La Marzocco Linea Mini high pressure gauge reading when idle - Page 2
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tlo (original poster)
- Posts: 9
- Joined: 15 years ago
Thanks for posting the video. I've turned the pressure down and it hasn't risen over 12 bar today. When I pull a shot thought it shows at just a little over 10 bar on the gauge. Even though it is a bit high, my shots came out quite nice. I am currently using Pallet Roaster's Benchmark. I haven't completely dialed it in yet but I must say it tastes quite good. Quite similar to what my VBM was pulling but has a smoother and fuller mouth feel. Once I am more familiar with this marchine I will lower the brew pressure a bit to see if it makes any difference.
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Peppersass
- Supporter ❤
- Posts: 3692
- Joined: 15 years ago
What the video shows would be normal for a GS/3 and I assume the Mini is setup the same way.
Does the Mini manual tell you how to adjust the bypass valve and the expansion valve? If so, do what it says.
On the GS/3, the rotary pump bypass valve is adjusted so that the coffee boiler pressure gauge reads 9 BAR. Then you run the coffee brew cycle for 10-20 seconds and turn it off. The pressure will rise past 9 BAR. Adjust the expansion valve so that the maximum temperature reached is 12 BAR.
Why does the pressure rise after a brew cycle? This happens because heat has been applied to the coffee boiler during the brew cycle. This increases the pressure in the boiler. However, the pump bypass valve limits the pressure to 9 BAR. When you turn off the pump, the bypass valve is no longer in play and the pressure will rise to whatever level the expansion valve permits.
weebit_nutty:
You said that the pressure increases to 15 BAR during backflush. I don't recall that happening when I had the stock rotary pump in my GS/3 (I use a gear pump now.) My recollection is that the bypass valve and expansion valve restricted pressure to less than 12 BAR. Maybe other GS/3 and Mini users can comment on their backflush pressure.
Note for techno-geeks:
What I've found with the gear pump, which doesn't have a bypass, is that there's a difference between how the expansion valve reacts to the relatively gentle pressure increase from water in the coffee boiler being heated and the much more powerful action of the pump. If I adjust the expansion valve using the slow post-brew pressure rise, it will not restrict pressure to 12 BAR if I backflush with the gear pump speed set too high. The pressure can easily exceed 14 BAR, which I believe is the max rating for the GS/3. This doesn't happen with the stock pump because the bypass limits pressure when the pump is running.
If I set the expansion valve to max out at 12 BAR when I backflush at nominal speed (9 BAR free-flow), then the machine will idle at too low a pressure and will actually drop in pressure after the brew cycle. This suggests that there's some hysteresis in the expansion valve such that setting it using the post-brew pressure rise is different from setting it during backflush -- at least with a pump that doesn't have a bypass valve.
My solution was to adjust the expansion valve to 12 BAR using the post-brew pressure rise and set the gear pump speed to lower than nominal when I'm backflushing (i.e., to a speed that limits pressure to less than 12 BAR.)
Does the Mini manual tell you how to adjust the bypass valve and the expansion valve? If so, do what it says.
On the GS/3, the rotary pump bypass valve is adjusted so that the coffee boiler pressure gauge reads 9 BAR. Then you run the coffee brew cycle for 10-20 seconds and turn it off. The pressure will rise past 9 BAR. Adjust the expansion valve so that the maximum temperature reached is 12 BAR.
Why does the pressure rise after a brew cycle? This happens because heat has been applied to the coffee boiler during the brew cycle. This increases the pressure in the boiler. However, the pump bypass valve limits the pressure to 9 BAR. When you turn off the pump, the bypass valve is no longer in play and the pressure will rise to whatever level the expansion valve permits.
weebit_nutty:
You said that the pressure increases to 15 BAR during backflush. I don't recall that happening when I had the stock rotary pump in my GS/3 (I use a gear pump now.) My recollection is that the bypass valve and expansion valve restricted pressure to less than 12 BAR. Maybe other GS/3 and Mini users can comment on their backflush pressure.
Note for techno-geeks:
What I've found with the gear pump, which doesn't have a bypass, is that there's a difference between how the expansion valve reacts to the relatively gentle pressure increase from water in the coffee boiler being heated and the much more powerful action of the pump. If I adjust the expansion valve using the slow post-brew pressure rise, it will not restrict pressure to 12 BAR if I backflush with the gear pump speed set too high. The pressure can easily exceed 14 BAR, which I believe is the max rating for the GS/3. This doesn't happen with the stock pump because the bypass limits pressure when the pump is running.
If I set the expansion valve to max out at 12 BAR when I backflush at nominal speed (9 BAR free-flow), then the machine will idle at too low a pressure and will actually drop in pressure after the brew cycle. This suggests that there's some hysteresis in the expansion valve such that setting it using the post-brew pressure rise is different from setting it during backflush -- at least with a pump that doesn't have a bypass valve.
My solution was to adjust the expansion valve to 12 BAR using the post-brew pressure rise and set the gear pump speed to lower than nominal when I'm backflushing (i.e., to a speed that limits pressure to less than 12 BAR.)
