water explodes when something else (a tea bag, powder) is added, at the Unwise Microwave Oven Experiments web site. That writeup notes that a pot being heated on the stove top has bubbles coming up from the bottom (where the pot is hotter than 100C and provides plenty of crevices for bubbles to form) and those bubbles are continually collecting more air as they rise. The evaporation of the state change at the bubbles' surfaces helps cool the entire volume of water. In the microwave, the water is almost uniformly heated throughout its volume to a superheated state. The bubbles visible are forming at or near the top of the mug or pot, and aren't providing any cooling in the depths of the mug. When a source of air bubbles is added (tea bag, powder) then there are suddenly thousands of bubble nuclei that allow the water to boil throughout its volume.

Getting back to Shel's question: the heating element at the bottom of the pot and the evaporative cooling at the surface of the water explain the uneven temperatures in the pot. I've always tested probe calibration by stirring the pot and moving the probe around inside the pot to even out temperature differences.

Edited for spelling and extraneous line feeds.