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Topic: Vac Pot Volcanic Eruption WARNING! (17 msgs / 349 lines)
1) From: sci
By way of warning, and of curiosity, I need to tell/ask you what happened to
my vacuum pot this morning. I have a Hario 5 cup tabletop model with a
butane burner like those that SM sells. As usual, I poured 190f water into
the bottom and put the moderately adjusted butane flame under it as I went
about weighing and grinding some Nic. Paca. DP.
After 3-4 minutes the water was not boiling. Odd. 5 minutes, no boil. Weird.
Then around 6 minutes with absolutely no warning, no little outlier boiling
bubbles, no slow ramping up to a boil, the whole bottom containter of water
erupted like a Vesuvius, spewing water all the way to the ceiling!!! The
explosion happened in a mere fraction of a second. BAM! Absolutely
terrifying! It just went from a peaceful serene calm water state to
EXPLOSION!! ??? *&% Fortunately I was alone and I didn't get burned. But I
could easily be in ER right now with a scorched face.
**So, please be warned vac pot users.**
Also, does any science geek here know what caused this? BTW, the clean,
bottom spherical glass was open at the top; nothing was obstructing it or
creating pressure. Can you imagine what would have happened if I had the top
sealed in place? I have never seen any open boiling pot, carafe, or whatever
behave this way. I have used my much-loved Hario many times, but this is the
first time this has happened. Since this is a tabletop model, meant for
presentation of the process, I sometimes do this for friends. While I'm sure
it would get a bunch of laughs if nobody got hurt, it would be a disaster if
somebody did. I don't want it to happen again. Please CC any replies to my
gmail address if possible.
[My usual method when I get the best results has been to pour hot water in
the bottom, get the water boiling with open bottom using burner, turn off
heat, put the top part on, relight flame, watch water travel to the top,
pour coffee, stir, reduce heat, wait 2-3 minutes, cut heat, watch draw down,
drink. This may not be the best method now, but I'm nearly certain I learned
it here.]
Have a wonderful day!
Ivan
the "coffee animal"
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2) From: Bob Hazen
I'm guessing you managed to super-heat the water.  This would be the same 
thing people have seen when nuking water explosively in a microwave.  If 
bubbles don't form, then the water super-heats and when you grab the cup or 
whatever, BLOOSH!
Not sure how to cure this in a vac pot bottom chamber.  Maybe it's "too 
clean" or you need some scratches in the glass.  In an open vessel, just 
sticking something in it like a chopstick will save you.  You saw the red 
flag - no bubbles.  If their aren't any forming, then beware.
Bob

3) From: Rich
There are 2 ways to prevent this.  Th easiest is to stir the water after 
it starts to heat over the flame and the other is to lightly abrade the 
inside bottom of the pot.  It only happens with nice perfectly smooth 
glass.  You have to have sites to initiate nucleate boiling. or you will 
get superheat and instantaneous bulk boiling.
I have little faith in the WiKi but this is a reasonable explanation of 
Nucleate boiling. http://en.wikipedia.org/wiki/Nucleate_boilingBob Hazen wrote:
<Snip>
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4) From: Michael I
Yes, if you just put a chopstick in the lower globe when you're boiling it, you'll not have this issue.  That's what the little chain dangling from the included filter on the Harios is designed to do - make sure the water doesn't superheat in the lower globe.  Of course, that only works if you use the filter, and if you heat the water to boiling with the upper chamber in place.
-AdkMike
On Dec 21, 2009, at 1:20 PM, Rich wrote:
<Snip>
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5) From: Yakster
Frightening experience.  I've not heard of this with Vac Pot, but have for
microwave.
Did you have the funnel in at that point?  I'd think that the funnel might
nucleate the bubbles? (don't really know what I'm talking about here).
-Chris
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6) From: Tom & Maria - Sweet Maria's Coffee
Geez - this is terrifying. So there was no filter in place (no glass 
rod, or no stock yama cloth filter?) I am going to go read the 
wikipedia entry and learn a little. Is it possible that if the glass 
surface temperature and the liquid temperature are too close to 
eachother this might happen, or does it have to do with physical 
surfaces ... anyway, I will read the wiki and maybe answer my own 
question.
Tom
<Snip>
-- 
-Tom
"Great coffee comes from little roasters" - Sweet Maria's Home Coffee Roasting
               Thompson & Maria -http://www.sweetmarias.com     Sweet Maria's Coffee - 1115 21st Street, Oakland, CA 94607 - USA
             phone/fax: 888 876 5917 - info_at_sweetmarias.com
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7) From: Ira
At 10:06 AM 12/21/2009, you wrote:
<Snip>
Dropping a glass marble or bead or even a piece of sand in the bottom 
will almost guarantee this never happens. If the container is clean 
and smooth and the heat is very gentle, this can happen, microwaves 
can cause this effect this on occasion. The marble will create a hot 
spot that makes sure boiling starts.
Ira
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8) From: Rich
Its a rate of heat energy input slow enough that the entire mass of 
water in the lower part of the pot is effectively heating all at once 
while the top layer sits on top of it and stays cooler.  Its an upside 
down stratification.  If you start with 190F water it is easy to do as 
the density difference is small so there are minimal stirring forces. 
Glass is a very good insulator.  The very smooth surface does not 
provide any place for the little vapor bubbles to form so they do not. 
The expanding and contracting bubbles keep the pot stirred.
If you want to see how this works go get a 6" or 7"  New test tube and 
3/4 fill it with water and then hold it still over a burner flame and 
heat just the bottom.  If its clean and smooth you will get one big 
belch of water and water vapor ejected from the test tube.  Its a 
standard lab accident.
Tom & Maria - Sweet Maria's Coffee wrote:
<Snip>
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9) From: Rich
True.  Go steal your kids marbles....
Ira wrote:
<Snip>
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10) From: Barry Luterman
Does pre -heating the water in a kettle and then transferring it to a
vac-pot avoid this problem?
On Mon, Dec 21, 2009 at 8:54 AM, Rich  wrote:
<Snip>
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11) From: Barry Luterman
Yes meaning pre heating the water will avoid this phenomenon?
On Mon, Dec 21, 2009 at 8:26 AM, Michael I  wrote:
<Snip>
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12) From: Mike Koenig
Ivan,
You've discovered the phenomenon of super-heating.  Mainly happens when you
have a very smooth container, and no movement, since there is no place for
bubbles to form.  Tiny bubbles need more energy to overcome the surface
tension of the liquid to expand than larger bubbles do,  so you can get a
situation where you need to get a few degrees above the boiling point of the
liquid to begin forming larger bubbles.  Unfortunately, in that case, once a
larger bubble forms, it helps the rest of them to pop, and you get explosive
boiling.   It doesn't happen as readily with water, as with organic
solvents,  but under the right conditions (which you've managed to create)
it will happen.
As others have said,  adding in some glass beads would be your best solution
for this.  You can swirl your pot a few times while it's heating to also
avoid it,  but if you've already super-heated your water, moving it will
release the bubbles and you will get a geyser.
While I was in school, I did an internship at a food and beverage analysis
lab,  and since I was the lowly student, I got stuck doing all the fat
analyses, which involved extracting fats with ether, and then evaporating
it.  Watching a dozen flasks of ether belch out into the fume hood because I
forgot the glass beads was a valuable lesson in lab skills. (and
fortunately, there were no open flames nearby)
--mike
On Sat, Dec 19, 2009 at 1:40 PM, sci  wrote:
<Snip>
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13) From: Dave Huddle
MANY years ago, when I was in the chemistry lab, we always added a few
clean 'boiling stone' to a vessle of liquid that was going to be
heated to boiling.
Clean bits of broken porcelain work well because the irregular surface
encourages bubble formation, preventing the bumping behavior you
described.
Dave
Westerville, OH
On Sat, Dec 19, 2009 at 1:40 PM, sci  wrote:
<Snip>
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14) From: Bob Hazen
Here's another link:http://en.wikipedia.org/wiki/SuperheatingBob

15) From: Ira
At 11:24 AM 12/21/2009, you wrote:
<Snip>
The only thing that will insure this doesn't happen is some roughness 
at the bottom or a foreign object to create a hot spot. Hot water may 
reduce the risk, but it won't stop it from happening.
Ira
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16) From: Kirk Janowiak
I resemble that remark, Dave, although my MANY years includes just  
last year in the High School lab setting as a teacher.
Boiling "chips" (whether they be chips, beads, lozenges, or teflon  
"pills") are a standard bit of lab equipment in all science labs where  
boiling in glassware will be done, but only now do they seem to  
catching on in the kitchen. You can purchase boiling rings, disks, and  
"stones," from several kitchen/cooking/"gourmet" shops and online. The  
ones sold for kitchen use are "pretty," but porcelain chips (say, from  
a broken old teacup or saucer) work plenty well and will last for many  
years. I do prefer the lozenge-shaped ones made of borosilicate glass,  
as they are a little larger and easy to fish out of the pan when I am  
finished.
Even with boiling chips, I have had glassware "explode" when high-heat  
was applied. This could usually be traced to a few tiny scorings from  
prior cleaning or a scratch/nick on the very bottom of the boiling  
flask. Believe me when I tell you that I boil in "used" boiling flasks  
only after a thorough inspection!
Kirk
(Janomac)
On Dec 21, 2009, at 3:24 PM, Dave Huddle wrote:
<Snip>
JanoMac
janomac
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17) From: Bob Hazen
<Snip>
Now >that< should have had a spew alert!  Hah!!
Bob
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