Pulling force for La Pavoni Europiccola

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uyeasound
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Postby uyeasound » Sep 10, 2012, 1:47 pm

I'm likely wrong, but I can't recall reading a post about exactly what it will feel like, on the lever, to pull a successful shot. I don't mean placing the thing on scales; i just mean how roughly to go about using the right amount of force throughout the pull. I'm no expert but i thought this might be of use to anyone who is in the position i was in, trying to improve with a naked pf, a few months ago.

My thoughts on the force required are, that with experience you will get to know, for a grind&dose&bean, the force required mid-pour. My advice is to use that same amount of force STEADILY from the very outset. As the flow appears, and then as you reach mid-pour, reduce the force so as to maintain the flow rate.

This would have helped me a while back: i was never certain whether to use more, or possibly less, force before the first drips appeared; and i suspect poor lever consistency was responsible for non-uniform flow from my naked pf.

As i say, i'm no expert, but my shots are pouring well and tasting great.
However, if i'm contradicted by someone with greater experience, well i'm open to suggestions and ready to learn too...

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KurtAugust
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Postby KurtAugust » Sep 10, 2012, 3:01 pm

Like cutting through cold butter with a knife?
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bostonbuzz
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Postby bostonbuzz » Sep 10, 2012, 7:15 pm

uyeasound wrote:exactly what it will feel like, on the lever, to pull a successful shot.


Not possible. "Knife through butter" sounds pretty good. I found that the pressure is generally less than you think it is. If you are pulling really hard, well, that's too hard... I find the best shots come when the pour is pretty slow by pump standards, yet I don't have to pull hard at all. I would also recommend starting out hard and quickly going easy when the drops are about to meet up and make a cone. This process is the most fun part of making espresso imho. :D
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homeburrero
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Postby homeburrero » Sep 10, 2012, 9:26 pm

bostonbuzz wrote:I would also recommend starting out hard and quickly going easy when the drops are about to meet up and make a cone.

uyeasound wrote: My advice is to use that same amount of force STEADILY from the very outset. As the flow appears, and then as you reach mid-pour, reduce the force so as to maintain the flow rate.


Two different approaches. I've always naturally tended to use the bostonbuzz approach, but will have to try being more patient with the initial force and see if uyeasound is on to something here.
Pat
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Anvan
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Postby Anvan » Sep 10, 2012, 10:43 pm

One confusion that happens sometimes, it's not a hot knife through butter or a knife through soft butter. So some people work this analogy in terms of cutting through a small block of cheddar cheese.

Another way of approximating the required pressure is to put both hands on the lever and ease it down using mostly just the weight of your hands and arms. If you feel yourself having to pull down very much, your prep is probably a little tight, and if the lever falls quickly with little or no resistance, it's too loose. And for sure, if you have to press down really hard and the lever "bounces" back up when released (like a shock absorber) then you know that the shot is essentially "choked" and that grind is too fine for the current dose.

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homeburrero
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Postby homeburrero » Sep 11, 2012, 4:47 am

Most of us have our idea of 'too soft' vs 'too hard' vs 'really hard' based on experience with the machine. But if you are uncertain about that I think you can benefit by putting your Pavoni on a bathroom scale and playing around with pulling against a relatively choked shot. Might work better than thinking about knives and butter and cheese.

This has been discussed on HB, with the advice that around 40lbs is right*. This is in my opinion too hard. It also looks too hard when I do a simple calculation. By my calculation a 30 lb pull is about right for 9 bar brew pressure.

Here's my calculation:

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9 bar = 90 N/cm**2 =  9.18 Kgf/cm**2

Pavonis have 44mm gaskets, so the cylinder radius is about 2.2cm giving an area of:

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2.2 cm**2 * 3.14 = 15.2 cm**2

So the force at the piston is:

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9.18 Kgf/cm**2  * 15.2 cm**2  = 140 Kgf

If you measure the distance between the fulcrum pin and piston rod pin (24mm), and measure the distance from the lever handle grip's end to the fulcrum pin(265mm) you will get a ratio of about 11/1, and a practical lever advantage of around 10/1 to the point an inch or so in on the handle, so ...

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140Kgf/10 = 14 Kgf  = 31 lbf at lever handle

When pulling 9 bar you are pulling with more than 31 lbf in a vector directed towards the machine's base (to keep it from tipping) but the downward vector measured by a bathroom scale would still be around 30 lbs.

*It's amazing how often you hear the 40 lbs figure for pulling 9 bar. I think this comes from an old alt.coffee post - http://alt.coffee.narkive.com/LVokoYhH/...so-network. That figure was a guesstimate, and used a lever advantage of only 8:1. I like mine better because it's based on the actual piston seal diameter, and an actual measurement of the lever advantage.
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uyeasound
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Postby uyeasound » Sep 11, 2012, 5:17 am

My confusion a while back was "ok, i know the feeling in my arms when i pull it correctly mid flow, but by then i've already messed up the start! How do you start it?"
And i've now come to the conclusion that steadiness, at whatever force you usually use, is best. Then when the drops appear you can perform your routine (which for me is a reducing pressure profile to maintain flow rate).
Levers are fun aren't they! I think i would be bored if i went back to a pump machine.

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yakster
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Postby yakster » Sep 11, 2012, 1:06 pm

uyeasound wrote:Levers are fun aren't they! I think i would be bored if i went back to a pump machine.


I strongly agree with this statement.

As for pressure, Dr. Gary likened the force required to be about the force required to press the plunger down on an Aeropress. This analogy helped me out when starting with my Gaggia Factory.
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