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How to determine brew pressure on Rancilio Silvia

Postby srossnz on Mon Oct 11, 2010 1:10 am

Ok, am a bit confused over the best way to determine pressure on my Silvia. Here is some of the conflicting info I have seen:

-250ml per minute from the return pipe is about 9bar (never heard anyone say this is the correct rate for a V3, some say 400ml/min , some say 500ml/min because of the style pump it is being vibratory, some say the measurement doesn't simulate a shot because you use a blind filter, there is conflicting info of when to measure some say after 200mls has poured, and some say warm or cold machine makes a difference in the measurement)
-get a portafilter gauge (some say you must build one with a valve to simulate a shot, that a straight gauge to the portafilter can be off as much as 4bar). I feel building or buying one could be a waste if it is not a true measurement.
-install an isomac pressure gauge on the Silvia, I can't seem to find a comprehensive guide or package deal to accommodate such a mod and I surely do not want to 'play around' with that.

So I am not sure where to go from here, my return pipe is putting out 300ml minute, but something seems off with my pressure, the shots are disgusting (although I am still wrestling with other variables). I have a pro-barista showing up in a week for home training that should help but they do not own a pressure gauge or anything. Maybe a V3 Silvia owner who is happy with their shots can tell me what their return flow rate test produces?
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Postby cafeIKE on Mon Oct 11, 2010 2:17 am

See The importance of brew pressure, purpose of adjusting OPV?
and OPV Flow on Calibrated 'Double'

300ml on a blind basket in 60s equates to ~9.75bar on an E5
150 to 200ml per minute could be returned on a pump with Ulka E5 flow characteristics on 'standard' double.

If numbers are close to this, pressure is not a likely culprit.
Pressure is much less critical than many assume.
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Postby HB on Mon Oct 11, 2010 6:26 am

To elaborate on Ian's point, refer to this graph:

Image

The short explanation: The more volume a vibratory pump displaces, the lower the pressure. The less volume a vibratory pump displaces, the higher the pressure, up to its maximum rated capacity (usually around 15 bar). Around double espresso flow rates, the actual brew pressure works out pretty close to 9 bar. For ristretto flow rates, see I still don't get it: Why adjust the OPV?

srossnz wrote:...something seems off with my pressure, the shots are disgusting (although I am still wrestling with other variables).

At brew pressures well above 9 bar, the taste profile of some coffees will become acrid and the roast notes exaggerated (e.g., burnt cedar and peat flavors). Based on numerous reports, the factory setting for the Rancilio Silvia is 11 bar. If pulled as a tight ristretto at that pressure, the shots could be harsh/bitter tasting. For most owners, I suspect the positive outcome of dropping the pressure is the reduction in channeling due to the increased "forgiveness factor", not an inherent issue with espresso at higher pressures.

That said, if you really want to know the brew pressure, it's not difficult or expensive to build a portafilter gauge and there's ample online hints:

Image
From Rancilio Silvia Pressure Gauge Test; also see Building a Portafilter Pressure Gauge

The DIY portafilter gauge above has a needle valve to simulate the flow rate of espresso. It's more accurate, but you can get a "good enough" measurement by simply not applying thread sealant so the fitting drips a bit.
Dan Kehn
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