La Marzocco Linea Micra owner reviews - Page 5

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#41: Post by PaulTheRoaster »

Shakespeare wrote:Does this adjustable bypass valve send excess water to the Micra water tank that is controlled by making adjustments in the valve?.
Therefore it is similar to an OPV valve except this bypass valve is connected to the pump motor output ?
No. There is no return line to the tank. The bypass valve sends water back to the input of the pump internally. The OPV opens at 12 bar and sends water to the drain.

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#42: Post by homeburrero »

PaulTheRoaster wrote:The bypass valve sends water back to the input of the pump internally. The OPV opens at 12 bar and sends water to the drain.
Yes. And they have different intended purposes.

The bypass valve on a rotary pump limits the pressure produced by the pump. It is used to adjust brew pressure.

The OPV, also called an expansion valve, limits the pressure inside the brew circuit even when the pump is not running. This is needed because the brew circuit has no air, and when water is heated it expands. Water is not compressible and that expansion could cause a dangerously high pressure if not for this expansion valve. It's normally factory set to around 12 bar on a rotary pump machine and not adjusted by users.

On a vibe pump machine you don't have a bypass on the pump, so you often do use the OPV / expansion valve to limit the pump pressure as well as the expansion pressure.
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#43: Post by JRising »

To be more precise, all OPVs are OPVs. The bypass in the rotary pump is an OPV, it just relieves pressure from the outlet to the inlet to "bypass" the rest of the circuit. An expansion valve is an OPV in a system placed in the spots in circuits where the directional control valves can close off all other exits, thus providing an exit for the pressure created by thermal expansion. And, of course, some machines have only the one OPV, so long as it's placed to function as the expansion valve it will do so.
Using a P-stat in the brew circuit is different.

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#44: Post by Shakespeare replying to JRising »

I have read some excellent comments and explanations on how these valves perform and flow...

We started on the prior page when a post was made about the P-stat and other comments that the Line Mini offerers the user some control of the flow pressure . That's how we gained all this knowledge.
Except I still not sure if any of these valves or other parts of the Mini offers the user some control of the flow to the group.


#45: Post by PaulTheRoaster replying to Shakespeare »

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#46: Post by JRising »

Shakespeare wrote: Except I still not sure if any of these valves or other parts of the Mini offers the user some control of the flow to the group.
By adjusting the OPV, you are setting a limit for the maximum pressure at that point in the system.
Any time the pressure gets higher than that setting, it is high enough to un-seat the valve against its spring which allows internal leakage through the valve. Any time the pressure at the rotary pump's outlet exceeds the set point, it leaks back to the inlet, or for a machine with a pump with separate OPV, it unseats the valve and leaks internally back to reservoir.

By having a maximum pressure that the pump can force on the brew circuit, you can limit the damage that the "ramp up to pressure" when the water begins flowing won't go so high as to be further compacting the tamped puck in the portafilter, or worse blowing it to pieces.

The pressure across the puck (something like 8.5 to 9 bar above the prep and atmospheric below, of course, it's falling into the air) matters. You want the water flowing over the puck for 20-30 seconds at least (some times much more) to properly extract all of the components. If the pressure is too high, the grounds must be very fine to slow the flow and the puck prep could break differently each brew attempt. The pump doesn't know any better, it will keep pounding water against the coffee prep until the pressure hits the pumps limit and stalls it or you blow a hose or fitting. Some sort of OPV is needed, even if it's just an un-adjustable one in there for expansion valve purposes, but the adjustable ones are nice for giving the user some control of maximum pressure.
People also know from experience that 8.0 - 9.5 bar is a decent pressure drop across the puck, so by setting a pump's limit to 9.0 bar, you can know immediately that your coffee isn't prepared correctly or ground properly if the water just runs through the prep as it would through a sieve.


#47: Post by mctrials23 »

Not a review as I have had the machine all of 15 minutes but a few observations.

The good

Fast heat up time
Very quiet machine
Coffee seems good from first shot
Steam is fantastic
App seems responsive and largely intuitive

The not so good

Wet pucks again :cry:
The machine is cramped
Not enough headroom with scales

To elaborate on a couple of these. I am coming from an LR lever machine where pucks are dry and simply fall out of the PF when you go to clean the PF. Wet pucks is annoying as I will have to use a knockbox again.

The steam is amazing. I never had any complaints about the LR steam power but I could never get perfect textured milk which is, i'm sure user error but I managed perfectly incorporated and textured milk first time on the micra. Very happy with that.

The machine is compact and due to the design where everything is recessed it even more apparent. I will get used to it but its cramped.

I have no issue with the plastic attachment for the PF but the very obvious downside to this is the bottomless aspect of it is a bit taller than it would need to be if it wasn't convertible. I can't get many of my mugs under it along with the acaia lunar scale. Not a massive issue and I will probably move to just not using scales that much after getting a feel for how the shots run but still a slight annoyance.

Finally, the machine looks lovely in the gloss white.

Edit: There is a slight smell coming from the machine but hopefully that will go after a few hours.

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#48: Post by OK31 »

mctrials23 wrote:There is a slight smell coming from the machine but hopefully that will go after a few hours.
Not sure if you saw the other thread but this seems to be common it's one poster noting that LM is sending a tech. You should check that thread.

The wet puck I have even on my Mini which is certainly annoying but seems like it may be due to the puck not being "done". I've had a few of same bean that ran very long and the puck was dry in those. Not sure how to "fix" but shots seem to taste fine.


#49: Post by BaristaBob »

One of my "wants" is 4 inches of clearance from the top of my lunar scale to the edge of my bottomless pf. Does this machine provide such clearance?
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#50: Post by mctrials23 »

Its not 4 inches. Less than 3.5.

Its 8.5cm and getting a cup under it comfortably means you probably want a cup that is about 6.5cm tall at most.