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VBM - quality not quite there

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  • #16
    Re: VBM - quality not quite there

    It’s the engineering skeptic in me that questions that wisdom

    The main problem that I have is the spring itself or to put it in another way, the springs resistance value
    The spring can control the speed of release depending on its tensile strength from fast to slow however that said it is a soft release

    If one sets the OPV at 9 bar you are telling the OPV to release at that value
    Looking at it another way? It will release pressure at the most crucial time. The optimum brew setting?

    So the users changes the grind size and tamp strength to compensate and ensure the OPV valve does not trip and release as set pressure is reached continuously

    The question remains what is the actual pressure after user compensation?
    It surmise that it is under 9 bar more likely 7 or 8 bar

    Ideally you don’t want the OPV to trip at all
    If it is set just above at the ideal bar level the actual brew pressure will be aprox 1 to 2 bars below.

    KK

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    • #17
      Re: VBM - quality not quite there

      Except vibe pumps have a running pressure of around 12 - 15 bar (depending on whether it is ulka or fluidotech). I spose the the theory of the OPV is to bleed off (albeit rather crudely) the excess pressure so as to achieve a relative constant pressure down stream of the OPV.

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      • #18
        Re: VBM - quality not quite there

        Depending on machine and pump combo you may have one or two pressure relief valves (OPVs). They are a standard item in industrial fluid or air control systems but there is some variations but they are simple and do a good job of maintaining pressure.

        All they are is a valve stem and an adjustable spring holding the valve closed when the down stream pressure is above the spring tension the valve cracks and some fluid is allowed to flow to waste when the pressure then matches the valve shuts again and this cycle continues as required.

        Providing the pump can maintain adequete flow for both the shot and waste requirements then the set outlet pressure can be maintained at the puck (assuming grind and tamp are correct) and it doesnt matter if the valve is open or shut you will still have pressure X at the showerhead. In an ideal world the back pressure from the puck would keep the valve shut and at 9 bar but you are never going to be able to creat a 9 bar puck with any regularity.

        In addition to this some rotary pumps have a PRV fitted with the same type of valve but instead of dumping to waste they recirculate the waste water internally to maintain a constant pressure downstream. Playing with these is a different matter as it depends on pump inlet pressure to set these.

        With PRV springs over time will develop a different set and require adjustment and in the long term generally need replacing (10+ years in most cases). The valve faces may need reseating prior to that or even the valve stems may wear out and cause the valve to stick.

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        • #19
          Re: VBM - quality not quite there

          maybe im mistaken , but the pump pressure remains constant at about 4-5 bars, and the OPV responds to the resistance created BY the puck. so if an OPV is set at 12 bars, essentially it is being told to keep the resistance pressure from the coffee puck at 12 bars.
          My point is that the 9 bar OPV, is ensuring that the pressure AT the puck (ie the brewing pressure) IS 9 bars.

          the real question is..... if commercial coffee machines are factory set at 9 bars, why are the home/ prosumer machines OPVs set at 12?

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          • #20
            Re: VBM - quality not quite there

            HI Soupy,

            the pump is a constant VOLUME (varies slightly with pressure in the case of these types) and pressure is created by restricting the flow either by using a valve or puck pressure.

            Pump pressure will need to be slightly higher than 9 bar to get to 9 bar at the screen due to other system losses. The OPV or PRV does not create pressure magically at the screen In the case of a plumbed in model inlet pressure from the mains + pump pressure is the total pressure supplied to the system so the pump may only be adding 5 or 6 bar depending on setup.

            Setting the PRV at 12 bar is fine providing you can create a puck that will cause a 9 bar brew pressure as the machine will still brew at 9 bar even with a valve set to 12. As this is not repeatable or consistant the OPV takes care of that variable so even if you grind really fine and tamp heavilly there is still only X pressure across the puck. This is why the valves are best set at 9 bar.

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            • #21
              Re: VBM - quality not quite there

              Taking into account everything that has been explained above, the important criterion is that where so-called "Vibe Pumps" are being used, the OPV pressure is adjusted to operate at between 8.5-9 Bar when there is a flow-rate through the Group of 120ml/Minute. This is a very specific adjustment and takes into account any variables that exist from one machine to another. This does not apply to machines employing Eccentric Vane or so-called Rotary Pumps.

              Adjusting the OPV on its own without taking into account the above flow-rate, is a pointless exercise and the reason that Chris and other coffee specialist professionals employ special tools and aids such as the "Scace Device".

              Cheers,
              Mal.

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              • #22
                Re: VBM - quality not quite there

                Can someone have a look at this photo and tell me whether this is the wrong coloured light bulb behind the light?

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                • #23
                  Re: VBM - quality not quite there

                  The bulb is orange, though Im colour blind.  Doesnt really matter what colour it is though does it?

                  The stainless in your photo looks coffee-coloured.  Id be more concerned about that. :P ;D

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                  • #24
                    Re: VBM - quality not quite there

                    Originally posted by Dennis link=1225344730/20#22 date=1225837929
                    The bulb is orange, though Im colour blind.  Doesnt really matter what colour it is though does it?

                    The stainless in your photo looks coffee-coloured.  Id be more concerned about that.  :P  ;D
                    Looking at your tagline you say you like shiny colourful objects - coffee is a nice colour! ;D

                    PS Im guessing it might be just the normal orange cast that incandescent lighting can throw.

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                    • #25
                      Re: VBM - quality not quite there

                      Looks as though the stainless steel is reflecting the light brown colour of my kitchen cupboards.

                      So do all Domobar Super machines have a green coloured light with an orange coloured bulb? Or was my machine assembled on a late Friday afternoon just before a big Italian football match?

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                      • #26
                        Re: VBM - quality not quite there

                        Originally posted by garbage link=1225344730/20#24 date=1225848672

                        So do all Domobar Super machines have a green coloured light lens with an orange coloured bulb?
                        Yep...its magic!

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                        • #27
                          Re: VBM - quality not quite there

                          You would think a machine in this price range would at least have a lens that matched the colour of the light bulb!

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                          • #28
                            Re: VBM - quality not quite there

                            Originally posted by garbage link=1225344730/20#26 date=1225852513
                            You would think a machine in this price range would at least have a lens that matched the colour of the light bulb!
                            And if it were the same colour then you could complain that its too hard to discern whether its on or off. Price of the machine has absolutely nothing to do with the choice of colours.

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                            • #29
                              Re: VBM - quality not quite there

                              Originally posted by garbage link=1225344730/20#26 date=1225852513
                              You would think a machine in this price range would at least have a lens that matched the colour of the light bulb!
                              If that is the biggest problem youve found with the machine, consider yourself lucky!  : If the coffee is good then thats all that really matters! Bulb colour is rather superficial...

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                              • #30
                                Re: VBM - quality not quite there

                                Originally posted by Dennis link=1225344730/20#27 date=1225852759

                                And if it were the same colour then you could complain that its too hard to discern whether its on or off.  Price of the machine has absolutely nothing to do with the choice of colours.
                                The orange lens/globe on the left hand side of the machine is easily discernible. Surely they could have done the same on the right hand side, but in a green lens and globe. Coupled with all the other points I raised in the original post, its a lack of attention to detail in my books, something that should not be happening on a machine that costs 5-10 times that of an entry level machine.

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