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  • #16
    Re: Pressure and flow

    This is exactly why i used our VST baskets when showing you the silvia, not the synesso basket that came with it. when we were using synesso baskets we were roughly a quarter turn coarser on the roburs. we had to tighten the grind up considerably when we switched to VST. Also i can confirm that a hotter temperature with all other variables the same will result in a faster flow rate.

    ill hold up the two baskets to the light when im at work next and come back with results.

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    • #17
      Re: Pressure and flow

      Dont expect exactly the same flow rate / volume / pressure / temperature to be delivered from two different groups on the same machine. All sorts of variables going on here!

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      • #18
        Re: Pressure and flow

        Originally posted by 2A222337222F4D0 link=1327651715/14#14 date=1327996226
        Thats really good info, Kevin - I assume you put the same amount in each basket.
        Well - I dont have the scales here in the Tea room at work - so the dosage was volumetric. But near enough. Pulling wild guesses out of the air - maybe the variation for dosage would be at most +/- 1 second all up on those measurements. Allowing for the human reaction time and the very minor dosage variation.

        Originally posted by 172C2328383E4D0 link=1327651715/15#15 date=1328004415
        ill hold up the two baskets to the light when im at work next and come back with results
        No real need to do that - the VSTs have visibly more holes, more evenly spaced. I have no kit to work out what the actual open flow area of the basket is (although some image processing software might be able to do this... hmmmm). The grinder adjustment is significantly finer if you dont want soup!

        Cheers
        /Kevin

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        • #19
          Re: Pressure and flow

          To remove the group as a variable in this test, it would be easy to do it a few times, switching the baskets between groups. Should be pretty evident after a few shots if one group flows faster.

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          • #20
            Re: Pressure and flow

            Originally posted by 526D557E7C741F0 link=1327651715/18#18 date=1328098110
            To remove the group as a variable in this test, it would be easy to do it a few times, switching the baskets between groups. Should be pretty evident after a few shots if one group flows faster.
            Both the groups run the same temperature and pressure. The only difference is the flow sensor programming.. So there is no variation (in terms of this experiment anyway).

            Having said that - I did actually repeat this experiment on the Expobar with the same result 25seconds vs 18seconds... About the same ratio. Which Id lay quids on is the ratio of the cross sectional areas between VST and Synesso baskets

            /Kevin

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            • #21
              Re: Pressure and flow

              Originally posted by 6465622F0 link=1327651715/19#19 date=1328161969
              Which Id lay quids on is the ratio of the cross sectional areas between VST and Synesso baskets

              You really think that the perforations have THAT large an effect compared to the resistance of the puck? I just cant see it...

              Gonzo

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              • #22
                Re: Pressure and flow

                Hi Gonzob. Sorry for the late reply. Just got your message. I really should check the messages more often!

                Anyway, I checked out your videos and can offer some insight into what is happening. All the variables discussed above influence extraction to various degrees, but the main variable is preinfusion duration. Ive done significant experimentation with preinfusion, and have discovered quite a number of variables that have a dramatic influence on extraction quality and taste.

                I could write quite a lot about my findings, but Ill keep it short for now. The basic principle is that the longer the preinfusion, the more the water soaks into the puck and individual coffee particles than compared to no preinfusion therefore requiring less pressure to penetrate the puck during extraction. Using the same grind size, no preinfusion period makes it necessary for the water to push through a mostly dry puck, therefore requiring more pressure to achieve a given flow.

                It might have gone unnoticed, but the time it takes from the moment that water contacts the coffee in your modified presso until you are able to get pressure into the system is a relatively long time. You have probably noticed that your shots reveal a more pronounced nutty flavour, which is characteristic of long preinfusion. You need to measure the time from the moment that water initially comes into contact with the coffee, and use that duration as a variable. Ive found that that variable is by far the most influencing variable for quality extraction.

                As for the pressure / preinfusion relationship goes, the longer the preinfusion, the lower the pressure required. The shorter the preinfusion, the higher the pressure required. Both extremes can produce great extraction, but will have different characteristics for a given coffee. Ive found that 9 bar over-powers and drowns out the more subtle flavours, but lower pressures without adequate preinfusion produces a flat featureless extraction. I havent graphed the profiles I currently use (I will at some point and put them on portaspresso.com), but I use pressures between 4 - 6 bar. Low pressures with the correct preinfusion period and the correct pressure ramp-up profile (another very significant variable easily controlled with the Rossa Hand Espresso, but maybe a bit harder with your presso; but I encourage you to try variations if possible) will produce the same or more body than a conventional 9 bar extraction, and the subtle flavours that generally go unnoticed by the masses will be evident.

                Pump or vibration frequency is another variation. Your pumping action could potentially pulsate the water as it penetrates the puck. I realise that most people will consider this insignificant, but Ive found that even the smallest and subtle fluctuations can influence flow and quality, and if people knew how much the pump in their electric machines influenced their coffee experience, they would seek out some type of manual or lever machine.

                Based on my experience, pumping your presso more than once will destroy your shot. The pressure ramp-up profile should be as smooth as possible. It can be a steep climb or flatter, but it needs to be smooth. Even very small fluctuations will totally destroy your shot. You can pause half way through the build-up, but dont fluctuate the pressure.

                If you can modify your presso to control the variables described above, you should be on a winner.

                Ive only lightly touched on the above variables, but Im more than happy to discuss them further if anyone is interested.

                Cheers,
                Ross

                Comment


                • #23
                  Re: Pressure and flow

                  Originally posted by 7A45585E4B595A584F5959452A0 link=1327651715/21#21 date=1328235343
                  Pump or vibration frequency is another variation. Your pumping action could potentially pulsate the water as it penetrates the puck. I realise that most people will consider this insignificant, but Ive found that even the smallest and subtle fluctuations can influence flow and quality, and if people knew how much the pump in their electric machines influenced their coffee experience, they would seek out some type of manual or lever machine.
                  This sounds like an argument for a hydraulic accumulator of some sort to deal with this. Id have to be convinced that this is actually measurable in real-world machines though. The actual hydraulics of the machine I use has the pump pushing water into a large boiler and the boiler would have headspace - and thus be quite nicely dealing with the otherwise "pulsing" nature of the pump. Plus the OPV in the circuit will also tend to squish out some of the leading edge of the pressure pulses. I cant help but have a gut instinctive feeling that while youd no doubt be able to recover a 50Hz signal from measuring the pressure, the actual 50Hz ripple youd see superimposed over the 9bar pressure signal would be very small indeed.

                  I might try to find a suitable pressure transducer to measure the Expobar to see how this actually looks.

                  Ross: do you have some data on this already?

                  /Kevin

                  Comment


                  • #24
                    Re: Pressure and flow

                    Originally posted by 68696E230 link=1327651715/22#22 date=1328239884
                    Ross: do you have some data on this already?
                    Electrical frequency is not the issue, its the mechanical vibration quite noticeable on many domestic and semi-commercial machines caused by pump design. Machines fitted with good rotary pumps would not noticeably suffer from this issue. I tend to think that if you can feel the pump working by placing your hand on the machine during extraction, there would be some level of pulsation through the water, which in my opinion could influence extraction.

                    The data is inferential, but Im sure it wouldnt be too difficult for someone with the time and motivation to setup and record the pulsation or pressure fluctuation (I wont say frequency so that no-one confuses it with that associated with electricity).

                    When I first made the Rossa, the quality of extraction made with it was dramatically better and smoother than my Bezzera. This certainly got the mind going to figure out why. The Bezzeras pump is what I consider quite rough, and I always wondered what effect it had on extraction, but was unable to test it. Once I made the Rossa, I was able to replicate the Bezzeras profile, but without the rough pump. I believed that I isolated the pump fluctuation enough for comparison, but one can never be sure. I have since made similar comparisons with many machines, and it seems that the smoother the pump, the more closer the shots are to the Rossa (and I imagine a lever machine; need to keep it objective!). The differences are noticeable both visually and by taste.

                    Ive also replicated a type of pulsation fluctuation with the Rossa, and the results start to resemble that of the Bezzera (and other similar machines). Of course there could be other variables at play, but Im quite sure that the slight pressure fluctuations make a noticeable difference (to me anyway). That said, maybe few people can taste a difference. Until I knew better, I thought my old machine was great. Everything is relative!

                    Originally posted by 68696E230 link=1327651715/22#22 date=1328239884
                    The actual hydraulics of the machine I use has the pump pushing water into a large boiler and the boiler would have headspace - and thus be quite nicely dealing with the otherwise "pulsing" nature of the pump.  Plus the OPV in the circuit will also tend to squish out some of the leading edge of the pressure pulses.
                    Maybe someone else could comment on this one, as I havent looked at the Expobar, but I wouldve thought that the pump plumbing associated with the extraction circuit would go through the boiler via a heat exchanger process. Either way, the pressure fluctuation would be there and measurable, unless of course the pump is of very high quality.

                    I imagine that this topic would be controversial, and really only apply to those seeking to push the limits in the pursuit of perfection.

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                    • #25
                      Re: Pressure and flow

                      Originally posted by 023D2026332122203721213D520 link=1327651715/21#21 date=1328235343
                      Based on my experience, pumping your presso more than once will destroy your shot. The pressure ramp-up profile should be as smooth as possible. It can be a steep climb or flatter, but it needs to be smooth. Even very small fluctuations will totally destroy your shot. You can pause half way through the build-up, but dont fluctuate the pressure.

                      Yes, Ive noticed, and others have commented that, with a standard Presso, if you "have another go" with the levers it stuffs it.

                      However, with my Pneu-presso I can hold the pressure very well. My videos show the machine in its infancy and when I was still new to it. Since then Ive fixed a few leaks and learned to use it, and I can now ramp it up, to say 2 bar, hold it there for a bit, then ramp it up to 6 bar, and hold it there. The shot volume with my machine is not limited by the stroke of the levers.

                      I digress. The issue here is that theres a big difference between machines with the same grind. KJM has pointed out that the shape of the basket is a big variable (and maybe the perforations...). Zaneus says temperature is a big player. Now Ross says the pre-infusion makes the world of difference. All back up their ideas with experience and logic.

                      And I thought it was going to be pretty straightforward....

                      Gonzo

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                      • #26
                        Re: Pressure and flow

                        Originally posted by 4D45445045482A0 link=1327651715/24#24 date=1328251749
                        And I thought it was going to be pretty straightforward....
                        Coffee? Straight forward? Doesnt it famously have 2+2=5 in here somewhere :

                        Originally posted by 7A45585E4B595A584F5959452A0 link=1327651715/23#23 date=1328248578
                        Electrical frequency is not the issue, its the mechanical vibration
                        No - that was my point. The pump will oscillate at the mains frequency so youll see a 50Hz (or 100, depending on the action of the pump) pressure fluctuation..

                        /Kevin

                        Comment


                        • #27
                          Re: Pressure and flow

                          Originally posted by 5D55544055583A0 link=1327651715/24#24 date=1328251749
                          And I thought it was going to be pretty straightforward....
                          Hi Gonzo,

                          I personally think it is quite simple, but we seem to make it very complex. With the right equipment, following a few simple principles will result in great coffee. Reaching the pointy end of quality takes a bit more care and attention, but it is certainly not out of reach.

                          In relation to the suggestions outline above, experimentation is king. Testing different baskets could prove difficult, but you should be able to figure something out. Temperature and water contact time should be easier.

                          I see further potential for your idea if you can design it to control all the applicable variables. The great aspect of manual operation is the ability to control those variables to a much higher level than the majority of conventional machines. I look forward to seeing further revisions.

                          Originally posted by 7170773A0 link=1327651715/25#25 date=1328266219
                          The pump will oscillate at the mains frequency so youll see a 50Hz (or 100, depending on the action of the pump) pressure fluctuation..
                          Hi Kevin,

                          This would make for an interesting experiment. Let me know if you end up doing it.

                          Cheers,
                          Ross

                          Comment


                          • #28
                            Re: Pressure and flow

                            Originally posted by 675845435644474552444458370 link=1327651715/26#26 date=1328497399
                            This would make for an interesting experiment. Let me know if you end up doing it.
                            I actually had a snarf around the labs for some pressure transducers. We have some at work, but theyre rather excessively humongously large.. The liquid ones I have access to also suffer from the problem of having a 38mm (1 1/2") BSP thread that the liquid goes in So Im out on a limb unless I go buy one myself. Which I might do yet, just out of prurient interest.

                            /Kevin

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                            • #29
                              Re: Pressure and flow

                              OK - Ross - there isnt going to be any measurements. High speed pressure transducers are basically outside my ability to fund.

                              The fastest (affordable) transducers have response times of 10mSec. To look at the pressure waves in the fluid you need to resolve a bit better than that - youre looking at primary waveforms of 20mSec and I suspect you want to look for harmonics at higher frequencies...

                              Having said that, however, a couple of engineers of the wet mechanical type listened patiently to my question about the pressure fluctuations and pointed out that the boiler with any headspace in it plus the group head (which would certainly have air entrained) will actually act as a low pass filter.

                              I do hate it when the bleeding obvious gets pointed out! :-[. Darn - I knew that too!

                              Assuming the espresso machine designers were on the ball, this will nicely filter the pressure fluctuations down to vanishingly small. It will certainly take out 50Hz (they said, after the second espresso and without measuring anything...).

                              This would also explain why there is no discernible difference in the taste from a vibe pump or a rotary in blind tasting.

                              /Kevin

                              Comment


                              • #30
                                Re: Pressure and flow

                                I wonder if the pressure fluctuations are more an effect of the thermoblock type machines, which dont have the whole suppression effect of a large-ish boiler?

                                Perhaps this is another reason why Boilers are Better? ;D

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