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If I bring 20 ounces of water (2.5 cups) to boil in a kettle, and promptly remove it from the heat, is the amount of water lost (by volume/mass) a negligible quantity?

To be more precise about my own intent for this knowledge; if a pour over guide for coffee brewing is indicating that there should be twenty ounces of water at 195-205'f, should I go through the steps of weighing out twenty ounces of was-just-boiling water, or will the displacement of water to steam be relatively non-impacting of the coffee extraction and final cup.

What are the various ways in which more steam will be displaced, and how can I conserve mass to minimize variability and improve consistency extraction to extraction?

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Some (possibly overly-fussy) cafés near me will actually put the pour-over pot and cone on a scale as they pour to ensure they end up with the correct amount of water (they also time it). –  Josh Caswell Jul 15 '12 at 21:50

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This depends on the time you need to bring it to boil (water evaporates long before it starts boiling), the surface area of the water, and your definition of "negligible". Best test it with your own kettle.

My kettle needed 2.5 minutes for boiling it, and lost 11 g out of 560, not quite half an ounce. It will probably lose some more while cooling. If you use an open kettle, a very wide kettle, or heat the kettle slowly, you will lose more.

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I added an explanation to qualify "negligible"; this is for brewing coffee by pour-over, will steam displacement be impacting of extraction –  mfg Jul 15 '12 at 17:20
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@mfg In rumtscho's measurement they lost about 2% of the volume - I expect based on the many factors involved this may vary from <1% to 5% or so. Even 5% water loss should not be enough to substantially affect the taste of the coffee unless your taste is very sensitive, but you can feel free to add 2% if you want to get a bit closer to the target (so 20.4 oz). –  D Coetzee Jul 15 '12 at 18:34
    
Actually, I lost more than 2%, this was measured the moment the kettle turned itself off - it was still giving off lots of steam. So probably more like 3-4% until cooled. –  rumtscho Jul 17 '12 at 17:34
    
Well, even at a 10% loss (2 ounces), you would still have three 6 ounce cups of coffee. –  mfg Jul 18 '12 at 16:30

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