Hi guys.

Over the months and years, various threads have included sundry curses and facts on single baskets, so I figured it may be helpful to have a thread on the topic.

The profile of the basket, to me, seems important, and I would have thought a relatively rectangular profile (such as on the LM) would be the way to go. Such a shape would allows even pressure to be exerted all the way through without any basket shape anomalies interfering. I always wondered why single baskets weren't the same design as a double, but half the depth, rather than being the irregular shape they are:


Double and single baskets from Rancilio Silvia


Double and single baskets from La Marzocco


Double and single baskets from La Pavoni, also used on Imat/Quaha and some others

On reflection, the reason seems obvious. A filter basket must have a certain combination of diameter and depth, matched to pump pressure, to allow a full load of typical espresso grind to produce about the right extraction time. A basket too deep for its diameter would require a coarser grind or a lighter tamp to offset the extra depth, and a basket too shallow for its diameter would need the opposite. That being the case, with a rectangular profile, there would only be one possible perfect depth for a given diameter basket, and I feel machine manufacturers make the double basket to basically meet this formula.

Because in the domestic market at least there's a demand for single baskets, they've had to make some compromises on that formula to allow a basket with half the coffee to produce roughly the same extraction times with the same grind as what's needed for the double. Considering the depth / diameter equation, this could be achieved by keeping the depth the same and halving the surface area (~diameter), or halving the diameter and maintaining the surface area (~diameter). Since the top basket diameter must match the double basket for correct fitment to the group, the option most choose is to reduce the diameter of the basket by roughly half and keep the depth similar. Of course, the opening needs to be the same size to allow correct fitment to the group, so they start at 58mm and then cut in to take it down to its final diameter which is around the 35mm mark.

This causes a plateau at the outside of the basket about 10-15mm from the top of the basket which complicates the tamping process. It's impossible to tamp all the coffee to the same pressure when the basket shape causes some areas to accept more pressure than others. Also, the different depths mean there are different length paths for the water to travel; water entering at the outside will have to run through the high pressure stuff at the outside, then come in and continue through the low pressure coffee in the bottom well; water entering in the middle takes the quicker path straight through the centre. And I'm sure there are other reasons too.


Suspected pressure in a double basket before and after tamping


Suspected pressure in a single basket before and after tamping

A final word: none of this means single baskets are impossible to produce decent coffee with, but it does make it more difficult, and it does depend on the machine. I produced acceptable results with the single in the time I had my Quaha, but I find it much harder with the Silvia due to its lower showerscreen.

Look forward to hearing what others have to say!


Greg

Greg,

I believe the above is an excellent explanation of the difference.....

It is also interesting to postulate where the water might flow...

Given that water always takes the path of least resistance (shortest route) through the puck....

In a double (assuming even tamping) water will flow uniformly throughout and will extract the coffee oils etc uniformly throughout the puck...... giving a good even extraction at all depths - and great tasting espresso.

In a single basket most water will flow through the centre 35 mm portion with little, if any, taking the longer route via the edges (the portion between 35mm and 85 mm because it needs to travel far further.... mostly through more compressed grounds....

Also assuming you get different density at different depths in the single basket (and I agree with you that is what you will have - unlike the double which will be uniform.) the water will spend longer in the top (more compressed) layer and travel though the bottom (less compressed layers) more quickly.

This will result in almost no extraction near the outer, possible over extraction near the top in the centre and under extraction near the base.... So in effect wasting the value of some grounds and producing a poorer coffee due to over extraction.

The one thing which I think maybe different to your drawings is the post tamp distribution on the single basket.... I think it will be more compressed in the region between 35 and 58 mm (less depth of coffee so more compaction) compared to the centre 35 mm where the increased depth will result in less compaction... and there maybe a type of fault line at around the 35mm mark where the two different compaction regions meet...

My 2c worth.

Poor effort at illustrating the above example!

JavaB wrote on Jan 26^th, 2007, 2:54pm:


Agreed. Drawings adjusted accordingly.

Greg,

Yep - that's what I meant...

And your drawing looks so much better than my feeble attempt...

Exactly Robusto. I did think that one alternative would be a tamper with a stepped base (I guess a convex tamper would come slightly closer to the requirement than a flat base) but using the double seems a much simpler (and cheaper!) option. Also, even if a stepped base tamper was used, the initial contact of the central extrusion with the coffee would cause some coffee to be displaced sideways rather than all downwards, ultimately creating a high pressure point at the edges when the rest of the tamper made its way onto the coffee.

Some doubles such as the Rancilios do have a slightly curved floor; others such as the LM use a flat floor. Over the ~25mm depth of the basket, I don't reckon a 0.5mm or so of curve is enough to be concerned about.

Greg

Just had a close look at the La Cimbali (genuine) baskets...

The double has vertical sides to within about 5mm of the base.... it then slopes in slightly to the bottom and the joint between the side wall and the bottom is quite sharp.

The single on the other hand is only a bit over half the depth... the sides are vertical to just below the level a full tamped puck would reach and they then slope inwards (at about 45 degrees) to the said 35 mm base...... so La Cimbali get half the dose by utilising half the depth.

Their thinking must therefore be that the water from the surface of the puck (all 58mm of it) would have less variation in the distance to travel to reach the perforations in the bottom (35 mm) - a maximum variation of 1.4 times in the worse case..... but this requires you to grind finer.

Although it sounds like they have thought about the best compromise.... I still don't use the single as the coffee produced is just OK (by my standards)

OK,

Here are the "real measurements"

ID at top 58.41 mm
ID at bottom 38.60 mm (bit bigger than normal)

straight sides down 9.61mm from top of rolled edge

basket depth - from top of rolled edge 17.31mm

so tapered edge is 7.7 mm high and 9.90 wide...

giving and angle of about 38 degrees (my eyeball 45 was just a tad out )


and you do have to grind finer

Viviane wrote on Jan 26^th, 2007, 4:43pm:


Sounds good Viviane. I give it a slight tamp after half filling to get some compaction in the bottom section. But then I have a different machine to you. I hate wasting coffee (money) as well. Are you happy with the quality of your coffees and how do your singles compare with your doubles?

That's ok for DIY roasters but what about those of us who don't. In my own situation I don't live in a capital city (don't know about viviane) and it is hard to buy cheap fresh beans.

I think maybe its time I started roasting.  

All,

At the end of the day what you choose to do is your own choice. There are clearly pros and cons of both methods, and the final decision you make will depend on many factors including machine, budget, location and personal preference. Having the correct information with which to base that decision, however, is invaluable, and that's the value of this thread.

Greg

barri wrote on Jan 26^th, 2007, 8:40pm:


Barri,

Seeing you are remote from Melbourne - which makes getting fresh beans harder, it would be even more advantageous roasting your own....

The green beans which Andy sells on BeanBay each month are fantastic... great range of different varieties from different regions.  They generally sell for about $10 per kilo. Roasting isn't hard to do- there are lots of threads where popper, corretto and various other "home built" and commercial roasters are discussed....

And the flavour from freshly roasted beans is so much better.... and as Coffeechaser said above - that applies to both single and double baskets....

I only use beans from a local roaster (within a day of roasting) when I miscalculate and run out of my own roasted beans...

barri,

Nope, unfortunately he doesn't - but I'm trying to convince him to sign up as a sponsor.

But the pricing of roasted beans is a problem. If I buy 1kg or more they are a bit over $20 a kilo - a good price but I often can't use that many beans in under 3 weeks- after which they are stale!

So I have to buy 250 gram bags which are about twice the price per kilo.... and that becomes less attractive!!

The beauty of home roasting is I do 300-400 grams at a time and that will last me two weeks or so..... and I can watch my supplies and roast some more if they are going too fast - like when my wife says " I'm having 10 people over for morning coffees in 3 days time"

But when she says tomorrow!!! - Off to the local roaster if I'm low

If you look at my post on the EM6910 on page 2 of this section I posted pictures of my EM pouring a single shot - I used the single basket - I did not cheat and use a double basket with single spout!  I get that quality pretty much every time from the single basket (and I use it infrequently).  I am no barista, so perhaps there is somthing about the machine's shower screen or somthing which gives it a good seeing to - the basket is a standard one that I sourced from Barazi coffee in Brisvegas.

Cheers

PS.  While I can continue doing that I will not be doing doubles for a single standard coffee

Vivi,

I can appreciate that you want to master the "single basket" challenge..... I guess I've always found problems compared to a double basket and stopped trying.....

When the instructor at the course I attended told the class (all of whom were baristi - except me) to throw away half a double rather than use a single basket.... other than the shocked and horrified look on some of their faces - it just reinforced by belief.... The instructor is the only full time professional trainer here in WA, and is very well respected by the coffee community - I think he is a judge  for the national Latte art competition - or something like that.

But enough about him..... I don't know the silvia very well- but if it's like my La Cimbali and a few other machines I've had... the water enters behind the showerscreen near the front of the PF - you can check by taking off the showerscreen and hitting the brew switch. The water gushes out at between about 5 and 7 o'clock.....

Now the shower screen disperses that somewhat.... but I wonder if there is still more water trying to flow through the screen in that area....

A single basket has a small amount of coffee near the edge of the basket... so I wonder if the water is  lifting that..... (basically going under the puck)..... and then the shortest route..... which water will always follow.... is down the side and out the bottom i.e. it creates a channel..... Because you are mounding in the centre and then tamping, you might have more compaction in the centre and less near the edge - making it easier to lift.....

Greg did an experiment some time ago whilst testing convex based tampers... many (including myself) had though that as you pressed down the tamper would touch the centre first and coffee would move sideways as well as down.... well it doesn't!! So if you have a mound in the centre and a flat tamper.... the centre will compress more.... and it will be less compressed towards the edges.... less compression... easier path for water... and channelling.

It's just a thought- and might be way off the mark.... but it's the best I can think of...

As a test you could probably tamp normally then rotate the basket 180 degrees in the PF before locking it in - if it is tamp related the channelling will also move.... if it follows my above theory- channelling will still occur at the same spots...

I've tried to illustrate the effect below (with apologies to Greg for wrecking his great work)

Barri,

Like I said in my opening post, I used to use a single on my Napoletana for years and was satisfied with its results. A double may be more forgiving than a single, but if you're happy with what your single produces then go right ahead and use it, that's all that matters.

JavaB wrote on Jan 27^th, 2007, 2:01am:


Probably anyway. What the test showed is that no movement occurs at the surface of the puck which is where we presumed it happened. The suggestion was made that perhaps the movement occurred within the puck, which is possible; the tests I did didn't investigate that option. However I would think if coffee grounds can't slide against a polished stainless steel tamper, they're unlikely to slide against rough irregular bits of ground coffee.

Greg

Some more interesting information on the La Marzocco Swift grinder....

It tamps as it grinds - but only to 8 lbs!!! They apparently did a lot of research into grinding / tamping and found.....

Quote.... (from CoffeeGeek)
First, ground coffee is a wide range of particle sizes within it's scope of "espresso grind", and those grounds form a kind of interlocking puzzle when compressed. The more pressure exerted on the interlocking pieces, and the tighter the overall fit is.

Second, and most crucial, they believed that in manual hand tamping, no matter how much pressure a person exerts on the top of the bed of ground coffee, by the time one gets down to the bottom third of the bed, almost all that pressure is eaten up, and the bottom portion of the formed puck is barely compacted any more by the tamping pressure. Because the Swift grinder continuously tamps as the coffee is ground and added to the basket, all the pressure comes to bear on the forming puck, a millimeter in height at a time.

Simply put, when you tamp by hand, you're tightening up the pieces of the puzzle at the top of the formed puck quite a bit; less so by the time you get midway down the puck, and very little, if any tightening at all by the time you get to the bottom few millimeters. The swift interlocks the grind puzzle pieces firmly and tightly from the get go, right through the entire formation of the puck. Result? Water hits a brick wall, even if the coffee is ground coarser.

Unquote...

This is very interesting.... the author concludes the most important part of tamping by hand is therefore at or very near the surface of the puck- and that is where the puck seals against the wall of the basket.... one tamp- with a level surface and a tight fitting (dare I say Pullman) tamper produces a good seal at the edge and even compaction across the top of the puck. Tapping (to dislodge loose grounds at the edge) will break this seal and, in all probability, it won't be restored with the next tamp.

Any weak spots - especially near the edge- will be exploited by the water (which seeks the most direct route- and the one with the least resistance - through the puck).

So from Greg's earlier experiment that a convex tamper doesn't displace grounds at the top sideways..... there would be even less tendency for it to happen further down in the body of the puck as the pressure is rapidly reducing.

What does this mean for us?

I guess it confirms the idea that the top surface prior to tamping needs to be flat (certainly with a double basket)..... but a single basket with it's typical stepped bottom profile? My guess is the best result would be achieved when the top of the uncompressed puck was a mirror of the bottom surface..... basically a flat "mound" in the centre of the same diameter as the bottom of the basket.

Regardless of whether this is correct or not, the top surface needs to be equally compressed from edge to edge so there is no weak spot(s) and it forms a solid uniform barrier to the water.

barri wrote on Jan 28^th, 2007, 11:36am:


barri,

Yep, that would be a logical conclusion.

You would have to make sure that it is a light tamp. When I was trying that technique (with a double) I found that with about 5Kg tamp for the first I would get separation of the two halves - sometimes.

Hey Greg,

It is annoying indeed... Happens to me if I "SLIGHTLY" underdose... I make sure I updosed as much as I can on the Singles... it is very annoying!!

Very informative post.


BUMP!

This thread deserves to be pinned!