RapidCoffee wrote:I have never found this to be the case. On the contrary: in my experience, it is more difficult to get a "pretty" pour with a triple basket than a double, or with a 53mm Spaz double (which is triple height) than a standard 58mm double. Levers, despite lower brew pressure and smaller diameter baskets, do not always yield the most visually striking pours.

(emphasis added)

I'm not ready to agree that less-than-striking visuals necessarily correlate with discernible flavor flaws in the cup.

The unevenness of the arm-applied pressure of a manual lever doesn't produce the clock-work cone-formation and evenness of flow that a mechanical pump produces as its pressure ramps up and remains steady; the manually produced flow will rarely be as perfect and even as a machine's. If those are the visual features you regard as most "striking"?

If dose, distribution, and compaction are not seriously flawed or excessive, once you have reached a very basic level of acceptability on the pressure (the bar is low, as it were ) most of the deliciousness to be attained in the cup results from a combination of brew water temperature, the right grind setting, and the inherent excellence of the roasted bean, IMO. Why do I hold this opinion? Because I've pulled delicious espresso at very low pressures and with wavering, inconstant pressure on manual lever machines; when the flavors in the cup were great, it had to be in spite of the vagaries of pressure and the less-than-striking visuals. Taste Disclaimer: flavor is much more important to me than body/mouthfeel.

Timo,

Thanks for your eloquence on the mechanical issues in this discussion. As for shot appearance and quality I feel unqualified to comment. However, since the "double pull and puck fracturing" issue has arisen it ugly head, an observations and a question.

During a very sloooowww pulling down of the lever after a second or even third pull on my Pontie Vecchio Export the two porta filter holes often continue to dribble closing off both holes. Also bubbles will close both holes. No way is atmospheric pressure getting to the bottom of the puck. Otherwise the coffee and bubbles would be sucked back up into the porta filter.

Pressure differential across the puck IMO is a viable mechanism for puck fracture. But this case appears to indicate there is no pressure differential across the puck. So what causes puck fracture?

I must add, I rarely if ever get puck fracture if I sloooowwwly pull down the lever.

Edit: Whoops. I incorrectly wrote raising the lever and have corrected it to read slooowwwly pulling down the lever.

Geoffrey

My apologies to all if I have hijacked this thread.

timo888 wrote:I'm not ready to agree that less-than-striking visuals necessarily correlate with discernible flavor flaws in the cup.
...
Taste Disclaimer: flavor is much more important to me than body/mouthfeel.

No argument there. But I believe that the "tall narrow basket = more forgiving" hypothesis is merely urban myth. I've always found taller baskets to be more susceptible to channeling, not less. I can't think of an easy way to test this, other than visual observation with a bottomless PF.

peacecup wrote:
In fact, I'd argue the opposite - that tall, narrow baskets (and narrow pistons, hence more pressure per pound of leverage) with a couple of pulls, makes it easier to get a better extraction. This basket design is certainly more forgiving in my opinion. Its almost impossible to pull sink shots with the PonteVecchio once one has had a little practice. The Caravel, because it is a manual lever, is a little more touchy. But the design concept, with the tall narrow basket, seems to be able to make the absolute most out of 14g of bean dust and glass of hot water.

PC

I agree wholeheartedly. I have been testing a Ponte Vecchio Lusso for the last few weeks and have found that it performs very consistently- and all the shots taste great. Seemingly I cannot burn the coffee. If anything the temperature scale may be a bit limited to the low side. I was really surprised- as I had been using my Sama Club for the weeks prior. Both machines are essentially identical in every functional detail- except for the group diameter: The Sama is 49mm- the Lusso 45mm.

I thought that the larger 49mm group would be a big advantage over the Lusso group. On balance I was wrong. With the Lusso (like the Caravel) tamping is a breeze- just fill the basket completely and tamp down hard- basically pack as much in as you can. The essential point is the grind size- as long as it is correct- a notch off choking- the shot will be super.

With the 49mm Sama tamping (and grind for that matter) seem far more important- and the shots vary a lot more. The Lusso is much more user friendly.

At the end of the day- I don't think the larger group will ever make a better shot than the smaller one- but it can make a stronger shot- simply because you can use larger doses.

I think the Caravel proves the point- 43mm seems to be the secret of it's awesome abilities. If only it could froth.

GB wrote: ... Pressure differential across the puck IMO is a viable mechanism for puck fracture. But this case appears to indicate there is no pressure differential across the puck. So what causes puck fracture?
...
I must add, I rarely if ever get puck fracture if I sloooowwwly pull down the lever.

I suspect each piston seal is designed to seal in one direction only, Geoffrey. Between the two opposing seals there is air. On the piston upstroke, the lower seal (closest to the puck) allows some of the air above it to pass by, mitigating but not entirely eliminating pressure differential at the puck. If the piston rises faster than the air can flow past the seal, the differential occurs. An hypothesis.

An experiment: are you able to pull the lever down (slowly) with the spout holes completely blocked?

Covering the porta filter holes is good follow up test. I have not done it because the porta filter is probably at 180 degrees F - ouch! In time I will try it with something other than my finger.

However, I think the solid streams and bubbles alone are very accurate indicators of air not going back through the puck. What I repeatedly have observed is that when the piston very slowly rises from the down position after a "pull" the porta filter continues to drain in two solid streams. Then bubbles occur and the streams slowly cease leaving almost static bubbles, which remain at the end of the upstroke. Consequently air cannot be going back thru the puck otherwise the streams would be sucked back into the porta filter and/or the bubbles would deflate.

Timo's hypothesis of balancing air coming from between the piston seals and entering the cylinder is plausible. Especially considering the lower seal geometry and its direction. Not that it matters, but I doubt that it is air. It is more likely water and steam, because the space between those two seals is open to the boiler every time it passes the grouphead water inlet hole. But to his point, be it air, or water and steam, any of these fluids could theoretically pass the seal into the cylinder.

But, because of the mechanics of the seal I doubt this is happening. The seal is quite stiff - having a fairly high durometer - and also has a high pre-compression. This means that a very high pressure differential is required for fluid to bypass the seal. It also means there has to be a very low pressure in the cylinder, which IMO would fracture the puck

I have different theory which was triggered by my stream/bubble observations..... When the piston is allowed to rise very very slowly some hot water in the puck and on the walls of the cylinder etc. change to steam. And as the piston continues to slowly rise more steam is generated. The steam expands compensating for the reduction in pressure caused by the piston displacement. And if the puck is permeable these gases migrate thru the puck creating a balance of pressure on either side. Alternatively, if the puck is not permeable then maybe the same steam generating mechanism occurs below it at a much smaller scale but adequate for the smaller volume of the porta filter.

Pardon the lengthy post.
Geoffrey

I don't know if this is really relevant but I was pulling apart a Pavoni group head the other day when I noticed that the water inlet hole is actually above the seals in the piston- even on the full up stroke. I was very surprised- I had never noticed it before despite resealing quite a few Pavoni groups. I looked on the net and found an explanation- that the water forces itself past the seals- and that Pavoni possibly did it that way to better disperse the water across the puck.

sorrentinacoffee,

If not relevant it certainly it is at least damned interesting. And it seems to refute my idea that water/steam does not get past the seals giving more credence to Timo's theory.

"The plot thickens"

Can you direct us to the information you found on the net?

Thanks
Geoffrey

I must say I was very surprised- I kept looking in again to see if I was missing something- the Pavoni was an older Europiccola group and I thought it may have been unique to that design. So I check a later Pro machine- the same. I couldn't figure how the water could get past the piston. It may have been on Franco's Italian Pavoni site that I found the explanation, not sure.

As for the Sama group- there is definitely water circulating in the spaces between the seals when the lever is at rest.

Geoffrey, I agree, the dangling bubbles are telling us that on a slow piston upstroke (PV spring lever arm downstroke) the pressure differential is not great enough to disturb the bubbles, and so neither should it be great enough to disturb the puck.

I don't know what kind of seals are on the Pavoni piston. But seal type is important. Some seals are unidirectional. The v-seal's seal is totally actuated only when the piston is moving in the direction in which the broader end of the seal is facing. That's why the v-seals are mounted facing in opposite directions. One seals on the piston's upstroke and the other seals on the piston's downstroke.

the pavoni has two V section seals- the bottom one faces down and the top one up.

Now that I think about it- I am not sure what is happening when the piston is at rest- what stops the space above the piston filling with water...

I don't think the water passes thru the gaskets at all. Water surrounds both ends of the piston.
Here's a pic of the my Pavoni Pro piston, at it's highest point inside the cylinder wall.

You can see the water inlet ( square hole) on the top edge of the cylinder. The water runs down the cylinder channel to exit into the cylinder just below lower piston gasket but above the front face of the piston.
This forces the water to run around between the cylinder wall and piston wall , filling the chamber evenly from the whole of the piston perimeter. Much like a toilet pan really. Wetting the puck from the outer perimeter to centre.
With the lower piston gasket facing forwards, and critically positioned on the inlet it allows me to do very small pumps to set up preinfusion in the puck; thus only one pull and no channeling for 1oz of liquid gold.

180degrees wrote:With the lower piston gasket facing forwards, and critically positioned on the inlet it allows me to do very small pumps to set up preinfusion in the puck; thus only one pull and no channeling...

What lies behind the word "thus" here is exactly what we're trying to tease out, only with a full pull, not a series of minimalist preinfusion pulls.

Let's walk through your scenario:

Piston full upstroke: lower piston seal is positioned so that water can enter the cylinder via the square notches.
Piston minimalist downstroke (just a couple of mm for preinfusion): a little water is forced through the filter.
Piston minimalist upstroke: with this operation, if the lower piston seal were perfectly sealed, it would create a small vacuum. And maybe it does. And maybe it's not disruptive enough to cause any problems because the coffee in the filter at this point is a deformable soggy medium, not a hard cake, that can be resettled with downward pressure and water-flow after it has experienced an insignificant up-suction.

What we need is a pyrex portafilter and basket

it would create a small vacuum

After the intial upstroke there's plenty of air caught inside the cylinder, partly due to the inflow of water going down the side walls of the cylinder, across the face of the puck and rising with the air trapped on top.
Especially noticeable on the first shot. The puck sits with 3/4 oz of water on top of it. The downstroke compresses the air, which applies pressure to the water forcing some of it into the puck. The minimalist upstroke just uncompress's the air, the weight of the water protecting the puck. At the end of the full downstroke there is always some bubbles from the compressed air, again especially noticeable on the first shot.
After the first shot the amount of air is always a lot less, and progressively so the more shots pulled. I would propose that the first shot, under pressure properly fulls the top of the piston which is now down and full of water and it is that water flowing lifted up by the piston,under pressure that is being forced into the cylinder inlet thru the cutouts on top of the cylinder wall flowing across the cylinder inlet and refilling the bore.
Why is the first shot so airy and subsequent shots less ? ---- because any air left in the outer wall of the cylinder is forced past the cylinder gasket on the second shot upstroke and out past the screen gasket effectively bleeding the head. We have a 2 stroke espresso machine.
Sorry it's a bit wordy, should really do a flow diagram.
Cheers

Very very interesting and I think if I understand it correctly, very different than the Ponte Vecchio design. But I am not sure I understand it correctly.

180degrees wrote about one square hole and a channel.

Timo wrote about square notches at the top of the cylinder.

Are they discussing the same thing?

Sorry about my thickheadedness. But does that protrusion on the left side of the cylinder contain the aforementioned square hole and the channel which allows the water to exit through another hole that is below the piston on its upstroke? Or is it entering the via the notches as Timo suspects?

Thanks
Geoffrey

hmm, quite different than my Europiccola group. It just has a small inlet hole near the top of the brass cylinder. No plastic.

180degrees wrote: ... After the intial upstroke there's plenty of air caught inside the cylinder, partly due to the inflow of water going down the side walls of the cylinder, across the face of the puck and rising with the air trapped on top. ...

Yes, some air remains in the cylinder.

Here's my understanding of the dynamic. It assumes, arguendo, that all seals are sealing 100%.

UPSTROKE
When the piston has risen to where its face (lower seal) is above the water inlet, much of the air inside the cylinder gets displaced by the inrush of water-under-boiler-pressure; the displaced air is forced out through the coffee in the filter, which is very porous at that point. Until the piston's downstroke closes the inlet, the boiler and cylinder form a continuum, and any air that does not get forced out of the cylinder gets mixed with the incoming water and compressed a little by boiler pressure. The pressure in the continuum will drop as some pressurized steam escapes through the filter, at least until the puck becomes wet and swells and effectively slows or even blocks egress. How much or how little pressure is in the chamber will depend on the p-stat setting and whether the heating element has kicked back on during the pull; other ancillary factors such as grind and dose and freshness and basket geometry will determine how much can be pushed through the coffee-in-filter by boiler pressure alone.

DOWNSTROKE
The downstroke compresses any air remaining in the cylinder as its contents are pushed through the coffee.
Some air remains above the saturated puck. The piston is in its lowest position and the bottom seal traps the air between the puck and the bottom of the seal. Residual pressure from the piston's compression may cause bubbling at the spouts (or on the bottom of the filter, if naked PF), the only egress . A small amount of air remains above the puck nonetheless; how much will depend on how full the basket is and on how much or how little air was forced out of the cylinder by boiler pressure during the upstroke.

SECOND UPSTROKE
When the piston rises, a slight vacuum is created. Whether the vacuum is sufficient to rupture the puck, and the severity of such rupture if it should occur, will depend on the amount of air that remains trapped above the puck, on the speed/force of the upstroke, and on the degree to which that trapped air remains compressed or has reached equilibrium with the atmospheric pressure on the outside of the filter.

PRACTICAL HYPOTHESIS
An overdosed heavily compacted basket is more likely to be ruptured by a second upstroke than a moderately dosed lightly tamped basket because the latter has a greater cushion of (stretchable) air above the puck, which mitigates the vacuum-effect. If the portafilter would sneeze, chances are good that a (slow) second pull wouldn't disrupt the puck.

I agree with peacecup on this issue: it's not that big a deal. I always do double strokes on my Sama and PV (and Caravel for that matter). Also I am not that careful about the second stroke. I never really have a 'ruptured puck'. Sometimes the top of the puck is a little dimpled- or cratered.

Also I generally fill the basket as much as I can- and certainly on the PV if I tried to quickly remove the PF immediately after the second pull- I would get a 'sneeze'.

Not wanting to hi-jack this topic maybe we should move to a new topic on this one sysop ?
--- but I wouldn't disagree with Timo either
But is it just me that has more air in the first shot ? if not then,
Presumably this extra air is coming from the inner chamber of the group head and as Timo says being ejected to the cylinder by boiler pressure as more shots are pulled.
Would be good to have a technique that would empty the air pre the first shot, it certainly reduces the volume and quality of pull.