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  • Giotto Evo expansion valve during shot

    I have a Giotto Evo.
    Machine is about 15 months old / has seen normal domestic use / has been cleaned regularly

    Past couple of weeks I have noticed the drip tray needing emptying more frequently. Subsequently noticed that water is flowing out of the expansion / OPV valve outlet (not the 3 way valve) into the drip tray
    when I’m pulling a shot.
    I know that one of the things the expansion valve does is help to bleed off a bit of water as the boiler comes up to pressure, but should not really drip too much besides that

    From my limited understanding I’m guessing it could be something like:
    1 – OPV (if it is even adjustable for ones attached to a rotary pump system?) is set too low or the setting has moved somehow
    2 – OPV / expansion valve has gunk / scale in it that is causing it to open under pressure
    3 – brew pressure (ie the setting on the rotary pump itself) is set to ~12 bar and is hence forcing the OPV to open when pulling a shot

    Am I on the right track with possible causes? Any advice appreciated
    NB - coffee still tastes good

  • #2
    Originally posted by chonski View Post
    I have a Giotto Evo.
    Machine is about 15 months old / has seen normal domestic use / has been cleaned regularly

    Past couple of weeks I have noticed the drip tray needing emptying more frequently. Subsequently noticed that water is flowing out of the expansion / OPV valve outlet (not the 3 way valve) into the drip tray
    when I’m pulling a shot.
    I know that one of the things the expansion valve does is help to bleed off a bit of water as the boiler comes up to pressure, but should not really drip too much besides that

    From my limited understanding I’m guessing it could be something like:
    1 – OPV (if it is even adjustable for ones attached to a rotary pump system?) is set too low or the setting has moved somehow
    2 – OPV / expansion valve has gunk / scale in it that is causing it to open under pressure
    3 – brew pressure (ie the setting on the rotary pump itself) is set to ~12 bar and is hence forcing the OPV to open when pulling a shot

    Am I on the right track with possible causes? Any advice appreciated
    NB - coffee still tastes good

    # 3 is probably the issue. Try setting the pump back down to 9 - 9 1/2 bar (with blind filter in place). It should not be at 12 bar - which is probably forcing the expansion valve to open and act as an OPV. They should be set ex-factory for this pressure (about 9 bar - in the "green zone" on the gauge), but if plumbed, line pressure will come into play - and you may need a pressure reducer in line if you don't already have one (especially if supply mains pressure varies).
    My guess is this will solve the problem.

    Comment


    • #3
      A system pressure of 9-9.5 is always too low according to our Scace. I'd recommend 10-10.5 which will give you 9 bar at the group.

      If the system pressure has always been high, the bloke who sold it to you should be doing this. It's part of a routine bench test and theres no excuse for lack of attention.

      Comment


      • #4
        Originally posted by Talk_Coffee View Post
        A system pressure of 9-9.5 is always too low according to our Scace. I'd recommend 10-10.5 which will give you 9 bar at the group.

        If the system pressure has always been high, the bloke who sold it to you should be doing this. It's part of a routine bench test and theres no excuse for lack of attention.
        Very slightly OT, but should a Giotto PP show 9 bar when either brewing or back flushing with a blind filter? The gauge on my PP only just creeps past 8, I'd say about 8.1-8.2. Is that too low?

        Also, am I right to assume that the pressure gauge on the PP won't be hugely accurate, it's not like they're controlling a nuclear reactor. What kind of tolerances do you normally expect from such gauges?

        Cheers
        Jonathon

        Comment


        • #5
          Too low Jonathon,

          See above comments.

          We received a batch like that. All of them required adjustment. ECA was made aware of this and have rectified the issue.

          Cheers

          Chris

          Comment


          • #6
            Why use 9 bar specifically as a reference point for setting pump or OPV pressure with a scace?
            Not wanting to open a can of worms, but there are quite a few comments (in other forums) that the "best results" (subjective of course) set blind, then checked with Scace 2 show pressures below 9 bar.

            Comment


            • #7
              Originally posted by Talk_Coffee View Post
              Too low Jonathon,

              See above comments.

              We received a batch like that. All of them required adjustment. ECA was made aware of this and have rectified the issue.

              Cheers

              Chris
              Thanks Chris, I'll speak to the shop I bought it from. I bought it in December 2012, PP v2, same batch do you think?

              Does it take long to fix?, Is it a quick adjustment or will I be on the Aeropress for a couple of weeks?

              Cheers
              Jonathon

              Comment


              • #8
                Should take about 2 minutes Jonathon :-)

                Comment


                • #9
                  Thanks for the replies...
                  Machine was bought from a sponsor and scaced for me before delivery.
                  The fact that I only started noticing it a couple of weeks ago led me to think that it may not be the brew pressure - unless it has somehow moved from its original setting

                  I'll be talking to them anyway...just wanted to see if I was on track with determining possible causes...

                  Comment


                  • #10
                    Originally posted by Jonathon View Post
                    Also, am I right to assume that the pressure gauge on the PP won't be hugely accurate, it's not like they're controlling a nuclear reactor. What kind of tolerances do you normally expect from such gauges?
                    I'll leave it for more knowledgeable people to confirm 100%, but my understanding of the brew pressure gauges are that the reading is not taken directly from the group head.
                    Instead it is taken from somewhere back down the line of the plumbing and hence there will be *some* pressure loss / difference between the gauge location and the group head?

                    Comment


                    • #11
                      Originally posted by Talk_Coffee View Post
                      Should take about 2 minutes Jonathon :-)
                      Thanks Chris

                      Comment


                      • #12
                        Originally posted by chonski View Post
                        I'll leave it for more knowledgeable people to confirm 100%, but my understanding of the brew pressure gauges are that the reading is not taken directly from the group head.
                        Instead it is taken from somewhere back down the line of the plumbing and hence there will be *some* pressure loss / difference between the gauge location and the group head?

                        Yes. It's complicated reading a dynamic system. A Scase pressure gauge useful to standardise actual group pressure characteristics between different machines.
                        It's usefulness to "calibrate" any particular machine to some theoretical standard is compromised by the fact that assumptions have to be made. One is that flow rate through the scase is typical of actual "optimum" performance through the puck during extraction, and there's a big problem in that if you assume that 25 or 30 seconds is ideal extraction time, then to achieve that with say 8.5 or 9 bar group pressure, then you'll need a different grind to achieve that for a 16 gram dose and a 22 gram dose - and that's going to have much more effect on flavour than the "chemical" difference in extraction directly attributable to different pressure. Another is that pressure at the group vs gauge will change according to how scaled up the machine is over time.
                        Some science can be useful in order to make an art less "dark" - but beware false prophets.

                        Comment


                        • #13
                          And you can wax lyrical as much as you like. Bottom line is that regardless of chosen contraption, the overwhelming majority of machines showing 9 bar at the gauge will be delivering well under that at the puck. Period.

                          Comment


                          • #14
                            Originally posted by Talk_Coffee View Post
                            And you can wax lyrical as much as you like. Bottom line is that regardless of chosen contraption, the overwhelming majority of machines showing 9 bar at the gauge will be delivering well under that at the puck. Period.
                            The OP's machine was apparently delivering at 12 bar at the gauge, probably about 11 bar at the group, and in his words:
                            "NB - coffee still tastes good"

                            Comment

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