So today, I was creating a starter for my Saturday brew day and I thought I'd check my digital thermometer while bringing the water up to boil. I had a pot of plain filtered water on my burner and I had the digital thermometer stuck in through the lid. I noticed a few bubbles showing at about 210.5° and by 212° there was a good bit of bubbling going on. However, I left the thermometer in the pot and I noticed that the temp went up to 216°.
So my question is, does this mean my thermometer is off? I had read somewhere that after water reaches the boiling point it is very hard to get it to increase in temperature (takes a lot more energy). Just for reference, I am at about 450 ft. above sea level.