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9Bar OPV operation in Lelit PL60

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  • MrJack
    replied
    As good a summary as any.

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  • DrJones
    replied
    Originally posted by MrJack View Post
    I hope that made some sense?
    Probably for a person with way more insight than myself .

    If I read you correctly, adjusting the OPV is the right (/only) way to go if I'm shooting for 9 bars. And only real down-side in doing this (according to Talk_Coffe) is that 9 bars may actually not be the holy grail...

    Right?

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  • MrJack
    replied
    Originally posted by Talk_Coffee View Post
    I'll just throw a cat amongst the pigeons.
    I've often wondered if the whole 9 barg thing might just have been a historical accident.

    Originally posted by DrJones View Post
    Let me be more precise:
    What I would like to achieve is 'optimal' parameters: App. 9 bars and 25 seconds. This seems impossible with the OPV set to 11/12 bars (the only way I can come down to 9 bars is by grinding so coarse that the water spits through like Niagara Falls).
    If you do that, you're OPV isn't really doing anything, because there isn't enough resistance for the backpressure at the OPV to be sufficient to overcome the spring force (i.e. for the valve to open). Your flowrate is therefore equal to the pump output at that discharge pressure. Due to the way a pump works (google 'pump curve'), changing the puck properties therefore alters both the pressure and flow through the group.

    In comparison, with the OPV open, the flowrate through the group should be whatever flowrate results in 9 bar pressure drop (or whatever your setpoint is) between the OPV and the basket. The OPV basically stabilises the pressure, such that changes in the puck properties really only alter the flowrate to the group.

    I hope that made some sense?
    Last edited by MrJack; 13 December 2013, 08:29 PM.

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  • TC
    replied
    I'll just throw a cat amongst the pigeons.

    Thanks to Rick Bond (aka Coffee Machinist), who made a Scace device to suit the San Marco lever group, we did some Scace testing of our Pompeii to see what it does.

    Result? The overwhelming majority of the extraction happens at 12Bar....This machine consistently produces some of the best shots I have ever had (and I've had a few )

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  • DrJones
    replied
    Let me be more precise:
    What I would like to achieve is 'optimal' parameters: App. 9 bars and 25 seconds. This seems impossible with the OPV set to 11/12 bars (the only way I can come down to 9 bars is by grinding so coarse that the water spits through like Niagara Falls).

    So (as I don't exactly understand your comment about dP) the question is: Are there - in regards to brew quality - any down sides in just reducing the OPV setting to 9 bars? This would probably solve my problem with 'the optimal numbers'.

    (Let's for a minute disregard the obvious security risks in fiddling with a pressured/electrical device and assume that the adjustment process is made by a professional).

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  • MrJack
    replied
    Of course. Achieving it might be difficult though. Why do you need to fix the grind dose and tamp?

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  • DrJones
    replied
    Originally posted by MrJack View Post
    lowering the setpoint will reduce flow through the group/puck, such that dP is equal to the available dP.
    I'm not sure what this means... I'm trying to achieve a longer brew time on the same amount/grind/tamp. Reduced flow should give me that, right? Any down side?

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  • MrJack
    replied
    Correct. Thanks for picking that up Barry! Auto-complete strikes again!? That statement should probably have read:
    [QUOTE ] Now, as the maximum flow a pump can provide is inversely proportional to the pressure (...), setting a lower pressure will reduce the total flow out of the puck.[/quote]

    Not the most clarifying statement, in hindsight!

    What is missing is that when you lower the OPV setpoint, you lower the 'available' pressure drop. Since pressure drop (dP) is proportional to flowrate and flow resistance, and as flow resistance is constant, lowering the setpoint will reduce flow through the group/puck, such that dP is equal to the available dP.

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  • Barry_Duncan
    replied
    Originally posted by MrJack View Post

    Now, as the maximum flow a pump can provide is inversely proportional to the flowrate (in vibration and rotary pumps anyway), setting a lower pressure will reduce the total flow out of the pump.
    Wrong. Setting a lower pressure reduces the flow rate through the puck. However it increases the total flow from the pump. The excess flow goes through the OPV back to the tank.

    Barry

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  • JetBlack_Espresso
    replied
    Last time I checked, they were the same hardware with a different calibration,

    charlie

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  • DrJones
    replied
    Originally posted by JetBlack_Espresso View Post
    (...) our advice is to get a 9 BAR OPV, and take it to your local tech to get it installed (..)
    Thanks Charlie (and thanks for your replies to my emails as well :-). Good service from JetBlack!

    I would think that the chance of getting it dangerously wrong is relatively low if I only adjust the screw outwards, right? But I wouldn't go ahead with the adjustment, if fx. the OPV is not meant for operation in the 9-bar range.

    Do you know if there is any constructional difference between the two OPV's (like a shorter spring on the 9 bar version), or are they just the same hardware that is calibrated differently from the producer?

    Leave a comment:


  • JetBlack_Espresso
    replied
    Hi DrJones,

    The Lelit models distributed by Jetblack including the PL60T V2 (and PL41TEMD, PL42EMI etc) are all spec'd with 9BAR OPVs. The one in yours will be set at 10.5BAR. It is possible to adjust these but it is a bit fiddly. The main reason we don't recommend it as a home "tweak" is that if you get it wrong and block the OPV, you have effectively disabled the safety pressure relief from the boiler. If you really want it done, our advice is to get a 9 BAR OPV, and take it to your local tech to get it installed,

    charlie
    Last edited by JetBlack_Espresso; 6 December 2013, 01:12 PM.

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  • DrJones
    replied
    I've had some OPV trouble with my PL60 Plus T as well. Mine goes to 12 bars on the gauge when pulling an ordinary espresso:

    Espresso at 12 bar pressure - YouTube

    Don't you guys think I need to adjust the OPV? And is this possible on Lelits (I've heard that the OPV is locktite'd shut)?

    Any experiences?

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  • MrJack
    replied
    The PDF may have a slight error.

    My understanding of the fluid dynamics are as follows (although I am more accustomed to working with compressible flows, so should be taken with a grain of salt):

    With the exception of certain pump types, the discharge pressure of a pump is determined by the pressure drop downstream (accounting for static head, frictional losses and velocity head). The pressure drop is proportional to the flow velocity (and thus flowrate) - the greater the flowrate the greater the pressure drop.

    What the OPV does is provides an alternative path for the water to flow ( as opposed to via the puck), which opens at whatever pressure will overcome the spring force.
    This limits the backpressure on the pump to whatever the "set pressure" is.

    The ratio of flow between the two branches will be determined by, and in proportion to, the relative resistance to flow along the two branches.

    So, when the pump starts, the flow will all pass through the line to the group, until the pressure reaches the set pressure. Then the opv will open and some flow will begin to exit back to the tank.

    Now, as the maximum flow a pump can provide is inversely proportional to the flowrate (in vibration and rotary pumps anyway), setting a lower pressure will reduce the total flow out of the pump.

    Reducing or increasing the resistance to flow through the puck will alter the ratio between the two branches, increasing and reducing the flowrate through the puck respectively.

    Clear as mud?

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  • Robbks
    replied
    Yeah, that gives a general overview on how they work.

    I'll sit down tonight and go through the physics of it all with regards to pressures, etc and how they would differ between a system with OPV and one without

    Leave a comment:

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