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VBM Domobar Junior brew pressure too high?

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  • VBM Domobar Junior brew pressure too high?

    So I've recently re-calibrated my grinder (Vario), which seems to be working a lot better now as there was no distinct change in the sound of burrs. I've been attempting to properly dial in my coffee and get a good result. I'm using an 18g VST basket, the last dose was 20g and took around 34 seconds to get 40g of coffee extracted. So obviously too long. I found the coffee was very sour though, which means you'd probably go a finer grind, but then the 34 seconds tells me to go coarser because it took too long. So I'm confused!

    While I attempt to figure this out and dial in the espresso properly for a better extraction, I've noticed on my machine that no matter the dose, the brew pressure always gets to 11 bars. I don't know much about brew pressure, but is this too high? If so, what should it be? Am I able to fix this on my own if required? The machine is probably 8 years old, is in really good condition otherwise and I have just added a new seal and shower screen to the grouphead. Could the brew pressure be the culprit here?

  • #2
    11 bar brew pressure is definitely a bit high, if it is 11 bar at the gauge is it prob close to 10 bar at the group. Ideally you want 10 bar at the gauge which should be 10 bar at the group. This can be changed by adjusting (using a flat head screwdriver) the OPV (over pressure valve) usually clockwise to increase pressure and anti-clockwise to decrease. Adjustment needs to be made with machine off and unplugged.

    The other variable is your temperature at group and usually Domobar machines (I have worked on some Piccolos and Super machines) require a pretty good flush of a few hundred mls of water before you run your shot.

    Here is a parts diagram (I think the OPV is item 12 on page 4);
    https://coffeesnobs.com.au/forum/equ...-bodywork-pump

    Comment


    • #3
      What happens if you put 18g in the 18g vst basket?

      Comment


      • #4
        Originally posted by roosterben View Post
        11 bar brew pressure is definitely a bit high, if it is 11 bar at the gauge is it prob close to 10 bar at the group. Ideally you want 10 bar at the gauge which should be 10 bar at the group. This can be changed by adjusting (using a flat head screwdriver) the OPV (over pressure valve) usually clockwise to increase pressure and anti-clockwise to decrease. Adjustment needs to be made with machine off and unplugged.

        The other variable is your temperature at group and usually Domobar machines (I have worked on some Piccolos and Super machines) require a pretty good flush of a few hundred mls of water before you run your shot.

        Here is a parts diagram (I think the OPV is item 12 on page 4);
        https://coffeesnobs.com.au/forum/equ...-bodywork-pump
        I wonder if it's easy to get to myself. I've only ever been comfortable changing the seal and shower screen on the grouphead.

        So 11 bar at the gauge is high - I'm reading online some suggest the brew gauge should be around 9 or 9.5.

        Can 11 brew pressure impact the taste/extraction?

        Comment


        • #5
          I'm not sure about taste, but I've found that if the pressure is too high it becomes less forgiving with dose errors etc as you will get channeling more easily.

          Generally I aim for 10bar on the gauge which will give you about 9bar at the group. It's common to assume 1 bar drop.

          I have a BZ10 and for a while it was set on 11 bar at the gauge. Had no issues with taste or extraction once everything was dialled in; however, once I dropped it down to 10bar it was more forgiving with dosing errors.

          Comment


          • #6
            Ideally you want to try and adjust the Brew Pressure such that you get between 8.5-9.0Bar at the coffee puck. This will about 9.0-10.0Bar on the pressure gauge...

            Mal.

            Comment


            • #7
              The sweet spot on my old Domobar Super is 9.5 bar at the gauge. Pretty sure that equated to 9 bar at the puck, last time I checked with my portafilter gauge. Any higher, the coffee has a real strong harsh taste. Any lower, it lacks punch & crema.
              The adjustment is very simple. Turn the brass adjustment screw slightly anti-clockwise to reduce pressure. I always do it with the machine running & blind filter installed. I can't say this is the correct way but it works for me. If you're not comfortable working with the machine dismantled, unplug the power, adjust maybe a quarter of a turn & retest. Note, if the valve starts to leak slightly as you back it off, don't panic, it just needs a new o-ring.

              Evan.

              Comment


              • #8
                Originally posted by ev View Post
                The sweet spot on my old Domobar Super is 9.5 bar at the gauge. Pretty sure that equated to 9 bar at the puck, last time I checked with my portafilter gauge. Any higher, the coffee has a real strong harsh taste. Any lower, it lacks punch & crema.
                The adjustment is very simple. Turn the brass adjustment screw slightly anti-clockwise to reduce pressure. I always do it with the machine running & blind filter installed. I can't say this is the correct way but it works for me. If you're not comfortable working with the machine dismantled, unplug the power, adjust maybe a quarter of a turn & retest. Note, if the valve starts to leak slightly as you back it off, don't panic, it just needs a new o-ring.

                Evan.
                Do you know of any guides to take it apart and get to the OPV?

                Comment


                • ev
                  ev commented
                  Editing a comment
                  I don't but I'm sure they'll exist. You really can't miss it. Big chunk of brass at the bottom of the machine that accepts a large flat blade screw driver. Try Googling Domobar Bypass. Plenty of photos online. I don't believe that OPV is the correct name though. I always thought that was the Over Pressure Valve which sits on top of the boiler.

              • #9
                Did a Domobar Junior today, with a brand new brew gauge installed, adjusting the OPV to get 9 bar on my scace indicated 9 bar on the machine gauge. They are very close.

                I've also noticed recently that some Domobar Juniors have been running extremely cold at the group which gives a distinct sour taste.

                In the last few weeks alone I've done three of them, all purchased around 2014-2015 and no matter how long I let them heat up, I could only get 75-80C from the group under extraction.

                They have an approx. 1.8mm restrictor in the HX circuit and with the factory set 1.1-1.2 bar boiler pressure, the boiler is only sitting at about 110-112C (normal HX boiler temp with an E61 is around 120C).
                I've had to up the boiler pressure to an indicated 1.5 bar to get 120C at the boiler and drill the restrictor out to 5mm. Once done, the machine is stable with extraction temps tight around 92.5-94C. No cooling flush required either.

                My customers have all bought these machine from box store sellers which I assume is the problem. The other problem is that all three had previously been in for service elsewhere.

                If you have a Domobar that was purchased from a box seller, I'd highly recommend getting it taken to a reputable service shop for an assessment as you could be missing out what the machine can actually offer.

                Comment


                • #10
                  Don't know if this useable on a VBM DbJr but here's a link to a Super Manual...
                  https://coffeesnobs.com.au/forum/equ...-user-s-manual

                  Mal.

                  Comment


                  • #11
                    Originally posted by noidle22 View Post
                    Did a Domobar Junior today, with a brand new brew gauge installed, adjusting the OPV to get 9 bar on my scace indicated 9 bar on the machine gauge. They are very close.

                    I've also noticed recently that some Domobar Juniors have been running extremely cold at the group which gives a distinct sour taste.

                    In the last few weeks alone I've done three of them, all purchased around 2014-2015 and no matter how long I let them heat up, I could only get 75-80C from the group under extraction.

                    They have an approx. 1.8mm restrictor in the HX circuit and with the factory set 1.1-1.2 bar boiler pressure, the boiler is only sitting at about 110-112C (normal HX boiler temp with an E61 is around 120C).
                    I've had to up the boiler pressure to an indicated 1.5 bar to get 120C at the boiler and drill the restrictor out to 5mm. Once done, the machine is stable with extraction temps tight around 92.5-94C. No cooling flush required either.

                    My customers have all bought these machine from box store sellers which I assume is the problem. The other problem is that all three had previously been in for service elsewhere.

                    If you have a Domobar that was purchased from a box seller, I'd highly recommend getting it taken to a reputable service shop for an assessment as you could be missing out what the machine can actually offer.
                    How can I measure the temperature under extraction?

                    Comment


                    • roosterben
                      roosterben commented
                      Editing a comment
                      You can source a multi-meter with a temp probe some normal multi meters like Flukes etc have an input for a heat probe (usually a K/T type thermo couple). For a rough temp guide you could use a milk jug thermometer but can't really rely on these as they are quite laggy. I found a cheap multi-meter on eBay with a thermo couple which cost under $20. I have compared it to my Scace and they read within .1/.2 of a degree so plenty accurate. Somewhere like Jaycar stock a few different multi-meters and I have seen models with heat probes in store before.

                      If they have a long flexible heat probe you can use a blind filter (or a basket with coffee in it) and just run the probe over the edge of the basket and lock in the portafilter. This is going to bend the probe but if you are careful and use the same positioning it shouldn't get damaged.

                  • #12
                    Originally posted by noidle22 View Post
                    I've also noticed recently that some Domobar Juniors have been running extremely cold at the group which gives a distinct sour taste.
                    Yes. Me too. Especially if scale has been a factor. Tested plenty at circa 85 deg at the group with a Scace. Am finding the 3.2-3.5mm seems to be the sweet spot for the ones I have seen.

                    Comment


                    • #13
                      I have the same issue (bar pressure about 11), I've had mine for about 9 years (got it second hand from Jet Black) and it's always been at that pressure. I tried to twist the OPV but can't seem to make it budge. I tried a flat head screwdriver, 10c coin. Nothing seemed to work.

                      I read somewhere else that you needed to apply a heat gun to it in order to change it?

                      Anyone been able to adjust the pressure on a Domobar Jnr HX?

                      Comment


                      • jchong
                        jchong commented
                        Editing a comment
                        Thanks ev. If cooking oil doesn't do it, any other thoughts on how to get it to loosen?

                      • ev
                        ev commented
                        Editing a comment
                        No idea, I've never had a problem adjusting/dismantling mine.

                      • jchong
                        jchong commented
                        Editing a comment
                        No worries. Got it moving with a bit more elbow grease. Thanks for the help.

                    • #14
                      So I took my Domobar Junior outer case off and was able to immediately see the OPV. I adjusted it and now the brew pressure gauge is around 9.5 when I'm extracting.

                      It seems to have a less sour taste to it too as I didn't make any real changes with my dialing in process. I don't understand how such a small change can make such a significant difference.

                      I guess the last step will be to simply check if the grouphead is running a bit cold but that seems like a little bit more of a complex process.

                      Comment


                      • #15
                        In terms of measuring temperature, is this the kind of multimeter which could work? https://www.jaycar.com.au/cat-iii-mu...ature/p/QM1323

                        How do I actually use this?

                        Also, I know it's not a good solution, but how can I use my milk jug thermometer in the mean time?

                        Comment


                        • Caffeinator
                          Caffeinator commented
                          Editing a comment
                          Nope- At a bare minimum you need a thermocouple over the puck and that's less than ideal.

                          What you really need is a Scace device and they're the best part of $1.3k. It's why you pay when you have a tech do a Scace calibration.

                          It takes time and the equipment costs.
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