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I have been a coffee drinker forever and no matter what coffee maker I buy, all of the carafes are terrible. Is there one that pours well? Or is there a trick I do not know about?

I notice that when the coffee level is lower in the carafe it pours better and spills less..

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This is correct and it has a reason. Actually three. Let me explain...

First, I want you to consider the shape of the Hario Buono Coffee Drip Kettle:

Hario Buono Coffee Drip Kettle

Or, a Turkish tea kettle:

Turkish tea kettle

As you can clearly see, both have separate spouts. The special design of the spout make the flow of the liquid as laminar as possible while you pour it.

This is our first concern, why don't coffee carafes don't have this special spout that starts from the bottom, makes a curve and ends at the top, apart from the body of the carafe?

  • In that case, the coffee at rest in the spout will loose so much heat and become cold rapidly. This is not what we desire. We want the coffee carafes to keep heat as stable as possible. So, we want to keep coffee all together, apiece, in one place together. That's why, coffee carafes have only one (not very nice) spout at the top. This is our first reason.

  • Here comes our second one, for liquids, or in general for fluids, you have a magic number, called Reynolds number. This number is (simply) kind of the ratio in between how particles are carried by the total motion of the coffee (inertial forces) and the resistance against being carried (viscous forces). When inertial forces are too much against viscous forces, your coffee starts to tremble by definition. This is what happens when you start to pour your coffee. Apparently, when the angle in between the carafe's spout and the vertical to the earth's gravitational vector is close to 90 degrees, Reynolds number will be bigger than when it is close to 0 degrees. This explains why emptier carafes have more laminar flow during pouring.

  • And finally, the third. Fluids also have another magic number called Weber number. This number explains why surface tension effects your pouring. As in the previous case, also surface tension adds an extra complexity to the flow while you pour with greater degrees. The surface tension forces your coffee to form droplets. As a result, instead of having a perfect laminar flow, you end up a turbulent and sometimes intermittent with droplets flow while you try to pour when your carafe is full.

There are also a few other interesting concepts like Ohnesorge number and others. But I hope these could explain it all.

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  • Much of the answer to this question is about "physics" and thermodynamics rather than "coffee", not that its reason enough for the Q&A to not be here because it pertains to coffee very much. Its interesting to see the difference too of the same carafe with Cold VS Hot liquid as well (part of the Weber Number and surface tension). I find the very bulbous carafes you see at restaurants, typically associated with BUNN, to be the next best thing outside of the turkish system, which is still by far the best. Jun 29, 2018 at 13:44

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