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9 Bar brew pressure, OPVs and the puck...

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  • 9 Bar brew pressure, OPVs and the puck...

    I think I got to get out more often... I feel like Im slowly morphing into a geek of sorts. I just cant stop tinkering with stuff, or thinking of tinkering with stuff... The latest thought project revolves around trying to achieve the ideal brew pressure of 9 bars and OPV mods come to mind.
    Ive been doing some thinking though, now, the law of thermodynamics would indicate that liquid under pressure will travel in the path of least resistance, which is what the OPV would provide so that the maximum pressure a vib pump can pump out is capped at 9 bars. However, the path of the water we are trying to push through would be the coffee in the basket tamped. So essentially, the puck is what provides the resistance (and therefore pressure). With an OPV set at 9 bars, if the coffee is ground too fine and packed so that it requires more than 9 bars to get water through it, nothing would come out at the head and the water would escape through the OPV instead back into the tank, something we call "choking". Similarly, if the puck was too coarse, or underdosed, it would produce coffee but a gauge would show that the brew pressure would not have gotten up to 9 bars.
    I suppose what Im getting at is, if the grind is set properly and tamped consistently, the 9 bars of pressure would be "provided" by the puck rather than the OPV... So even if an OPV was at a higher setting of 12 bars, you would still get 9 bar brew pressure at the extraction.
    I suppose the value of the OPV set to 9 bars is that it helps you determine where the limit is (dosing, tamping pressure) before it chokes but I wouldnt see how if the dose and tamp is kept the same, the espresso made from OPV of 9 bars would taste different to the OPV of 12 bars seeing as it would have provided the same resistance.
    Is that logical?

  • #2
    Re: 9 Bar brew pressure, OPVs and the puck...

    Yep that sounds logical,

    You need to get out more often, and BTW bean bay attendances dont count!

    You do assume black and white operation of the system though, when puck pressure is close to but just higher than the OPV, there will be some interplay" between the puck flow and the OPV bleed.

    So an OPV setting close to optimun brew pressure [9 Bar] will be more forgiving of the grind/dose/tamp variants and allow you the odd ristretto.

    I dont get out much these days either.......
    I recall reading of some experiments [was either HB or CG] where triac dimmer circuits were quite effectivly used to control the power to and hence the pressure generated by the vibe pump.
    I think Jim from PID kits was involved...... you may want to a bit of searching on this.


    • #3
      Re: 9 Bar brew pressure, OPVs and the puck...

      The original purpose of the fitment of an OPV or PLV(Pressure Limiting Valve) to espresso machines with Vibe Pumps, was not to control the pressure at which espresso shots are poured but to protect the pump and the high pressure circuit from being subjected to excessive pressures.

      Excessive pressure will cause a Vibe Pump to stall and eventually cause damage to its internals, plus the possibility of burning out the Pumps Armature Coil. Machines that use an Eccentric Vane Pump, or a Rotary Pump (as it is more popularly known within the coffee industry) dont suffer from the same problems as the Pump Housing is designed to include a Bypass Valve that allows liquid from the High Pressure side of the Pump to recirculate back to the Low Pressure side when ever the Bypass Valve pressure setting is exceeded. These are essentially self controlling.

      As you can probably appreciate though, if this situation was allowed to continue unabated for an extended time, the temperature of the water that is being continually recirculated like this will slowly but surely continue to rise (due to friction losses, etc) as high as boiling point (and even beyond) until the pump seals fail and the pressure can be released. So, even in the case of this pump design, there are limitations.

      Anyway, hopefully Ive explained the gist of why the OPV or PLV exists.....