What are, roughly, the effects of too high or too low brewing pressure ?

by **Ton** on Fri Sep 09, 2011 11:22 am

My Expobar Brewtus IV with vibe pump was delivered with a factory setting of 12 bar (measured with a blind filter and read on the gauge). The shop were I bought it told me that, because I have a vibe pump, the usual factory setting is 11-12 bar opposed to the often mentioned 9 bar.
Meanwhile I read that it can be interesting to experiment with lower pressures and use taste as a guideline and also that gauges are often not very accurate.
But what is roughly the effect of a too high or too low pressure ? In other words, what is to be expected when the pressure is not optimal ? Gives a too high pressure for instance more bitterness ?
Thanks in advance.
Ton

by **another_jim** on Fri Sep 09, 2011 12:50 pm

There's not much difference at 14 gram doubles and lots of headroom. A taste test for a large group of espresso drinkers at the University of Salamanca found they slightly preferred 9 bar shots over 7 bar shots, and preferred both over 11 bar shots. The 11 bar shots were perceived as too bitter, the 7 bar ones as slightly too soft. These effects can magnify when using higher doses.

These results are based on a rotary pump, that has a distinctly faster ramp up in pressure, and much less of a pump curve (pressure fall off when the flow increases). The soft or bitter taste is affected by these, so Italians mostly set vibe pumps to higher pressures (11 bar against a blind filter is the actual standard). An alternative is to set the pump at 9 bar but use larger diameter group jets than in the same group tied to a rotary pump.

by **Intrepid510** on Fri Sep 09, 2011 5:00 pm

Yeah you might want to bring it down a bar, but its static pressure you are seeing so you want somewhere between 10-11 when testing.

by **Ton** on Fri Sep 09, 2011 7:12 pm

Thanks for the replies.
Meanwhile I lowered the pressure to 10,5 bar, against the blind filter (and read on the gauge). Then I measured the pressure with a manometer portafilter which I had still lying around (bought it for use with my Rancilio Silvia). With that I measured 9 bar.
Is the difference because of the inaccuracy of one or both instruments or because the measuring takes place on different places ?
Anyhow it struck me that I had immediately better results with a 18 gram VST basket. With that basket I have been struggling the last few days to get decent pours. Is this just a coincidence or could it indeed be a result of the lower pressure ?

by **another_jim** on Fri Sep 09, 2011 7:25 pm

Ton wrote:Is this just a coincidence or could it indeed be a result of the lower pressure ?

Depends. Can you make three identical shots in a row that taste the same? :wink:

by **boar_d_laze** on Fri Sep 09, 2011 7:49 pm

another_jim wrote:A taste test for a large group of espresso drinkers at the University of Salamanca found...

A. E. I. O. Universidad de Salamanca, ole! Ole!! OLE!!!

Sorry. Couldn't help myself. Hail to thee oh alma mater, and all that.

BDL

by **another_jim** on Fri Sep 09, 2011 8:15 pm

And sillier yet; it was the food science department.

However, they have, to date, published about five articles on taste testing various aspects of espresso making. The results are not dramatic, but instead confirm conventional wisdom and techniques. Nevertheless, actual sound evidence is better than the usual hearsay.

by **Ton** on Sat Sep 10, 2011 7:34 am

Ton wrote:Meanwhile I lowered the pressure to 10,5 bar, against the blind filter (and read on the gauge). Then I measured the pressure with a manometer portafilter which I had still lying around (bought it for use with my Rancilio Silvia). With that I measured 9 bar.
Is the difference because of the inaccuracy of one or both instruments or because the measuring takes place on different places ?

Does anyone know an answer to this part of my question ?

by HB on Sat Sep 10, 2011 8:22 am

Part of the difference could be due to inaccuracies in the gauges, part of it due to where the two gauges are measuring pressure (onboard gauge measures before the gicleur (orifice) in the grouphead, portafilter gauge measures after the gicleur), and part of it due to the behavior of pressure relief valves under different flow rates. Greg elaborated on this latter point in Why difference in pressure between blind filter and brewing? from the FAQ:

gscace wrote:Now lets examine why there is a difference in pressure between pumping against a blind filter and when brewing coffee. When water delivered by a pump is forced through the pressure relief valve, the plunger within the valve is forced away from the plunger seat, as needed to accommodate the volumetric flow rate through the valve. When this happens, the new equilibrium pressure between the water and the pressure relief valve plunger is higher than the cracking pressure because the piston has moved away from the seat and the spring is further compressed. The change in pressure is dependent on the amount of liquid flow through the valve, the diameter of the plunger, the geometry of the outlet orifice that is uncovered by the plunger's motion, and the spring constant. The pump produces excess flow rate at 9 bars, compared to what is needed to brew espresso. This means that there is always excess water flowing through the pressure relief valve to some degree when the machine is brewing coffee. Vibe pump systems using relatively long-traveling pressure relief valves, such as the one being discussed here, produce large pressure differences between actual brewing and pumping against a blind filter. In the case of pumping against the blind filter, all of the pumped water flows through the relief valve. When brewing, a large fraction of the initial water flow is absorbed by the coffee cake at the start of the extraction process. Then a smaller, but still very substantial fraction of the total flow exits the system through the group and into the cup. Thus the equilibrium pressure imparted by the relief valve is less when brewing than when pumping against the blind filter.

by **Ton** on Sat Sep 10, 2011 8:33 am

Thanks Dan. That clarifies it.
Am I right then when I assume that with the current setting (9 bar on the manometer portafilter), and of course taking in account possible inaccuracy, I am somewhere in the ballpark ?


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