October 19, 2004

Paging Mr. Wizard

Riddle me this: I've got a small coffee-maker in my office. (Mr. Coffee 4-cup, if you want to know.)

When I make coffee, I fill the pot up with cold water and pour it into the top of the machine. Easy. No problem.

But when I pour the first of the brewed coffee out of the pot and into my cup, it dribbles all over everything. Every. Freakin'. Time.

I cannot figure out why this is.

I thought it might have something to do with the coffee having a slightly greater viscosity than the water, causing it to "stick" more readily to the underside of the spout.

I also thought it might have something to do with the shape of the spout being very slightly altered by the pot alternately holding cold and hot liquids.

Either way, I'm getting tired of having to clean up the mess every time. And the poor table on which the coffeemaker sits is beginning to show signs of wear and tear.

Anybody have an explanation for this phenomenon? And, more importantly, anybody have a way to fix it?


UPDATE: Ladies and Gentlemen, Mr. Wizard is in the House. Reading the comments below, I feel like Mr. L.B. Gumbee. Oooh, my... brain... hurts...

Posted by Robert at October 19, 2004 02:19 PM | TrackBack

Basic hydrodynamics. I imagine that you pour the water into the coffee maker much more quickly than when you pour coffee into a cup.

Surface tension causes the fluid to adhere to the carafe's surface. When the fluid flow rate is slower, the energy in the flow is not sufficient to overcome the effect of the surface tension: you get a dribble. When you pour faster, the energy level in the flow exceeds the effect of the surface tension: no dribble.

Solution A: pour faster.

Solution B: some sort of barrier (like a speed bump) just inside the lip of the spout, which would cause the fluid to accellerate as it passes over it, like "rapids" in a river.

Posted by: George Gaskell at October 19, 2004 03:36 PM

I thought it might be that, too. I've tried correcting for speed of pour as best I could, to come up with comparable rates, but it doesn't seem to make that much difference.

Posted by: Robert the LB at October 19, 2004 03:41 PM

My Dear Robert LB, one word, old boy...Krups.

Posted by: Enoch Soames at October 19, 2004 03:59 PM

You can't tell much about flow rate by visual observation. You would need to measure it.

When you pour water into the coffee maker, you are typically pouring the whole contents of the carafe in there, so you probably tend to tilt the carafe at a much steeper angle without even thinking about it. In contrast, pouring a single cup of coffee is a much more delicate operation (since you are pouring liquid into a smaller recepticle, and it is hot so you are probably more anxious about spillage), and would therefore result in a much shallower pouring angle. A shallower pouring angle produces a lower flow rate.

Also, the pouring angle can be deceptive. A comparison of the trigonometric values will tell you what kind of variation in the flow rate you will experience. Let's say you compare two tilts -- Tilt A, where you tilt the carafe from zero to 20 degrees (for a range of 20 degrees). In Tilt B, you tilt it from 20 to 40 degrees (also a range of 20 degrees). You are going to get a far greater increase in flow rate in Tilt B than Tilt A, even though the range is the same.

Try pouring regular water into your coffee cup as you would do with hot coffee. I suspect you will get similar dribbling.

Posted by: George Gaskell at October 19, 2004 04:00 PM

Alas, my dear Mr. Soames, two words: government salary.

Posted by: Robert the LB at October 19, 2004 04:26 PM

I have the exact same problem with my 12-cup Mr. Coffee at home. Weird. And I've tried the speed pouring experiment, too, with no success.

Posted by: jen at October 19, 2004 05:51 PM

Just drink out of the pot. Problem solved.

Posted by: Matt Navarre at October 19, 2004 06:34 PM

Do you have the same problem with a brand new carave? A slight film of coffee stain might inhibit boundary layer seperation. Try simmering the carafe in a large enamel or stainless kettle, with 1 part in 10 vinigar in the water, and then in clear water to rinse.

One of Hitler's cronies, I think it was Goering, had a recipe for coffee which began "In a previously unused coffee pot..."

Posted by: triticale at October 20, 2004 12:18 AM

Forget the science. You are doomed to an eternity of dribbling first-cup pours, I'm sorry to say. As a long-time Mr. Coffee user, I have had that identical experience. The only way I found to alleviate it is to brew less than a full pot. Why? Dunno. Less messy, though. I am currently using a severely bottom-of-the-line off brand coffee maker that does the same thing. Must be a Mr. Coffee knockoff. Once, in a previous life, I had a Krups...things were better, then.

Posted by: rick at October 20, 2004 12:10 PM
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