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During the boil and more pertinently afterwards when I'm cooling the wort I have noticed that when I stir the thermometer around the temperature will rise (sometimes up to double digits) but when I hold it still the reading drops. Naturally when I hold it still long enough the fluctuations level out to give a more consistent reading. However I'm still curious why agitation causes a spike in the reading and if it's related to the thermometer or if it's just due to temperature imbalance within the wort.

For reference I'm using a digital thermometer which only allows me to read 4 inches into the top.

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up vote 4 down vote accepted

The heat of your wort will not be uniform throughout the kettle, it's going to be at its hottest at the bottom of the kettle - where the heat source is, and its coolest at the surface.

The act of stirring agitates the wort and causes some of that hot wort from below the surface to rise to the top and mix with the cooler wort, hence the rise in temperature you're seeing.

For consistent readings, (like you've noticed), it's best to keep the thermometer stationary.

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