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jpender
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jpender
Joined: 11 Jul 2011
Posts: 722
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Thu Mar 29, 2012, 6:24pm
Subject: Moka pot pressure
 

What's the typical pressure range for a standard moka pot under normal use?

I've seen the number 1.5 bar on the internet in a number of places (e.g. "An Espresso Glossary") but I could find no reference to any explanation or actual measurement.

Does anybody know?
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NobbyR
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NobbyR
Joined: 10 Jul 2011
Posts: 2,061
Location: Germany
Expertise: I love coffee

Espresso: Poccino Opus One, Ariete
Grinder: Eureka Mignon Istantaneo
Vac Pot: N/A
Drip: Melitta Linea Unica de Luxe
Roaster: N/A
Posted Fri Mar 30, 2012, 1:58am
Subject: Re: Moka pot pressure
 

The brewing pressure you found is just about right, which is why you cannot really brew espresso with a moka pot, because 1.5 bar is too low for proper extraction.

Also water temperature is too high for expresso (around 100C): When the pot is placed on the stove steam will build up and eventually will reach high enough pressure (1.5 bar) to force the boiling water up the funnel through the ground coffee and into the upper chamber, where the coffee is collected.

 
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jpender
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jpender
Joined: 11 Jul 2011
Posts: 722
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Fri Mar 30, 2012, 11:12am
Subject: Re: Moka pot pressure
 

Nobby, thanks for the response.

How do you know that the pressure in a moka pot is about 1.5 bar?
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greggb
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greggb
Joined: 7 Jan 2007
Posts: 157
Location: Mississauga, ON, Canada
Expertise: I love coffee

Espresso: Bezzera Selenia
Grinder: Rocky; Capresso Infinity
Vac Pot: Yama SY5
Drip: Aeropress; Melitta; Brikka
Roaster: Behmor
Posted Fri Mar 30, 2012, 11:34am
Subject: Re: Moka pot pressure
 

I'm not sure that there is any significant pressure involved in moka pot brewing. The pressure in the  water reservoir needs to be just enough to overcome gravity and the minimal amount of resistance offered by the coffee. I imagine it varies depending upon the size of the pot and the coffee (grind / amount / compaction).

On the other hand, the Bialetti Brikka variation on the moka pot is designed to develop a certain amount of pressure. Therefore it's reservoir walls are thicker.

Gregg.
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 722
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Fri Mar 30, 2012, 2:05pm
Subject: Re: Moka pot pressure
 

Gregg, no significant pressure? As in just a small fraction of a bar or less?

I don't think gravity has much effect unless one has a really tall moka pot. A height of 10 cm adds only 0.01 bar. It comes down to the resistance of the grounds but is there any way to estimate that? Coffee grounds are complicated and dynamic.

Illy found that with the constant pressure in espresso brewing the flow rate dropped over time as fines migrated in the cake and formed a more compact layer. In a moka pot, assuming the same sort of thing happens, an increase in cake resistance would result in higher pressure. Illy doesn't have any nice round formulas relating grind, tamping, pressure and flow rate. Even if espresso afficionados knew how these parameters related (do they?) I'm not sure one could extrapolate to the coarser grind, no tamping, low pressure moka pot case.

I'm curious because there is a relationship between pressure and temperature in a moka pot; 1.5 bar would be undesirable.
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greggb
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greggb
Joined: 7 Jan 2007
Posts: 157
Location: Mississauga, ON, Canada
Expertise: I love coffee

Espresso: Bezzera Selenia
Grinder: Rocky; Capresso Infinity
Vac Pot: Yama SY5
Drip: Aeropress; Melitta; Brikka
Roaster: Behmor
Posted Mon Apr 2, 2012, 4:07am
Subject: Re: Moka pot pressure
 

I guess the point I getting at is that the moka pot method, to my mind, is not a pressure based method of brewing. According to an Illy video I saw, it reaches about 1.1 bar.

There are a number of schools of thought on how a moka pot should be used. Some use tamping and finer grinds in an attempt to raise the pressure, but this is not the traditional way a caffettiera is used. And yes I agree, raising the pressure raises the temperature, which as far as I'm concerned would be undesirable.

Gregg.
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 722
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Mon Apr 2, 2012, 9:34am
Subject: Re: Moka pot pressure
 

Gregg, in the Illy video could you tell if the 1.1 bar represented a typical peak value or an upper limit?

Of course it depends on grind/tamping. I've brewed with an espresso grind and the safety release popped every time. But such a fine grind is on the fringe of what most people normally use. Illy actually sells cans with a moka grind, something between espresso and drip.

I did an experiment:

I took off the safety valve and attached a hose through which I could direct pressurized air. Then I filled the water chamber with near-boiling water, assembled the pot (with coffee loaded) and applied pressure sufficient to achieve what I judged (by eye) to be a "normal" flow. That is, one that was continuous yet slow, so it would take about a minute to push out about 150 ml of water.

I tried this with 6 different grinds. Five were from my Kyocera grinder (adjacent steps that I've labeled #0 through #4) and one was pre-ground Peet's from the supermarket (labled "PG"). The #0 was very fine, the #2 was in range for what I use as a moka grind, and the #4 was on par with the Peet's. No tamping.

In order to maintain the same flow rate I had to steadily increase the pressure over the minute or two it took to push out the water. This is why there is a range of values for each grind.

#0: 7-16 psig (0.5-1.1 bar)
#1: 5-14 psig (0.3-1.0 bar)
#2: 5-10 psig (0.3-0.7 bar)
#3: 7-9  psig (0.5-0.6 bar)
#4: 2-3  psig (0.1-0.2 bar)
PG: 2-4  psig (0.1-0.3 bar)

From this I would venture to guess that my typical use of a moka pot rarely reaches even 1 bar, but admittedly this test was somewhat artificial.

jpender: pump_moka_pressure_test.jpg
(Click for larger image)
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blekk
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Joined: 10 Jun 2012
Posts: 2
Location: Australia
Expertise: I love coffee

Posted Sun Jun 10, 2012, 2:07am
Subject: Re: Moka pot pressure
 

Hi all,

First post and I thought this would be the place to do it :)  I was wondering if anyone knows what the Bialetti safety pressure valves are rated for, eg is it 1.5 bar (22 psi) before it's released?

Cheers all
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jpender
Senior Member
jpender
Joined: 11 Jul 2011
Posts: 722
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Mon Jun 11, 2012, 12:25pm
Subject: Re: Moka pot pressure
 

blekk Said:

Hi all,

First post and I thought this would be the place to do it :)  I was wondering if anyone knows what the Bialetti safety pressure valves are rated for, eg is it 1.5 bar (22 psi) before it's released?

Cheers all

Posted June 10, 2012 link

G'day blekk!

I have not seen a rating but it wouldn't be difficult to measure.

For a very rough estimate you could place the valve on a scale and then push on the plunger until it starts to open. Then measure the diameter of the orifice. To get a better idea you could attach a bicycle floor pump with a decent gauge to the top of your pot's coffee chimney with some plastic tubing and hose clamps.

Bialetti sells two different sizes of safety valves.
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blekk
Senior Member


Joined: 10 Jun 2012
Posts: 2
Location: Australia
Expertise: I love coffee

Posted Tue Jun 12, 2012, 4:10am
Subject: Re: Moka pot pressure
 

Hi jpender thanks for the reply.  I have a Brikka and it has the 15mm safety valve for which I assume has a higher rating due to the design of the Brikka but I would think it would only be about 0.5 bar more then the standard Moka pot.

That's a great idea setting up a test bench btw, Im not sure why I didn't think of it (Mech Engineering student)..... I might give it a try once I finish my exams this week.
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