My kettle claims to hold ninety-four degrees. A cheap probe thermometer says it holds anything between eighty-eight and ninety-seven, depending on how full it is and how long it has been sitting.
For months I blamed my technique for cups that came out differently on different mornings. The variable was sitting on the counter the whole time, displaying a confident number.
What the probe showed
Full kettle, freshly boiled, set to ninety-four: the water in the spout was three degrees cooler than the water at the element. Pour out half and wait two minutes and the gap grows. By the time the water reaches a room temperature brewer and a room temperature bed of grounds, the number on the display has very little to do with what the coffee sees.
What helped
- Rinsing the brewer with hot water before the bloom, which costs nothing.
- Brewing with the kettle at least two thirds full, so it cools more slowly.
- Choosing one fill level and staying with it, so at least the error is the same every day.
I have not bought a better kettle. Consistency turned out to matter more than accuracy: a machine that is wrong by the same amount every morning is a machine you can work with.