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SirCharles
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Joined: 3 Feb 2012
Posts: 11
Location: Oregon
Expertise: Just starting

Posted Fri Feb 3, 2012, 10:56pm
Subject: Brikka Theory
 

Hello, curious what others here think about this...

I'm new to the Brikka, and while it seems to be a promising way of making a good cup of coffee, it certainly seems to be a finicky device. In playing with it, I've had some insights that I'd like to share, one of them being that if it is effectively a pressure cooker, how does it get around the superheated water not burning the grounds?

I've attached a picture of the Brikka. Actually, the design I have is the improved model; there is no funnel gasket, the funnel is SS, and the lid has a hole on the top for steam to escape. Oh, and there is no water cup, you put water up to an H2O mark in the top part and pour it into the bottom part.

There is lots of talk about how the water level is critical. Also about how you are not supposed to tamp the grounds. Aside from the extraction reasons, look at the design of pot. As the water approaches boiling, it is forced up into the grounds, up the coffee pipe, to where it is stopped by the pressure weight. This water is out of the bottom of the pot, so my conjecture is that it does not get to as high of a temperature as the water in the bottom of the pot. In fact, I wonder if the volume of water is such that when the water hits the weight, if the grounds are of the correct volume, the base of the funnel is just above the water line in the water container. Note also that the funnel in the newer model is SS, not as good of a conductor as AL, which again serves to insulate the grounds from the heat.

The heat then increases the pressure in the water container. Once the pressure reaches 2 bar, the weight is raised, and the steam pressure pushes the water out the top. In other words, the superheated water, in theory, never makes it to the coffee.

This explains some things to me. If you tamp the grounds, not enough water makes it out of the water container, because the coffee takes up too much volume. In this case, superheated water does make it though to the grounds when the pressure weight releases, resulting in burned coffee. In this case, the crema looks like Guinness; dark and bitter. When things are right, the crema looks more the typical golden color.

Anyway, it's very clever... I think. It also explains the mysteries people seem to have with the Brikka in that tamping the grounds has an extreme effect on the effective water level, which has a direct effect on the superheated water hitting the grounds.

Thoughts?

SirCharles: brikka_index.jpg
(Click for larger image)
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 626
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Mon Feb 6, 2012, 2:46pm
Subject: Re: Brikka Theory
 

There is air above the water and below the weight that will get compressed (and heated) as the temperature in the water container rises. Since that air starts out at 1 atmosphere (1 bar) it would reach 2 bar when compressed to about half it's original volume (a bit less since it will be hotter as well). At the same time there is water vapor that is adding to the total pressure so the weight would release before the air is compressed that far. Without knowing the specific geometry of the Brikka I would guess that the hot water will be infiltrating the coffee grounds before the pressure reaches 2 bar and the weight releases. But how much exactly and how hot will that water be? It's difficult to say.

The initial water level matters because it affects the air pressure in the head space of the water container as the water level changes. The higher you fill it the hotter the water temperature needs to be to overcome the head space air pressure loss.

I have a question for you: How did you determine that the Brikka releases at 2 bar? And is that 2 bar absolute or 2 bar above atmospheric pressure?
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SirCharles
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Joined: 3 Feb 2012
Posts: 11
Location: Oregon
Expertise: Just starting

Posted Mon Feb 6, 2012, 11:43pm
Subject: Re: Brikka Theory
 

2 bar is posted on a number of sites. Here is one.

The air getting compressed as you say assumes the weight creates an airtight seal and not just a watertight one. There is evidence it is the latter in that in normal operation coffee leaks out prior to the valve releasing.
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 626
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Tue Feb 7, 2012, 10:51am
Subject: Re: Brikka Theory
 

Oh, I see. I was wondering how good a seal it made. It's interesting that it leaks the air and holds back the liquid.

That website doesn't say where it got the value 2 bar. I searched and couldn't find a reliable source (Bialetti doesn't appear to mention it). I found one post (on coffeegeek) by someone who measured the weight and the diameter of the orifice and those numbers are consistent with 2 bar gauge pressure.

I wonder how hot the water gets before the weight lets go? If it holds up to 2 bar gauge that sets an upper limit of 134C (water vapor). But of course there is air in the water chamber as well. In my own moka pot, the air space doubles by the time the water first reaches the top of the pipe. If the Brikka has the same relative dimensions that would imply a maximum temperature of about 125C. Pretty hot stuff to be pushing through the grounds, even if they've already been mostly extracted at some other(?) temperature. But I suppose the idea behind the Brikka isn't optimal temperature control but rather the production of pseudo-crema.

It's definately an interesting device. I'd love to read a more definitive explanation of its function. It's too bad Bialetti doesn't provide one.
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SirCharles
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Joined: 3 Feb 2012
Posts: 11
Location: Oregon
Expertise: Just starting

Posted Tue Feb 7, 2012, 4:40pm
Subject: Re: Brikka Theory
 

I wonder how hot the water gets before the weight lets go?

Well, again, this is all theory, but the water is one temperature when it steeps the grounds and fills the pipe, which ought to be way lower than 125C; no reason to think it is any hotter than in a Moka, which ought to be just below boiling. Let's say the water is the perfect level to put the base of the funnel just at the water level when the water hits the value. Now air and water heats up in the base, creates pressure, and forces the valve open.

This is all theory, mind you, but the thinking is that the superheated water never touches the grounds.

Note also that it is very easy to manually release the valve with, say, a wooden chopstick.
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jpender
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jpender
Joined: 11 Jul 2011
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Location: California
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Posted Tue Feb 7, 2012, 5:46pm
Subject: Re: Brikka Theory
 

I see what you're saying about what you imagine the internal dimensions of the Brikka to be. I know that isn't the case with my standard moka pot but that doesn't prove anything about the Brikka.

Assuming you have a decent scale, would you mind measuring the internal volume of yours?

Even if what you suspect were true, when the valve opened there would still be superheated water in the bottom chamber. And this would be in direct contact with the water in the lower end of the tube (and not far from the coffee). So even if the bottom of the tube were at the level of the water, some fraction of very hot water and superheated steam would still pass through the grounds when the valve opened.

I understand that we're only speculating, especially me since I don't even have a Brikka.
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SirCharles
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Joined: 3 Feb 2012
Posts: 11
Location: Oregon
Expertise: Just starting

Posted Tue Feb 7, 2012, 7:34pm
Subject: Re: Brikka Theory
 

So even if the bottom of the tube were at the level of the water, some fraction of very hot water and superheated steam would still pass through the grounds when the valve opened.

Right, which is why a lot of people recommend pouring the shot as soon as the valve opens and that the very last bit is very bitter if you don't.

I have a very good scale, but measure what? How much water goes in? See, the effective volume of the water (critical in my theory) is also a function of the volume of coffee grounds. And also, if the effective water volume is too low, then the valve opens very late because it takes massive steam pressure going through the grounds to do it. When this happens, the coffee boils out of the thing and is burned to a crisp.

I think it is a deceptively clever, highly temperamental device, but hopefully a little less mysterious.
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 626
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Wed Feb 8, 2012, 12:44pm
Subject: Re: Brikka Theory
 

SirCharles Said:

I have a very good scale, but measure what? How much water goes in? See, the effective volume of the water (critical in my theory) is also a function of the volume of coffee grounds.

Posted February 7, 2012 link

You could get a rough estimate by using a ruler on the primary parts but weighing water is the easiest way I know of to measure the volume somewhat more accurately. Well, volumes actually, and some use of a ruler as well since there are a number of measurements that are needed. As you pointed out the effective volume of the grounds should be taken into account. The water volume also increases with temperature (4% for 25C to 95C) but I'm going to ignore that here.


Here's how I measured my pot... in 9 easy steps and with a cup of coffee:

1) Tare the empty base of the pot. Fill the base up to the level you normally do
and weigh the water inside (A0 grams).

2) Drop the basket (dried off) in and pull it back out and measure how far up the
tube it got wet (H1 cm).

3) Measure the diameter of the basket tube (D1 cm).

4) Measure the diameter of the basket top (D2 cm).

5) Measure the depth of the basket (where the coffee sits) (H2 cm).

5) Tare the empty base and basket together. Fill the base to the brim, drop in the
basket, and top it up so that the water level is at the brim. Weigh this water (A1 grams).

5) Tare an empty container. With your finger on the bottom of the basket tube, fill the
basket to the brim with water. Pour this water into the container to measure it (A2 grams).

6) Tare the complete empty pot (base, basket, top). Place all three pieces under water in
a large bowl or basin and work it around to eliminate all the air. Screw it together under
water and lift it out vertically. Now, holding the weight/valve closed, pour out the water
in the upper collection chamber. Weigh the water still in the pot (A3 grams).

7) Tare the basket and fill it as normally with ground coffee. Weigh the grounds (C1).

8) Brew a pot! Enjoy the coffee. Now weigh the wet coffee puck (C2).

9) Weigh the water that remains in the bottom of the pot (A4 grams).


Assuming your measurements are in grams and centimeters,

Initial water volume: V0 (ml) = A0 (g)
Initial head space in the base: V1 = A1 - A2 + H1*(pi)*(D1/2)^2 - V0
Volume of water in the grounds: VW = C2 - C1
Volume of the coffee grounds: VC = H2*(pi)*(D2/2)^2 - VW
Initial air space below the top: V2 = A3 - V1 - V0 - VC
Volume of unused water: V3 = A4

For my pot (filled to just below the valve as per instructions):

V0 = 178 ml
V1 = 56 ml
VC = 28 ml
V2 = 38 ml
V3 = 28 ml

As you can see V2 << (V0-V3). That is, the initial upper air space is much smaller than the available brew water which is why the coffee begins emerging long before the water chamber is exhausted.

But that's my pot, which is not a Brikka (still mysterious to me).
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SirCharles
Senior Member


Joined: 3 Feb 2012
Posts: 11
Location: Oregon
Expertise: Just starting

Posted Thu Feb 9, 2012, 1:01am
Subject: Re: Brikka Theory
 

Wow! That's an interesting calculation you've got there. I'd need to find an accurate ruler to do it.

But really, look at it the other way. There are many YouTube posts of people claiming that Brikka makes great coffee; how could it do that and push superheated water through the grounds?

Take a look at this video, the most wicked Brikka video I've seen.
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 626
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Thu Feb 9, 2012, 3:44pm
Subject: Re: Brikka Theory
 

SirCharles Said:

There are many YouTube posts of people claiming...

Posted February 9, 2012 link

I was hoping for something a bit more objective in the way of evidence.
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