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  • The problem with single baskets

    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 werent 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 theres a demand for single baskets, theyve 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 whats 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. Its 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 Im 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

  • #2
    Re: The problem with single baskets

    nice graphics greg!

    Comment


    • #3
      Re: The problem with single baskets

      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! :-[

      Comment


      • #4
        Re: The problem with single baskets

        I think you do have to modify your tamping technique when using singles. To get an even pressure right throughout both sections of a single you may to have tamp twice. A light tamp at the top of the 35mm section then dose, collapse, compress, firm tamp and polish. This works for me but I understand why its easier with a double and why their recommended. I do tamp differently when I use a double.

        I also think the machine makes a difference due to showerscreen height as Greg mentioned with the silvia. The 6910 can make great singles provided you adjust your tamp which I know over complicates the issue but works. Maybe the "cheaper" domestic machines are more forgiving with single baskets.

        I still think that single baskets have their place in the market and I would not rule them out.

        Comment


        • #5
          Re: The problem with single baskets

          Originally posted by JavaB link=1169781510/0#2 date=1169783655
          Greg,

          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....
          Agreed. Drawings adjusted accordingly.

          Comment


          • #6
            Re: The problem with single baskets

            Originally posted by barri link=1169781510/0#3 date=1169786071
            I think you do have to modify your tamping technique when using singles. To get an even pressure right throughout both sections of a single you may to have tamp twice. A light tamp at the top of the 35mm section then dose, collapse, compress, firm tamp and polish.
            To do it properly youd need a 35mm tamper and a 58mm tamper otherwise you wont be able to exert enough pressure on the middle bit uninhibited by the outer edges. But that would also create a non-monolithic puck which isnt ideal.

            Originally posted by barri link=1169781510/0#3 date=1169786071
            This works for me but I understand why its easier with a double and why their recommended. I do tamp differently when I use a double.
            Thats the main thing. As I said in http://coffeesnobs.com.au/YaBB.pl?num=1169693192/26#26 recently, the main thing is that people know the limitations of techniques or equipment and what the next steps are if they want / need to go there.

            Comment


            • #7
              Re: The problem with single baskets

              Greg,

              Yep - thats what I meant...

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

              Comment


              • #8
                Re: The problem with single baskets

                Well done Greg. That would be exactly it. The base profile of the tamper should as close as possible match the bottom of basket profile, and, of course, the single basket has a peculiar stepped shape.

                Even the double may be cause for some concern. A tamper has a 90º edge, whereas, for srength, the basket is curved. That means some uneven compression of the puck in that vicinity.

                --Robusto

                Comment


                • #9
                  Re: The problem with single baskets

                  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 dont reckon a 0.5mm or so of curve is enough to be concerned about.

                  Greg

                  Comment


                  • #10
                    Re: The problem with single baskets

                    I use a singlebasket. Im a chef, so I hate wasting food, and I mostly make coffee for one person. Ive been working on having more coffee in the centre of the basket before I tamp, and this seem to do the job. My routine is: half fill, collapse with three taps, fill with a mound of coffee in the centre of the basket, tamp once and polish with only the weight of the (Pullman) tamper.

                    Id be interested in peoples thoughts on this.

                    Comment


                    • #11
                      Re: The problem with single baskets

                      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 dont use the single as the coffee produced is just OK (by my standards)

                      Comment


                      • #12
                        Re: The problem with single baskets

                        Interesting JavaB. How deep is the 45 degree section of the Cimbali baskets? If its a 58-35mm difference, my trigonometry would tell me its about an 18mm depth which would seem the majority of the basket depth. But your reaction suggests otherwise. Unless its a flatter angle than 45 degrees?

                        If its more like a double basket profile but half the depth, I would expect youd need to grind finer than you would for the double otherwise youd get the same results as you would by only half-filling a double, i.e. a gusher.

                        Comment


                        • #13
                          Re: The problem with single baskets

                          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

                          Comment


                          • #14
                            Re: The problem with single baskets

                            Sounds about right. As I suggested in the opening post, any single basket is going to be a compromise; if you keep the grind the same youll have to mess up the profile; if you tend towards a double profile, youll have to take the grind down to compensate for the reduced depth.

                            Or just use a double basket

                            Comment


                            • #15
                              Re: The problem with single baskets

                              Originally posted by Viviane link=1169781510/0#9 date=1169790202
                              I use a singlebasket. Im a chef, so I hate wasting food, and I mostly make coffee for one person. Ive been working on having more coffee in the centre of the basket before I tamp, and this seem to do the job. My routine is: half fill, collapse with three taps, fill with a mound of coffee in the centre of the basket, tamp once and polish with only the weight of the (Pullman) tamper.

                              Id be interested in peoples thoughts on this.
                              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?

                              Comment

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