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Coffee Pots That Spill

09/21/2009 8:11 PM

A while back, several of us got off on a discussion about all the %$%^! cheap coffeepots in the standard coffeemakers that spill coffee down the side instead of pouring into the cup. So, I got interested and started doing some experiments. Her's what I found:

When I fill the coffeepot with water from the faucet filter, it almost never does this.

The coffee runs down the side about 20% of the time.

Once it starts running down the side, it's hard to stop.

If I wipe the side of my finger just underneath the spout, the coffeepot does not spill.

Cold coffee rarely runs down the side.

Ideas? My thought is surface tension, but I can't quite square evrything.

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#1

Re: Coffeepots that spill

09/21/2009 8:21 PM

Cohesive forces are weaker at elevated temperatures. Therefore the water spout that was coherent at room temperature is diverted downward at the lip because the rest of the water can't pull it along with the stream, and into the cup??????

Bernolies Principle? The top of the stream is moving faster? It's in there somewhere.

Never mind. I've been into the peach hooch from another post.

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#11
In reply to #1

Re: Coffeepots that spill

09/23/2009 5:41 AM

Bernolies Principle?

That would be Bernoulli's principle I believe you may be refering to.

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#2

Re: Coffeepots that spill

09/22/2009 3:41 AM

Guilty is the "Coanda effect". You can see it if you let water pour from a faucet and come and touch the stream with the back of a spoon.

The coffee pots spill because the geometry is not made according to this effect but only for aesthetics. The speeds are small and the adherence to the wall important if the radius is too big so that the flow follows the wall and does not reach the cup!

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#3

Re: Coffee Pots That Spill

09/22/2009 5:37 PM

I raid the fridge every so often for a cool glass of milk, 3L milk container size, and I have noticed that when pouring it into the glass the very first drop misses the glass, the direction being towards the milk container, so thinking about the question, I speculate that when I tilt the container the inertia of the milk acts horizontally across the surface of the milk towards the container opening, the molecules resisting shear causes a turning moment for a split second, enough to cause the first drop to miss the glass. maybe a coffee pot acts the same? a small deflector below the spout might cure it.

Regards JD.

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#4
In reply to #3

Re: Coffee Pots That Spill

09/23/2009 12:02 AM

I think it's hesitation factor from having tipped a container to abruptly causing milk to enter and then exit the glass that precipitates the drop on the counter pre-pour.

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#5

Re: Coffee Pots That Spill

09/23/2009 12:04 AM

The mystery deepens ...

The cause: The spout it's not designed for it's function. (in my opinion)

(I'm going out on a limb here since there's no scientific explanation for this one and it's seemingly the opposite of what one should exactly do.)

The solution: drill a not too large of a hole (1/8" Ø) on the spout depression at about 3/4 the length of the spout on the "exiting end" of it. The location and size it's more of an art than a science, so start small and increase the size of the hole by very

little increments.

Why does it work? God knows ... could that the edge of the hole works as a vortex generator and accelerates the fluid directing it? Don't know, this could be a thread on itself.

All theories welcome.

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#7
In reply to #5

Re: Coffee Pots That Spill

09/23/2009 1:31 AM

I use that technique to keep the tip of exhaust pipes clean too

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#8
In reply to #5

Re: Coffee Pots That Spill

09/23/2009 2:31 AM

A fast liquid flow over a hole open to air should draw air into the stream. Perhaps this drawn air forms a protective sheet under the liquid flowing into towards the spout edge. This air sheet could prevent liquid drips.

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#23
In reply to #8

Re: Coffee Pots That Spill

09/23/2009 3:20 PM

Eh? Ya mean it softens the flow as in aeration?

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#26
In reply to #23

Re: Coffee Pots That Spill

09/24/2009 3:19 AM

Either that or it may form an air sheet between the flow and the conduit. Depends on lots of parameters (such as conduit smoothness) and probably hard to predict or calculate.

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#6

Re: Coffee Pots That Spill

09/23/2009 1:25 AM

"Once it starts running down the side, it's hard to stop.

If I wipe the side of my finger just underneath the spout, the coffeepot does not spill."

Both of these make me tend to agree with the surface tension. Fingers tend to be oily (mine more so than most...), and a line of oil would break the surface tension.

Post #5 is very interesting... I'd like to hear more on that!

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#9

Re: Coffee Pots That Spill

09/23/2009 3:05 AM

I gave you the explanation for the reason the flow follows the wall. Now the explanation why after you wipe with your finger it does not do the same.

It is not a surface tension problem but what is called "wettability" which is a consequence of "sympathy" of flow and wall surfaces. Depending on the stuff in contact the fluid will wet or not the wall. For instance water will wet a glass wall but not a "greased" one. Wiping with your finger you put on the wall a very thin layer of grease and this modifies the contact. The best example of not wettability is quicksilver on glass. This know physical phenomenon is quantified by the drop contact angle which is >90° for wettable fluid-wall couples and<90° in the contrary case. Do not forget that grease (as a general meaning) is water repellent!

I am quite surprised how participants do not read what was written before and in the case they do not know what it means not have a look at an available information source. I am also surprised that surface tension can be used in stead of adhesion.

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#17
In reply to #9

Re: Coffee Pots That Spill

09/23/2009 7:39 AM

I'm not sure I understand your point. Surface tension and wettability are like two sides of the same coin. Have you not read Quo Vadis?

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#10

Re: Coffee Pots That Spill

09/23/2009 5:14 AM

It was'nt always like that......my excuse I thought I loss the touch. . what a relief.

........and since its so damm wide spread.....How come it took so long to bring this problem forward..........

I never really stop to think about it, I thought it was just me. I tryed pouring faster. that did'nt work. Good disscussion.

Heres the reason from Mr. Coffee Web Site.

http://www.mrcoffee.com/FAQ.aspx#Coffeemakers

Why does my decanter dribble when I pour with it?

1.) Try pouring the coffee more slowly.

2.) Rarely, the spout on glass decanters is misshapen. Contact an Authorized Service Center for replacement.

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#12

Re: Coffee Pots That Spill

09/23/2009 5:48 AM

In fact I gave a simplified explanation since there is more in it : it depends as well of the boundary layer as a supplementary factor in the attachment of the flow to the wall.

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#13
In reply to #12

Re: Coffee Pots That Spill

09/23/2009 5:55 AM

Ok you supplyed the reason why it does that,

But I am sure it always was not like this (or maybe not just never noticed it).

How can one stop this, Longer Pour spout, Sharper Edge, Snaller Angle Between Pour Spout and Neck?

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#14
In reply to #13

Re: Coffee Pots That Spill

09/23/2009 5:59 AM

The pour spout less bend downwards and sharper to avoid even at the edge an attachment. I shall mail a picture. But I have no time now to do it it will be tomorrow or the day after.

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#33
In reply to #13

Re: Coffee Pots That Spill

09/26/2009 3:51 PM

I promised an answer as soon as I shall have the time to do it here it is now with a few pictures so that it does not stay only a "paper work". I used a spoon to demonstrate the flow attachment and a glass to indicate the importance of the edge radius. The spoon was used with two contact surface conditions and the pictures show the differences: in a 1st test the spoon was cleaned and dried so that no trace of grease was present when it was brought in contact with the water flow, in the second test the spoon was lightly oiled so that the surface was less rough due to the oil thin film and the friction coefficient was also reduced since the contact was fluid-fluid and not fluid-solid. The conditions influences can be seen as well at the contact between flow and spoon as when the flow leaves the spoon surface.

Spoon dry, flow turbulent at leave edge

The final sketch shows the differences and how it seems that the edge has to be. Of course this requires more optimisation work and can be considered ONLY as a rough approach.

Small radius and big angle between leaving speed and wall= no attachment

Spoon lightly oiled with sun flower oil = less turbulence and the flow leaves the spoon earlier and at a lower angle with the vertical

An other picture with the not oiled spoon = turbulence and leave at edge

Flow attached at small angles. The glass has a quite big radius and no angle between wall and flow direction= attachment

Even at big angles the flow continues to attach to the wall.

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#34
In reply to #33

Re: Coffee Pots That Spill

09/26/2009 5:39 PM

Did some experiments too. The Radius R is the big factor. I took a water jug and cut the top off. Even full, the water did not attach to the sharp edge once it got going. I cut a little vee in the plastic and it worked even better. Picked up a ceramic coffee cup with a thick wall and a nice round edge, and it attached until almost empty.

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#35
In reply to #33

Re: Coffee Pots That Spill

09/26/2009 7:13 PM

g.a.

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#15

Re: Coffee Pots That Spill

09/23/2009 6:49 AM

wow

whats with all this smoke in the in this room

can hardly see anything

Bernoulli's principle

I think he was my next door neighbor

my kid had him for math

was he the guy that wore those weird tee shirts

I would think twice about taking

my long haired

stapel gun face

under sized waist

kid to work

and asking him to make coffee

while the security cameras where off line

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#16

Re: Coffee Pots That Spill

09/23/2009 6:51 AM

my my you guys think to much

just have your parkinson's med's readjusted

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#18

Re: Coffee Pots That Spill

09/23/2009 8:34 AM

This is an example of a convoluted and therefor hard to understand principal. In fact if the propagates of the principal determine anyone has understood it they quickly change the laws of physics so the cause and effect can not be traced out.

Gp=Sp x Cd -(Pt x Af)

Gp: gross profit

Sp: sale price

Cd: consumer dummy factor

Pt: product testing

Af: actual function

As you can see any Pt (product testing) or Af (actual function) decreases Gp (gross profit). This is a pervasive and unbreakable law called the sixth law of thermodynamics and marketing.

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#19
In reply to #18

Re: Coffee Pots That Spill

09/23/2009 9:04 AM

Figures don't lie,

There seems to be enough variables there to enable lairs to figure.

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#20

Re: Coffee Pots That Spill

09/23/2009 10:19 AM

How about trying implementation of the distilled water bottle with tube mechanism to the coffee pot? You can exert better control and avoid spillage. Is it?

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#21

Re: Coffee Pots That Spill

09/23/2009 10:29 AM

If my memory serves me correctly, the ideal TEA pot was designed by Laithwaite of Linear electric motor fame, (or was it Fred Hoyle?), and featured in a book of his. It had a number of features for providing the correct flow of liquid and balance in the tipping thereof, and allegedly was a perfect pourer. It was ceramic but did not make it into commercial heaven.

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#28
In reply to #21

Re: Coffee Pots That Spill

09/24/2009 10:18 AM

http://assets.cambridge.org/97805214/41063/sample/9780521441063ws.pdf

This is why I keep coming back to this site, when I was 13 I attended a Christmas Lecture in Birmingham Town Hall with my school, the lecturer used this example; the pdf could could almost be a transcript of the lecture - maybe it was? Laithwaite and teapot on scroogle yielded the above link. The designer from the text was an Italian engineer, so maybe our lecturer was Laithwaite? Wow. The dates match, that would have been about 1973.

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#32
In reply to #28

Re: Coffee Pots That Spill

09/25/2009 4:56 AM

Thanks Hux. I was suffering from false memory syndrome. I should have googled eric laithwaite teapot before I jumped in. Up came the book by EL about inventions, that I had read, and indeed it was the Italian engineer and the German pot maker .

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#22

Re: Coffee Pots That Spill

09/23/2009 1:33 PM

I like this type of question, it can be applied to a lot of other fluid flow situations. I'm going with Nick Name on the coanda answer. Look at how much effort has gone into design of spouts for laundry detergent, bleach, paint cans, etc. Consider how many containers dribble or spurt for lack of proper design. There is a lot of room for improvement out there, and good answers from CR4 will hopefully filter down to the manufacturers eventually.

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#30
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Re: Coffee Pots That Spill

09/24/2009 8:05 PM

GA. I haven't even started on the &&^%*& teapots where the spout gets so hot that, when you go to pour, the liquid flash boils and scalds most of your hand. Grandma used to have 'em with long spouts to cool that, but we got smart and "improved" what we didn't know about.

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#24

Re: Coffee Pots That Spill

09/23/2009 4:05 PM

"I'm a little teacup, short and stout. Turn me up, and pour me out."

Whatever you do, don't get a Black and Decker Coffee maker.

When it got to be a mysterious science for the design of a coffee pot and it's pour spout, you gottah wonder if we are not really moving backwards.

Certainly, up until recent times you could pretty much pour liquids, even coffee, out of most any old thing with a spout of some description.

Big pot, big spout, is the rule.

Small vessel to small amount works in the lab, though small to large works better than large to small.

Lab rats know.

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#25

Re: Coffee Pots That Spill

09/23/2009 4:35 PM

I have to admit this is an issue I had not a thought of and try as I might to duplicate this trouble my coffee pot refuses to dribble.

I have in the past trashed the decanter from the cheap coffee maker and replaced with a proven pourer

Poor pours make excellent gifts...

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#27

Re: Coffee Pots That Spill

09/24/2009 8:39 AM

I thought the inventor of the dribble cup went on to design coffee pots and is just messing with us.

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#29

Re: Coffee Pots That Spill

09/24/2009 7:10 PM

Ok...nobody has hit on the problem I have had with my coffee pot. As you pour the coffee out, it hits the lid with the hole in the center and screws up flow.

My solution to my type of problem: Lift the lid with a well placed thumb while pouring coffee, almost no dripage from pot thereafter. If it does it just sizzles nicely on burner, wafting java essence in the air (and to think some people pay for coffee scented candles!!).

Drew

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#31

Re: Coffee Pots That Spill

09/24/2009 8:11 PM

At any rate I have designed an experiment to test my theory. I will report back unless I am called away to the G20 to explain to them how they royally screwed up my savings.

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