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Thread: Rocket Brew pressure guage readings ..can you explain ?

  1. #1
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    Rocket Brew pressure guage readings ..can you explain ?

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Rocket Giotto PP V2 with vibe pump.. lift the brew lever with no PF inserted and the group flushes with the brew gauge reading zero ( as you might expect with no puck to create pressure )
    Rocket R58 with Rotary pump..repeat as above , no PF, group flushes, but the brew gauge reads 9-10 bar ? ..why ??
    I suspect this may be the same on the Evoluzione V2 with rotary pump also ?

    How would you know if your grind or dose was way off if the brew gauge reads 9 bar even with no PF ??

  2. #2
    Coffee+carbon=heaven Mono's Avatar
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    Hi B52, I'm sure I have read somewhere about this oddity just can't quite remember. I currently have a vibe pump VBM [so I will have a look this evening] and am expecting an Evo very soon [and I will have a check of that]. Will report back if it is the same as what you see. The more I think about the more I'm convinced it was explained in a [much] earlier thread :-(

    Steve

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    Member sando's Avatar
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    Hi B52

    I have the Evo-Rotary pump same observations...sometimes (+- 50% and only when idle for a few hours).

    My take on it is as follows.

    When the unit is on and idle for a few hours (with or without Pf) there can be/is a build up of pressure within the system plumbing on the brew side.
    With the brew lever down and the water in the system expanding the pressure should rise??

    My logic says that this is good and is an indication that the valve/system is shut off and not leaking. (could be wrong).

    Solution: By lifting the brew lever just enough to open the valve but not activate the pump....pressure is released, gauge drops to zero, and a few drops water emerge.

    As you need to flush the HX before actually pulling a shot, no problem really.

    If anyone has a technical explanation it would be good to hear.

    cheers

  4. #4
    Senior Member specialpants's Avatar
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    Same thing with Duetto - pump stays at a consistent pressure. The main thing is that it will never exceed 9bars at the puck. Excess pressure bleeds back elsewhere.

    You can just go by what's coming out. eg colour, thickness, extraction volume/rates etc.

  5. #5
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    The short answer is, the rotary pump can deliver a much higher flow-rate than the vibe pump and so the gicleur causes a pressure buildup in the brew circuit.

    A longer answer - before the brew water gets to the puck it goes through a small orifice that restricts the flow (called the "gicleur"). Typically the pressure gauge is before the gicleur and so it measures the pump pressure, not the pressure at the puck. If you run a vibe pump machine with an empty group, it can produce a maximum of about 600 cc/min and you don't see much back pressure from the gicleur. The rotary pump on the other hand produces about 1200 cc/min under no load and the restriction effect of the gicleur becomes significant and the pressure quickly builds to the bypass pressure set on the pump. Hope this makes sense.

    Pete

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    Oops - missed part B of your questions. My answer - it would taste bad.

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    Pete39, .. i follow your thinking, but..
    1) if 600cc/min gives zero back pressure, then double that flow is unlikely to ramp up the BP to 9 bar ( or more if the PV is set to 9 bar)
    2) why would they select a pump with such a high flow capacity compare to the vibe pump ?

    If its just to show max pump pressure, there must be another restriction between the pump and the puck for the gauge to read pump pressure when flushing ?
    ......why do Rocket only configure the gauge that way on the Rotary pumped versions ??

    Note: .. i have seen rotary machine's where the gauge gives an "true reading" of the pressure at the puck.

  8. #8
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    Hi B52,

    On point 1, I'm too lazy to do the physics, but that's what happens.

    On point 2, I think the rotary pumps are designed to accommodate two and three group machines.

    I think some machines have the pressure gauge on the group side of the restrictor and so read brew pressure (I think the GS3s are like this but don't have one!). I think most are not like this because you get troubles with fines getting into the pressure gauge line.

    Pete

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    You are probably correct, but i suspect they must change the size of the gicleur between the vibe and rotary set ups, otherwise the flow rate at the group would be hugely different during pre-infusion.
    They could have used a 1/2 speed motor to reduce the flow for the domestic machines....or a flow restrictor between the pump and the "T" for the gauge. ?
    And it seems a bit pointless to have a gauge that simply reads constant pressure at the pump .... I find it useful to watch the pre-infusion pressure build and time taken..it lets me know my grind & dose is still correct.



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