Post By TampPolice
Post By MrJack
Post By Duffybeers
Post By LeroyC
Post By MrJack
Post By LeroyC
Brewing at a higher pressure for machines with smaller pumps
Hello Fellow Snobs
I came across this interesting video today on YouTube in which Luca Bezzera (great grandson of the fella who is known to have made the first espresso using a pressure system) is giving a few blokes bit of a tour around their Bezzera factory. See link to the YouTube video below.
What I found interesting was that he said that the 9bar pressure was meant to be a setpoint for machines with larger pump sizes e.g rotary pumps. He then says that the optimum pressure for small vibratory pumps should be set to around 11bar.
Now if this was just another bloke reviewing an entry level machine, I would have said Mate, don't reinvent the wheel but considering his position I thought I should at least give it a try just out of respect.
So I did and after about three attempts, got my dosing right to maintain around 9.8bar pressure and managed to pull the best shot I have ever poured in six months of owning my BES920 (see stats below and the video in the second link below).
The taste of espresso was definitely more lively and acidic and I instantly noticed the difference. Also ended up with a good looking puck (images in the second link). From now on, this will be my setting for a single shot for my usual beans. I may increase the brew time by another second though.
I'll play around tomorrow to get a tad more pressure and will also experiment with the double basket.
My first thought was to post this in the BES920 owner's thread but I also wanted to hear some experiences from people who own some decent Italian machines with small Vib. pumps (no offence to Breville of course)
Do you set your dosing and other settings to get a decent shot at 9bar or if you have a machine that allows you to brew at specific pressures, have you experimented with higher pressures?
Would you agree with Luca Bezzera that small pumps should be set higher?
The only logic my engineering brain (non hydraulic) can think of is that the pressure gauge typically measures the pressure in the boiler and a small size pump may not be able to maintain the same pressure in the brew head due to some losses in the pipework along the water path but it may be possible to maintain the brew head pressure same as the boiler pressure with a larger pump.
Stats from my test shot:
Machine: Breville BES920 Dual Boiler
Basket: Single cup 15g Breville stock (non pressurised)
Grinder: Mazzer Mini Electronic Mod A
Beans: Medium roast with good acidity (Brazilian/South American Blend) air roasted on Monday by a local roastery (5 days ago)
Water: Sydney Inner city tap water with Breville filter in the water tank
Shot Size: 14.1g
Grinder Setting: Very slightly finer than my usual
Grind Time: 10.25sec
Tamp Pressure: Approx. 30% more than my usual 9bar brew
Brew Temperature: 93deg
Pre-Infusion: 7sec at 60%
Total Brew Time (inc. Pre-Inf.): 31sec
Pressure: 9.75bar (as shown on the gauge)
Shot Volume: 26.2ml
The OPV operating pressure for my machine is approx 11.5bar.
Referenced YouTube Video:
My test shot video and photos:
Makes zero sense.
The pressure is the result of the flowrate and the resistance to flow; you won't achieve the pressure if the pump can't overcome the resistance at a given flowrate.
The results would be no different if you had a rotary pump.
I read somewhere on here a couple of days ago someone talking about how rotary pumps add to the line pressure but vibratory pumps kind of stop the flow and create their own pressure entirely (I'm poorly paraphrasing as I can't remember exactly), and so rotary pumps should have be set to 9 bar but vib pumps should be set to closer to 11 because it will drop off to about 9 by the time it gets to the puck. The point was that 9 bar was the right pressure at the puck but the pump or OPV needed to be set differently to achieve that.
The difference between a rotary pump and a vibe pump, capacity aside, is that the vibe pump pressure increases as the flow rate is reduced, whereas a rotary pump holds a steady pressure. Inversely, as the flow rate accelerates during the extraction, the pressure on a vibe pump tends to decline. What I observe on my HX machine, where the manometer sits after the brew solenoid valve, is a similar decline in pressure. I think it is the combination of this inherent pressure -flow decline (to which the OPV responds by partially closing while trying to hold the pump pressure) and the increased pressure loss of the tubing / HX / flow restrictor. The bottom line is that for the puck to see 9 bar throughout the extraction, the OPV should be set higher, typically 10,5 bar. While the Bezzera manual even specifies 12 bar, I left mine adjusted at 11 bar on the blind filter and I am happy with it.
Yea mate I think you are onto something here.
My first real espresso machine was a brasilia club with no OPV (think butt ugly silvia). Brutally unforgiving but when done right made a great shot. After that I bought a BDB and could never get a great shot out of it. The only thing that the brasilia club could do that the BDB cant is high pressure. I'm pretty gutted if the only think I was missing with the BDB was upping the OPV pressure to get a great shot.
I have recently upgraded my machine to the linea mini, but of all the e61 hx machines I tried I liked the bezzera mitica the best overall. It's a rotary pump model, so not too applicable here except to say that I think they know a thing or 2.
People get confused about how, when and where the pressure reading is taken. I've owned a couple of Gaggia Classics and spent a fair bit of time on the Gaggia users forum and this is the most common mistake people make when they're trying to adjust their OPV. The best way to read the pressure and make adjustments is with a proper portafilter pressure gauge with a needle valve so you can take the reading with flow through the group. Using static pressure readings is ok, but it's always an approximation and will differ for every machine. When I adjusted both my Classics I was only able to read static pressure at the group so set the OPV to roughly 10bar each time. By all accounts this would've given me an extraction pressure of roughly 9bar, but it could be anywhere between about 8 and 10bar. This is still any good region to operate in for espresso and it gave me awesome shots on my machines so I was happy. So just to be clear - static pressure and dynamic pressure are different. They always will be and should be.
Originally Posted by degaulle
A lot of people say this, but it is not actually true. The pump curve for a rotary pump shows the same inverse relationship as vibe pumps.
Interesting, that graph I wasn't aware of, I must confess. I wonder If the means of controlling the brew pressure plays a role here. An OPV will open or close more or less proportionally to the static pressure that is applied once this pressure is high enough to overcome the OPV's spring force and that is how it controls the pressure. If the flow speeds up, the pressure starts to fall, thus the OPV should do the opposite. A proportional controller at best, but hardly a precision instrument. The internal bypass on a rotary pump should work essentially the same, shouldn't it ?
The other difference between V and R is the rate at which pressure builds. The static pressure on a rotary pump builds faster, while the vibe pump has more of a pressure ramp to it. In either case, however, a flow restrictor should prevent that this pressure is instantly applied to the coffee bed, so it is not disrupted.
An OPV isn't supposed to operate as a PCV, even though it can fill that role to a certain extent.