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Thread: Pressure and flow

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    Senior Member gonzob's Avatar
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    Pressure and flow

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Ok, all you gurus...

    I have a Silvia with a Synesso 18g 57mm ridgeless basket. I have adjusted the pump opv to 9 bar.

    I have a Pneu-presso with a 51mm, ridged, double basket.

    I grind (with my Sunbeam - yes Im going to replace it at some stage...) to get a 25sec pour on the Silvia.

    When I grind with the same settings, use the Pneu-presso, same quantity of grounds, and tamp with the same force (as near as I can), I get a 25 sec pour using only 4.5 bar. If I up the pressure to 5 bar it shoots out faster.

    Theres more crema on the Silvia, but both coffees taste equally strong (no tracking thru the puck, I assume).

    Why do I get the same flow with half the pressure? Is it that the much higher pressure from the Silvia is compressing the puck even further? Or maybe the higher temp?

    Gonzo

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    Re: Pressure and flow

    My thoughts are that there are possibly a number of factors:
    1. Different number/size of perforations between the baskets.
    2. Same tamp pressure on a deeper basket (presuming the smaller diameter basket is deeper) may result in a less compacted puck.
    3. 9bar at OPV may not be 9 bar at shower screen.
    4. Not sure how the pump functions exactly, but perhaps the flowrate is limited?

    My money is on point number 2.

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    Re: Pressure and flow

    Lever machines tend towards having deep and narrow baskets as opposed to wide and shallow as this presents less of a flow restriction as the puck expands. Id hazard a guess that most levers are extracting at around 6 bar compared to the 8 or 9 bar of pump driven machines, and the principle is the same for your presso baskets.

    Id also agree that the VST basket perforations are more restrictive than the presso baskets by design.

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    Senior Member gonzob's Avatar
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    Re: Pressure and flow

    Hmm. Im running all these ideas through my mental model of the situation and this happens:

    Perforations: I cant see the perforations being the "bottleneck". If theres no coffee the water just goes straight through. The puck is the bottleneck.

    Tamp pressure: If I put the same force on each basket, the larger diameter basket gets the lower pressure. Thats the one that flows the slowest (Silvia). Cant see it....

    Pump Pressure: My pump pressure was measured at the portafilter, after the shower screen, with a Greg Pullman-style gauge. Its not that.

    Flowrate: Both machines are capable of pumping much more water than ends up in the cup. Not that.

    Less flow restriction as puck expands: I dont understand the mechanism here. If each puck expands by, say, 10% then whatever depth the puck was will increase by 10%. That should increase the resistance to flow by...10%?? It should be the same effect for both. Any more info on this?

    Gonzo

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    Senior Member GregWormald's Avatar
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    Re: Pressure and flow

    Quote Originally Posted by 49414054414C2E0 link=1327651715/3#3 date=1327757633
    Less flow restriction as puck expands: I dont understand the mechanism here. If each puck expands by, say, 10% then whatever depth the puck was will increase by 10%. That should increase the resistance to flow by...10%?? It should be the same effect for both. Any more info on this?
    That applies only if there is room for expansion between the grounds and the shower screen. Once the grounds hit the shower screen, the expansion can only go into the empty spaces between the grounds--i.e. the path the water takes. Less path, higher pressure needed for same flow.

    Greg

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    Re: Pressure and flow

    Quote Originally Posted by 2D25243025284A0 link=1327651715/3#3 date=1327757633
    Hmm. Im running all these ideas through my mental model of the situation and this happens:

    Perforations: I cant see the perforations being the "bottleneck". If theres no coffee the water just goes straight through. The puck is the bottleneck.

    Tamp pressure: If I put the same force on each basket, the larger diameter basket gets the lower pressure. Thats the one that flows the slowest (Silvia). Cant see it....

    Pump Pressure: My pump pressure was measured at the portafilter, after the shower screen, with a Greg Pullman-style gauge. Its not that.

    Flowrate: Both machines are capable of pumping much more water than ends up in the cup. Not that.

    Less flow restriction as puck expands: I dont understand the mechanism here. If each puck expands by, say, 10% then whatever depth the puck was will increase by 10%. That should increase the resistance to flow by...10%?? It should be the same effect for both. Any more info on this?

    Gonzo

    From a fluid flow perspective, EVERYTHING is a restriction. Any reduction in flow area will create a pressure drop.

    With regard to the deeper puck, this is how I see it. When you tamp, the grounds will be more compressed at the top, and less at the bottom. This is because the tamping force will be progressively absorbed by the grounds, and the walls of the basket. This is in part due to elasticity in the grounds.

    In a deeper (and narrower) puck, this is more pronounced. So at the base of the narrow basket the grinds will be further apart, and under less compression, than those in the larger diameter basket. This will partially offset the effect of a decrease in driving force (lower pressure).

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    Senior Member gonzob's Avatar
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    Re: Pressure and flow

    Yes, MrJack, you are right about the pressure drop. However, I suspect that the pressure loss across the perforations would account for 0.1 bar of the 9 bar available.

    And your comments about the tamping effects I accept also. However, were talking about a difference in diameter of 57 to 51 mm. Its not much, so I find it hard to believe that such a small diameter change would make such a large difference in flow/pressure characteristics.

    Try this: when Miss Silvia hits the puck with water, the first thing that happens is the top part of the puck gets hot and wet (steady...) and this slurry then plugs any further quick permeation of water, so that when, all of a sudden 9 bar arrives, the puck is put under a lot of compression. 9 bar gives around 300kg of force at that diameter. This is probably 20+ times the force I exert when I tamp.

    So Im thinking now that all the tamping does is make sure there are no crevices for water to by-pass the puck. The high pressure from the Silvia simply compresses the puck so much that you can get bugger-all water through it. I suspect that if I reduced the setting of the OPV to, say 4.5 bar, Id end up with about the same amount of flow. Its counter-intuitive but it makes some sense.

    Thoughts?

    Gonzo

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    Re: Pressure and flow

    Remember, when there is coffee in the basket, youre not talking about open perforations; grinds will protrude into the perforations, and alter the flow path. If you could somehow remove the basket altogether and check, I think youd find it accounts for more than 0.1 bar dP.

    I think the way you described the pressure rise at the puck is a little simplistic, and giving a false impression of what really happens. I doubt the puck gets hit suddenly by a stream of water at 9 bar. If it did, tamping would be pointless as the puck would be heavily disturbed.

    Rather, I think the channels leading to the shower head will progressively fill with water, and the pressure at the top of the puck will rise quickly, but not instantaneously. The water will likely have penetrated a fair way into the puck before full flow occurs.

    Also, I doubt water would just hit the puck and compress it as though it was impervious. Its like the difference between putting a bucket under a high flow tap, in comparison to a sponge.

    That said, most of the systems I look at dont involve solid particles (beyond fixed beds), and most of my experience is with high pressure gas; so in this case Im really just guessing based on my understanding of fluid flow.


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    KJM
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    Re: Pressure and flow

    Gonzo - interesting thoughts. I tried the following experiment with the two machines I have here - the Expobar Minore III and the Nuova Simonelli Oscar.

    Both set to 9bar pressure at the puck with the right flow through.

    Both give identical (as near as I can time it) with the same VST basket. Both give different flows with their own respective baskets.

    So the basket design does influence the flow in a measureable way. The other thing that might be happening is the brew temperature is likely to be different. Different temperatures will alter the rate at which the system extracts.

    As you extract, the extracted stuff (ie coffee ;D ) leaves the puck and flow restriction drops. I did measure this once - the flow increases significantly. I have always put this down to the equivalent of the really fine material being flushed from the filter bed so the filter bed is coarser.

    I think the actual dynamics of the extraction process are actually quite complicated. Id hate to have to model it!

    /Kevin

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    Re: Pressure and flow

    Good point Kevin (and interesting results)! Probably also that the viscosity of the fluid (which significantly influences frictional pressure losses) decreases as the oil content falls. :)

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    Senior Member gonzob's Avatar
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    Re: Pressure and flow

    Kevin, thanks for the info about the baskets - my two machines are so different theres no way I could do that.

    MrJacks earlier point about the loss of tamp effect as you go down the puck may be counteracted by basket shape. My Synesso basket has a nice rounded corner on the bottom, but the presso one is square. It may mean that the bottom layers of grounds are squeezed even tighter by the radiused corner and thus the flow is slowed.

    Yes, MrJack, the flow does increase in speed with time. I thought it was just the water finally getting to the bottom of the puck, but it may well be the viscous effects of the oils.

    Brew temperature: yes, I thought that might be an issue - again, with my equipment I can only do rough comparisons. Certainly with higher temps youd expect to get more oils.

    Good discussion!

    Gonzo

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    KJM
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    Re: Pressure and flow

    Quote Originally Posted by 69566E45474F240 link=1327651715/9#9 date=1327879258
    Probably also that the viscosity of the fluid (which significantly influences frictional pressure losses) decreases as the oil content falls.
    Unless youre using a Yemeni :o Theyre just thick-to-the-end! I hadnt actually considered the viscosity to be an issue, but of course it is.. Cripes - Id really not like to model this stuff!

    The other thing I noted is that the gauge on the Expobar does drop slightly as the shot progresses. Thereby lending further credence to the nett flow increase.

    /Kevin

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    Re: Pressure and flow

    Quote Originally Posted by 49484F020 link=1327651715/11#11 date=1327893909
    Cripes - Id really not like to model this stuff!

    /Kevin

    Oh, I hear you there. CFD on a batch liquid/solid extraction process...*shudder*

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    KJM
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    Re: Pressure and flow

    Quote Originally Posted by 043B03282A22490 link=1327651715/12#12 date=1327899472
    Oh, I hear you there. CFD on a batch liquid/solid extraction process...*shudder*
    Unless you have PhD [s]slaves[/s] students to do it ;D

    But on a more serious note - I tried to have a bit of a play using the Wega in the Tea room today. The machine has flow sensors and tries to deliver a precise volume (but fails!). I put my VST basket and the synesso basket head-to-head. The synesso shot ran for 25Secs (the grinder is set for this basket) and the VST ran through in 15. Same grind. So further proof that the actual basket does affect this stuff.

    It is nice and easy to do these experiments with a 2-group machine!!

    Anyway..

    /Kevin

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    Senior Member gonzob's Avatar
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    Re: Pressure and flow

    Well, I guess that explains it (well my initial query anyway).

    Thats really good info, Kevin - I assume you put the same amount in each basket.

    That is a really large difference in time! When you hold the VST and Synesso up to the light, do the perforations in the Synesso look more restrictive? I know its not a good test, but I thought Id ask anyway.

    Gonzo

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    Senior Member Zaneus's Avatar
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    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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    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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    KJM
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    Re: Pressure and flow

    Quote 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.

    Quote 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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    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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    KJM
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    Re: Pressure and flow

    Quote 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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    Senior Member gonzob's Avatar
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    Re: Pressure and flow

    Quote 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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    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

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    KJM
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    Re: Pressure and flow

    Quote 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

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    Re: Pressure and flow

    Quote 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!

    Quote 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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    Senior Member gonzob's Avatar
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    Re: Pressure and flow

    Quote 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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    KJM
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    Re: Pressure and flow

    Quote 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 ::)

    Quote 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

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    Re: Pressure and flow

    Quote 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.

    Quote 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

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    KJM
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    Re: Pressure and flow

    Quote 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 :o :o 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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    KJM
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    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

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    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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    Re: Pressure and flow

    Quote Originally Posted by 7B7A7D300 link=1327651715/28#28 date=1329291870
    OK - Ross - there isnt going to be any measurements.* High speed pressure transducers are basically outside my ability to fund.*
    No worries Kevin. It would have been an interested test, but my gut feel is that the electrical frequency would be too fast to have any noticeable effect anyway. Im more interested in the effects of mechanical vibration that can be felt by placing your hand on the machine during use. This effect would normally be very hard to isolate and test, but I might end up being able to test it during the R&D of another project. One of these days Id like to design a conventional electric machine (and as automated as possible) that incorporates the Rossa Hand Espressos fundamentals. If I manage to pull it off, it would certainly shake up the home espresso scene. So in the process of R&D for this project, I will be able to directly and objectively test the effects of mechanical vibration on espresso quality. If I dont die from old age before I get to do the test, Ill post the results.

    Quote Originally Posted by 6D6C6B260 link=1327651715/28#28 date=1329291870
    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.
    This would be true for any machine that pumps directly from the boiler. Air is a great dampener, but machines that use a heat exchanger through the boiler on the line that feeds the grouphead will not have any air in the system. Any air in the line would be pumped through the coffee puck as the water comes through. My electric machine uses a heat exchanger through its boiler, and I imagine that the majority of good machines would use a similar design. As previously noted, I havent seen the internals of many machines, so maybe a more knowledgeable person could comment on the use of heat exchangers.

    Quote Originally Posted by 6A4753414E4F4841675260475243260 link=1327651715/29#29 date=1329375246
    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?
    Thermoblock machines have a whole bunch of issues all to themselves largely resulting from the desire to design and build a cheap espresso machine. That said, Im sure the vibration from their pumps would not be helping their cause.

    Now that would be an interesting experiment! I might even dig out and test my old Sunbeam during the pump isolation test described above. Will definitely post those results.

  32. #32
    KJM
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    Re: Pressure and flow

    Quote Originally Posted by 033C2127322023213620203C530 link=1327651715/30#30 date=1329391333
    No worries Kevin. It would have been an interested test, but my gut feel is that the electrical frequency would be too fast to have any noticeable effect anyway. Im more interested in the effects of mechanical vibration that can be felt by placing your hand on the machine during use. This effect would normally be very hard to isolate and test, but I might end up being able to test it during the R&D of another project. One of these days Id like to design a conventional electric machine (and as automated as possible) that incorporates the Rossa Hand Espressos fundamentals. If I manage to pull it off, it would certainly shake up the home espresso scene. So in the process of R&D for this project, I will be able to directly and objectively test the effects of mechanical vibration on espresso quality. If I dont die from old age before I get to do the test, Ill post the results
    Surprisingly - vibration is dead simple to measure! You can just grab a piezo element and attach it - then look at the output with a DSO.. A cheap piezo beeper from Jaycar for example will have the element in there with 2 convenient leads attached ;)

    Alternatively (and not quite as responsive) - you can measure this with an Android phone 8-) There is an app in the marketplace that does this quite well...

    Neither of these cheap and cheerful techniques give you any sort of measure of the energy in absolute terms, but if youre looking at a vibrating pump you can pretty much work it out from the mass of the slug, distance moved and frequency..

    Cheers
    /Kevin

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    Re: Pressure and flow

    Quote Originally Posted by 576875736674777562747468070 link=1327651715/30#30 date=1329391333
    Air is a great dampener, but machines that use a heat exchanger through the boiler on the line that feeds the grouphead will not have any air in the system. Any air in the line would be pumped through the coffee puck as the water comes through. My electric machine uses a heat exchanger through its boiler, and I imagine that the majority of good machines would use a similar design. As previously noted, I havent seen the internals of many machines, so maybe a more knowledgeable person could comment on the use of heat exchangers.
    Many of the commercial machines out there use what I call a Test Tube exchanger. On these models the grouphead attaches directly to near the top of the boiler. Where it attaches there is a Test Tube that sticks down into the boiler at an angle. Fresh water is injected into the test tube at its top and leaves the test tube via a pipe/tube that extends from the grouphead down to the bottom of the test tube. With the test tube being a closed system other than the inlet and outlet it does not fill up with water and so the air in it provides a good buffer against any potential surges.


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    Re: Pressure and flow

    Quote Originally Posted by 556A7771647675776076766A050 link=1327651715/30#30 date=1329391333
    One of these days Id like to design a conventional electric machine (and as automated as possible) that incorporates the Rossa Hand Espressos fundamentals.

    Just make a small air compressor with a "profile" controller for the pressure, remove the handle on the Rossa and replace it with a pneumatic fitting, et voila` a Pneu-Rossa.

    Gonzo

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    Re: Pressure and flow

    Quote Originally Posted by 18191E530 link=1327651715/31#31 date=1329395071
    Surprisingly - vibration is dead simple to measure!* You can just grab a piezo element and attach it - then look at the output with a DSO..* A cheap piezo beeper from Jaycar for example will have the element in there with 2 convenient leads attached Wink

    Alternatively (and not quite as responsive) - you can measure this with an Android phone* Cool* There is an app in the marketplace that does this quite well...
    Thanks Kevin. Ill look into these options if I get to a point where I need to put some numbers against the vibration. At this stage Im more interested in determining the effect of vibration on espresso quality, which requires isolating the variable and tasting the difference. This will inevitably be a subjective measure, but I guess it will be a starting point at the very least.

    Quote Originally Posted by 19322532233B3A3F36530 link=1327651715/32#32 date=1329416259
    Many of the commercial machines out there use what I call a Test Tube exchanger. On these models the grouphead attaches directly to near the top of the boiler. Where it attaches there is a Test Tube that sticks down into the boiler at an angle. Fresh water is injected into the test tube at its top and leaves the test tube via a pipe/tube that extends from the grouphead down to the bottom of the test tube. With the test tube being a closed system other than the inlet and outlet it does not fill up with water and so the air in it provides a good buffer against any potential surges.
    That sounds like a heat exchanger with some form of air dampener incorporated; albeit a unique description. If your description is accurate, it would suggest that other designers have been confronted with the effects of vibration on espresso quality.

    What exact name and model of machine uses this system?

    Are you certain that air remains in the system during extraction? If so, would you describe the system in more detail?

    Quote Originally Posted by 343C3D293C31530 link=1327651715/33#33 date=1329423970
    Just make a small air compressor with a "profile" controller for the pressure, remove the handle on the Rossa and replace it with a pneumatic fitting, et voila` a Pneu-Rossa.
    Thanks for the idea Gonzo, but I would never encroach on another persons idea or design. Furthermore, I have no intention to complicate the Rossas simple design. I think its pretty well perfect as is!

    The machine I have in mind will not look anything like the Rossa. It will be completely different other than its ability to provide the user with total control over the variables. As Im sure people can understand, all else will remain a tad secretive for some time to come.

    In a move to redirect this thread back onto topic, did you manage to test the effects of filter basket shape, temperature and water contact time on pressure and flow?

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    Re: Pressure and flow

    Quote Originally Posted by 69564B4D584A494B5C4A4A56390 link=1327651715/34#34 date=1329453997
    That sounds like a heat exchanger with some form of air dampener incorporated; albeit a unique description. If your description is accurate, it would suggest that other designers have been confronted with the effects of vibration on espresso quality.

    What exact name and model of machine uses this system?

    Are you certain that air remains in the system during extraction? If so, would you describe the system in more detail?
    Looking at the hydraulic circuit for my machine it appears I had the inlet/outlet from the HX reversed, which means the air pocket inside the HX tube is smaller, but it is still present. Additionally there is also some air pockets in the grouphead itself to help in the dampening.

    Cimbalis as well as I believe Faema and Pasquinis (Owned by Cimbali.) use this design in addition to Im sure others as well. Download the manual at the bottom of this post to see the hydraulic circuit used in the Cimbali machines: http://coffeesnobs.com.au/YaBB.pl?num=1247055437/14#14


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    Re: Pressure and flow

    Quote Originally Posted by 6B54494F5A484B495E4848543B0 link=1327651715/30#30 date=1329391333

    This would be true for any machine that pumps directly from the boiler. Air is a great dampener, but machines that use a heat exchanger through the boiler on the line that feeds the grouphead will not have any air in the system. Any air in the line would be pumped through the coffee puck as the water comes through. My electric machine uses a heat exchanger through its boiler, and I imagine that the majority of good machines would use a similar design. As previously noted, I havent seen the internals of many machines, so maybe a more knowledgeable person could comment on the use of heat exchangers.
    I have to agree here. At 9 bar any air left at in the system (if any) is going to be either heavily compressed into a small volume, forced out with the water, or dissolved in it. Probably not going to be much damping effect. I wouldnt think there would be too many places for air to hide in there anyway (shower screen excepted)?

    The boiler is a different story though. Im presuming that the pumps are generally upstream of the boiler?

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    Re: Pressure and flow

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    Quote Originally Posted by 4E7149626068030 link=1327651715/36#36 date=1329532073
    The boiler is a different story though. Im presuming that the pumps are generally upstream of the boiler?
    Every machine Ive ever seen has the pump on the inlet side. So the pump operates on room temperature water to avoid problems.

    Im also pretty sure the E61 head designs end up with entrained air in the top as a consequence of the way they do the pre-infusion.

    Got to rush!
    /Kevin



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