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Last chance for Oscar!

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  • Last chance for Oscar!

    Hi all

    After a short stay Oscar and his friend Grinta are about to be evicted. Thrown on the street!

    Both arrived from a previous home where he sat unused for a couple of years because they "made crap coffee". Cash exchanged they came home, a home with many years of coffee experience, had a good clean, seals replaced, valves checked and all was good. And thats where the nightmare started!

    After a kilo of beans this thing is wrong but I can't work out why. Basically the closest to good pour and crema characteristics etc etc happens in about 10 seconds. Before anyone jumps in I know its not! By much experimentation using almost pulverised Greek style grounds of 14g I can get a 30g shot in 20 seconds but that is savagely over extracted. Temp is good, heat up is fine its just like this thing is a fire hose blasting through the group. One thing i have recorded is the free flow water rate is almost 150ml in 10 secs - as I remember a test used to be around 75ml in ten seconds.

    So everything points to an excessive flow issue but how and why? Everything is standard and I know of the various mods out there but this is way wrong from the outset.

    Anyone any thoughts?


  • #2
    I looked at buying an Oscar a while ago and read lots about them not having a "proper" OPV due to where its fitted in the hydraulic circuit and the different ways you could mod them to fit one. But what you describe suggests that maybe the non proper OPV is blocked or not functioning correctly at all? If it sat for a few years how much scale was / is in it, the OPV could be stuck closed so you are getting full pump pressure on the puck. Can you beg borrow or steal a portafilter pressure gauge or jury rig a pressure gauge into the brew water circuit to see what your current brew pressure is?


    • #3
      Sounds like symptoms of stale coffee beans. What are the beans you were using? Try to get something fresh roasted (<2 weeks roasted).

      I had the Oscar and it's a good machine capable of decent shots. You probably know already, since Oscar is an HX machine, it requires cooling flush from idle temp to brew temp. If I recall correctly, you flush the group head until the flash boiling is gone, and followed by about 2 sec flush.

      The OPV may not be set correctly, but it doesn't matter. You should still be able to get shot at 9 bar for 60ml/30sec, the Ulka pump is rated to give 9bar pressure at the specific espresso flow rate.

      Excessive water flow rate is not a big issue as opposed to low flow rate. The excessive water is going to be resisted by the coffee puck anyway. So it does not really matter other than affecting your grind requirement(coarser).



      • #4
        Scale was good ausdb - valves were cleaned and inspected but hardly any scale

        Samuel - always fresh beans in this house! As to your other points yes you'd think. I've been to ridiculously fine grinds on two grinders and cannot stop the flow. There's no evidence of channeling but I'm seeing evidence of high pressure and high flow - it theoretically can't work like that.


        • #5
          Cool! Apologize for the misassumption on the coffee beans used.

          In that case, for the next step I would do is to check the dose (not too high). I'd make sure there is enough headspace with the coffee dose - to avoid the grouphead grinding into your well prepared coffee puck. But you said there's no evidence of channeling so maybe you've got that under control.

          Also, from my first hand experience (with pressure profiling Rossa PG), if the coffee is going to choke at 9bar, 15 bar water pressure wouldn't penetrate the coffee any better than at 9 bar. Rest assured it's not the pressure nor flow problem.

          If you can, I would go even finer on the grinder until it is almost choked. Judging grind fineness by eye is almost futile because it's relative (different machine & grinder). For a proper espresso flow, grind from flat burrs is much more powdery than those from conical burrs (more granules). Not sure what was your previous grinder for comparison, but if you're using conical grind as your reference, it will throw you off definitely(grinta is a flat burrs so by default it needs to produce more powdery/talcum grind for proper flow). I used to dial in by touching the coffee grind, and that's what I observed. Also, since NS Oscar is a semi-commercial machine, the grind requirement is usually finer than home machine.


          • #6
            Thanks Samuel - after another day and 1/2 kilo I think I've found whats going on - or at least one thing!

            The extraction starts fine but seems to fall apart about 12 secs in. After spending some time looking at each puck it looks like I'm getting very even side channelling all the way round the basket. The top outer edge of the puck is actually rolling in towards the centre, as though the grounds are pushing up and in on expansion. Never seen it before - now to think how to deal with it!


            • #7
              Hi all

              Need to revisit this as 2 kilo's and numerous baskets, doses, distribution methods tamps grinds etc later I can't get on top of this problem. Basically the pour is perfect for the first 10-12 seconds. Within a second or two of that the pump surges as the pressure is released and the flow hugely increases and blondes up. In making changes I can finely control that first pour from the old fashion 60ml in 30 flow down to droplets down to choking the machine for 10-12 secs. But the end result is always the same. The surface of the puck is as clean and dry as you would want it - no sign of centre channelling.

              The only conclusion I can again draw is dramatic side channelling. I've run out of ideas now - any one any thoughts?



              • #8
                I had an Oscar and saw similar issues initially... Good flow at first and then gushtastic.

                Your diagnosis is right - side channeling. That's what it was for me.

                Why? In the Oscar there is actually no OPV. What some of the other have termed the OPV is actually a HX over pressure safety valve. It is attached to the group head itself, just behind the plastic case. It's purpose is to release excessive pressure in the HX circuit, and is thus set to around 16 bar. It is set to this pressure so that it will not open during a shot, it's not designed to.

                Therefore you are getting the full pressure from the vibe pump - which would be roughly 13-14 bar.

                Illy did a test where pressure and flow of espresso wee tested, and flow rates of espresso declined with pressure both lower and higher than 9 bar. So if your pressure is above 9 bar, you should theoretically see lower flow through the bed of coffee.

                What is happening is that for the first 10 seconds, solubles are pulled out of the puck, making it less solid and more prone to channelling. With such a high flow and high pressure, the water is finding and taking advantage of the weak spots in the puck, something that is far less common when running 9 bar.

                Solution? For me it was to install a proper OPV on the cold side of the brew water line. Not an easy job.

                If you don't want to do that - and I'm assuming you don't - then be very careful and more precise with your distribution and tamping. Give the puck less chance of developing weak spots. I found this helped until I did the OPV mod.

                What I didn't try but think would work, would be Matt Pergers nutating tamp technique. Watch his 2013 WBC routine, and try that tamp. I think it should work - really quite well in fact.

                Let us know how you go.


                • #9
                  Thanks Bames - I read some of your previous posts during my troubles.

                  I've come to a few conclusions during the journey - basically an Oscar/Grinta partnership can't be pushed to far. You can get the old 60ml in thirty secs happening fairly well but if you actually want to go by ratio you open a can of worms. I've found the standard basket is not conducive to doses lower than 16 gram and and lighter roasts are out of the question with this combo. Maybe my patience was reward the arvo with a great pour off 19 grams!

                  Anyway I'll try the Pergers tamp and keep playing but never has the kitchen Sunbeam been so warmly welcomed in the morning. It only makes ok coffee but it does it without fuss or drama day in day out. Big call to swap out for Oscar at this stage.


                  • #10
                    Agree with Bames...

                    If the Oscar OPV (or lack thereof) causes brew pressures to get too high, and make achieving consistently good shots so difficult, I would install a decent OPV in a heartbeat. It will be like night and day to what you are experiencing now, and so much easier to use. In essence, OPVs were originally installed in reciprocating pump machines to protect the pump against what is called "Closed head stalling". However, a much better alternative has been the introduction of adjustable OPVs that not only protect the pump, but also create the opportunity to establish a Maximum Brew Pressure, a la Rotary Pump machines that include a bypass valve within the body of the pump housing to achieve this.

                    Given that decent OPV units are not that expensive, it makes a lot of sense to fit one into the Oscar and thereby turn it into a machine that is not only a pleasure to use but much more capable...



                    • #11
                      Bames gave the nutating a good work out and it makes for significant improvement. The best results I had are with the Easy Tamp, nutating on the spring pressure and bottoming out on the tamp are a couple of revolutions. Still not perfect but pour speed is far more even. Grind can go a notch looser too.

                      Thanks for the suggestion!


                      • #12
                        Yep, the nutating tamp action is a good one; have been using this myself for more than a decade before it was called the "Nutating Tamp".

                        Nice to hear that things seem to be going better for you...



                        • #13
                          Maybe we should rename it the 'Maltating Tamp' ;-o.......
                          after all there's nothing 'nu' about it. ;-)


                          • #14
                            It's amusing though sometimes, when a more technical name for a description of doing something in a particular way, takes on a life of its own and some kind of 'aura'...



                            • #15
                              Hmmmmm.... especially if it's uttered by 'a name'.