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natedenay
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Joined: 15 Jan 2009
Posts: 3
Location: France
Expertise: I like coffee

Posted Thu Jan 15, 2009, 4:33am
Subject: Bialetti Moka & Brikka Brew Temperature and Pressure
 

Does anybody have accurate numbers on the pressure and/or temperature that the Moka and Brikka Brew at?

Since the Brikka holds pressure, the water is clearly heated beyond the atmospheric boiling point, but how far beyond? Does it make 2 bar? More?

The Moka would go a bit over boil as well. How far over would depend on if you pack your grounds and if so, how tightly.  
I would really like to find a source for these numbers and I can't seem to locate one. I'd like to know how far over optimum the brew temperature is.

I suppose it could be easily determined if you put a temperature probe through the wall of the lower chamber, then you could measure temperature and calculate the pressure based on that. Or pressure sensor could be fitted. Has anyone done this?

It seems you could get a Brikka style pot up to 15-16 bar without too much problem with a bit of a re-design, but then you'd be brewing with, what, 200 degree water? (almost 400F) Not good 'eh? My bike tires hold 130psi and 16 bar is only 100 psi more, making it strong enough would not be the problem.

Thanks for any help!
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Rancilio
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Joined: 12 Dec 2008
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Location: New Jersey
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Espresso: Giotto Premium
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Posted Thu Jan 15, 2009, 2:46pm
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

Hello Natedenay,
Obviously you don't have to be Italian to use a Bialetti brikka to make espresso coffee, but many do including myself, when I don't feel like turning my Giotto Premium ON, I use the Bialetti brikka and the coffee that comes out, is not bad at all.

I have the Brikka 4 cups and as usual I fill the filter to the top with fine grind coffee and lightly tamp it, than I fill the base with fresh water up to the line mark on the inside wall, set the filter on top of the base, clean the thread with a rag before screwing the top part of the coffee pot onto the base.

A very simple process occurs when the water reaches a temperature above a boiling point,  a lot of steam accumulates in the sealed chamber, which it has no place to go but up through the coffee filter holes and coffee grind, thus creating the extraction.
I never had any interest in mesuring water temperature inside the chamber or pressure, and I still can't figure out what kind of project you have in mind.
Mr.T

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natedenay
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Joined: 15 Jan 2009
Posts: 3
Location: France
Expertise: I like coffee

Posted Fri Jan 16, 2009, 12:53am
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

I understand what occurs inside a Bialetti machine, it's certainly a clever little design. Physics and my breakfast tells us that it works.

What I am concerned with is the precise temperature at which it occurs. To build steam pressure to cause the water to rise, the water needs to boil, right? That requires 100C temperatures, so we're at least 5C above optimal brewing temperature of about 95C.

Now, how far beyond 100C the water is able to heat is a function of pressure, the higher the pressure, the higher the boiling point of the water, the further from optimal brew temp we are. So a tighter pack in a Moka should result in higher brew temps, and a Brikka should create an even less ideal brewing temperature, albeit at a slightly-closer-to-optimal pressure. I heard someone mention 2 bar in a Brikka, or twice atmospheric pressure. That would indicate the water is at about 120C(!) though that is based on a quick internet search for a vapor pressure calculator... and I'm late for class now so I can't chat anymore on it!

Ideas?
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zippy
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Joined: 7 Apr 2010
Posts: 4
Location: Portland, OR USA
Expertise: Just starting

Posted Wed Apr 7, 2010, 12:36am
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

Sorry to revive an old post but I ran across this and started wondering.

Short answer: Yes, a 4 cup Brikka makes about two bars at least.  That comes out to boiling at about 130C/268F.

Long answer:  The Brikka works just like a pressure cooker.  A small weight over a small hole creates added pressure in the boiler.  I took mine apart and measured a few things.  The orifice at the top of the outlet spout is a 1.83mm diameter circle.  The weight weighs about 55 grams.  Do the math and it takes about 29.75 PSI or 2.05 bars to push the weight up and let the coffee come out.  At that pressure the water is boiling at 130C/268F.  Of course, that's for just water and doesn't count the added resistance of the screens and the coffee grounds the water has to push through.  So, safe to say around 2 bars minimum, probably a little more.

Of course these calculations are based on rough measurements, scale only measures whole grams and 6 inch micrometer for the orifice diameter, and the specific weights and measurements of your own unit may differ.  Still, I'd say it's safe to say about 2 bars and 130C for the 4 cup Brikka.

Scott
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JGG
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JGG
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Posted Wed Apr 7, 2010, 10:02am
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

I suspect it is not quite as simple as consulting a saturated steam table for 2 bars pressure.  Following is my understanding (if incorrect, I would welcome corrections):

In addition to water vapor pressure, there will also be pressure resulting from the heating of the confined air.  So the actual water vapor pressure will, I think, be well below the 2 bars (or whatever the actual pressure turns out to be -- the estimate of 2 seems pretty reasonable to me).

Until the trapped air is vented, allowing the gas inside the chamber to become saturated steam, the tabulated relationship between steam pressure and temperature does not hold.  This is the reason that steam boilers on espresso machines are equipped with valves that release the "false" pressure caused by expanding air during initial heating.

None of this really matters much in terms of using a Bialetti, but it is kind of an interesting physics problem I think.

Jim
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zippy
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Location: Portland, OR USA
Expertise: Just starting

Posted Wed Apr 7, 2010, 11:20am
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

Hadn't thought of that Jim but it makes sense.  Also since the exit to the chamber is at the bottom, that air can't even begin to expel until after all the water has pressed through.

There is a little coffee that dribbles out before the boiling point, probably from expanding air and water as it heats.   Once you hit the boiling point the whole thing kind of explodes into a dynamic and chaotic system that would be really hard to model.  As you said, an interesting thought experiment in physics.  

Thanks for the insight,

Scott
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zippy
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Location: Portland, OR USA
Expertise: Just starting

Posted Thu Apr 8, 2010, 8:57am
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

I thought about this some more.  The pressure calculation is what it is: 2 bars.  That's just math and physics.  It doesn't matter if it's water, steam, or another gas or liquid in the chamber.  The simple pressure cooker type valve means whatever comes out needs 2 bars of pressure to do it.  If the water or air expands a little before full boil it still only comes out once the pressure hits 2 bars.  The pressure can get higher if the screens and coffee grounds make more resistance or if, like at boiling, you get radical pressure increases and you just can't evacuate the contents of the chamber fast enough to keep it at a 2 bar euqilibrium.  At that point the restriction of the outlet orifice is the limiting factor, not the pressure needed to open the valve.

As to the boiling point and whether or not we're at 130C with 2 bars of, that may be affected by the combination of air, steam, and water in the chamber.  Still thinking about that part.

What do you think Jim?

Scott
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JGG
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JGG
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Posted Thu Apr 8, 2010, 1:15pm
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

zippy Said:

What do you think Jim?

Posted April 8, 2010 link

I think partial pressures are important (which is really unfortunate because I have purposely avoided this topic for many years).  I think that until the partial pressure of the water vapor equals 2 bars, then the relationships in the steam tables don't hold.  Once the gas in the pressure cooker has become saturated steam, and the partial pressure of the water vapor = 2 bars, then I think the 130C becomes correct.

An example that I witness every day is while pulling a shot on a single boiler espresso machine.  The boiler pressure jumps to 9 bars as a result of the pump running.  But the temperature remains unchanged at around 225F.  When the pump is turned off, the pressure in the system slowly drops from 9 bar down to normal (around 1 bar, gauge, IIRC), while the temperature holds at ~225F.

Jim
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zippy
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Joined: 7 Apr 2010
Posts: 4
Location: Portland, OR USA
Expertise: Just starting

Posted Thu Apr 8, 2010, 4:20pm
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

Therein lies the difference between a pressure cooker and the Brikka.  The opening on a pressure cooker is on top, so the steam displaces the air fairly soon after boiling starts.  In the Brikka the air and steam are both trapped until the water up to the level of the exit tube leaves the chamber.  At that point the bit of water at the bottom starts leaving directly as steam.

BUT, the chart I was looking at was water boiling temperatures at pressures.  It says 100C at 1 bar and that's when water boils in open air, basically the entire earth's open atmosphere which will never get to saturated steam but we get 100C boiling.  Now if you drop a loose fitting lid on the pot you get a saturated steam environment but since the lid is loose no pressure build up, so still 1 bar and 100C boiling.

With that in mind I'm not sure the ratio of steam and air in the chamber and partial pressures matter at all.  It may just be the overall pressure that matters.  If I'm wrong and the partial pressures factor in then I would guess that the boiling temperature is above 100C but below 130C, probably closer to 130C.

I can see why you've avoided discussing this. ;)

Scott
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JGG
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JGG
Joined: 31 Mar 2006
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Location: Kentucky, US
Expertise: I like coffee

Espresso: PID Silvia; PID Alexia
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Roaster: Hottop D w/HTC+TC4C
Posted Thu Apr 8, 2010, 5:31pm
Subject: Re: Bialetti Moka & Brikka Brew Temperature and Pressure
 

Sounds like its time for me to admit I don't know what the heck I'm talking about :-)  

Jim
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