Greetings everyone. I'm a newbie and this is my first post, so please be gentle with me ;-)
I got curious about espresso and how it's made and have spent the past few days reading up. I've got to a question which bugs the hell out of me and hope you can help. I'll start with what I understand so far.
The process of making espresso requires passing water at the right temperature (90-96C) and at the right pressure (8 to 10 bar) at the right flow rate (60ml per 20-40s roughly) through a puck of coffee. Now as far as I can tell, the puck is essentially the place where the pressure drops from the brew pressure down to the atmospheric pressure of your espresso cup - i.e. when you say "brew at X bar" it means "make the pressure drop across your puck equal X bar for the duration hot water flows through the puck".
How is this pressure drop generated? Well, it seems the vast majority (by number) espresso machines use an Ulka vibratory pump. How does this pump make pressure? As near as I can figure it out, there's a little piston that moves in an out at the AC supply frequency, which each piston actuation forcing a small amount of water from the low pressure water reservoir into the high pressure brew supply.
So here's the thing I don't get. The vibratory pump by definition cannot generate constant pressure. The pressure it generates must range from zero (just before the piston injects the new load of water into the brew system) to twice the average pressure (just after the piston has actuated), over the time of a single AC wave. In fact, it's a little more complex than that as there's also an OPV valve, which may well truncate the pressure at a certain level (though I'm not sure if the time response of the OPV valve is quick enough for AC), but let's leave that aside from now. On average, I can well see how the vibe pump is generating a 9 bar pressure drop across the puck. But in reality this pressure drop is made up of 18 bar half the time and 0 bar the other half of the time. So what's the actual "brew pressure"? And can it be that important if it can have such a wide range during the time you're pulling your shot?
The theory of operation and the practical reality of vibratory pumps are both discussed in great detail here and on home-barista.com. However, finding exactly what you want to know can be an exercise in crafting search terms. There are two or three manufactures of these pumps and their web sites offer some good information, too.
There is an interesting discussion on HB about adding a household dimmer control to the pump mains feed in order to attenuate pressure.
Hope you find what you want, sorry not to be more precise.
And welcome to the family.
I play go. I use Macintosh,. Of course I ride a recumbent.
What I mean by that is it is easy to get tied up in small details (yes they do matter) and miss the big picture.
The pressure drop is achieved by the bed of coffee being semi permiable (sp) On one side of the coffee bed (puck) you have the pressure from the pump as the water slowly flows through the bed of coffee which is offering resistance to flow, the water starts out at the regulated pressure from the pump and makes it's way through the coffee. By the time it reaches the other side of the puck, it is at room pressure, the coffee bed it's self is what offered the resistance and as the water works through the puck the pressure required to move through the coffee becomes less and less as the water gets further and further through the puck. The natural result is the coffee is at 1 bar pressure or atmospheric pressure at the exit from the puck.
The pump cycles at 60 cycle, this is 120 changes of direction or pulses per second. It is not a piston but a rubber chamber that gets bigger and smaller as one wall vibrates in and out of the chamber because it is attached to the rod which is in the center of the coil that is "the pump" There is a one way valve to let water in and a one way valve to let the water out so it can only flow one direction. The chamber pulses much faster than the water can flow, the result is that the pressure is built up in the chamber. The OPV drains away anything OVER the desired brew pressure either back to the water tank or to the drain system, most machines cycle to the water tank.
Yes the water is pulsing before the OPV valve but it is pretty constant after the OPV valve so you have a stable pressure at the coffee puck.
The role of the grinder is to grind the beans to a fine consistency that is tailored to the age of the beans, roast, humidity the volume of coffee grounds etc, so that the regulated pressure of 9 bar takes apx. 25 seconds to make either 1 or 2 shots of water pass through the bed of coffee, depending on how much you prepared to make.
Vibe pumps are very common on starter machines and can be found on more expensive machines too. The Rotary pump is used in commercial and many higher end prosumer machines but the rotary pump costs much more than a vibe pump.
In real life, my name is Wayne P. Anything I post is personal opinion and is only worth as much as anyone else's personal opinion. YMMV!
Feed the newbs, starve the trolls and above all enjoy what you drink!
I guess what you're saying is that while the pressure at the exit of the vibe pump pulses, the combination of the OPV valve and all of the volume of water between the exit to the vibe pump and the coffee all act as a damper (or a Low Pass Filter) which smooth the pressure waves to mostly a constant component by the time you get to the coffee. This explains why there isn't a strong body of opinion which believes that vibe pump coffee tastes fundamentally different in nature to rotary pump coffee - at least that I've found so far.
And I see what you mean about the coffee puck experiencing a pressure gradient across it. This just raises another annoying question though - if I could make the puck area twice as big (and hence the puck half as high) could I get away with half the pressure difference across the whole puck (but still the same pressure gradient as before) and still get espresso? But you're right - I probably am overthinking it!
billc Senior Member Joined: 15 Jan 2009 Posts: 130 Location: Seattle, Washington Expertise: Professional
Espresso: CC1 - GS - GS3 - GB5 Grinder: Baratza - Mazzer-Marzocco Drip: My own Creation
Posted Sat Jan 18, 2014, 8:49am Subject: Re: Vibe Pump Pressure Variation
Hey! You are not over thinking this at all. You are asking all of the right questions that are fundamental to the design of espresso machines.
You are correct about the vibe pump and calblacksmith has identified most of the physics behind it. Since there are one way valves in the pump and water flows only one way, inside the pump you have a pressure variation from 0-max on each stroke. Just outside the pump this is dampened somewhat because the pump flow rate is greater than the flow rate through the coffee. This is what creates the pressure differential. If no coffee then the flow rate of the pump is the same as the flow rate through the group head and the differential is zero (hence zero pressure on the gauge).
A rotary pump has such a high flow rate that even with no coffee there is still a difference in flow rate but is compensated with an internal bypass.
The pressure gradient through the puck is quite interesting. The diameter of the puck is some what limited by the materials we use. This is because of the force the water exerts on the portafilter at 9 bar pressure. You will notice that lower cost machines have smaller diameter portafilter baskets. This is mostly due to a lack of OPV and the machine allows max pressure form the pump to be exerted on the portafilter. Additionally the materials used in the group are aluminum or aluminum alloy (to make it less expensive).
There have been larger portafilters but the performance has not been great. There is a relationship between the diameter and the height of the coffee cake to make perfect espresso. This is why it is hard to duplicate the results of a double basket with a single basket. Those who have invested in the experiments keep the results secret because they reveal some pretty interesting stuff. And usually provide the company with valuable information about the roast and quantity of coffee to use for their recipes.
Posted Sat Jan 18, 2014, 5:13pm Subject: Re: Vibe Pump Pressure Variation
The pressure gradient through the puck is quite interesting. The diameter of the puck is some what limited by the materials we use. This is because of the force the water exerts on the portafilter at 9 bar pressure. You will notice that lower cost machines have smaller diameter portafilter baskets. This is mostly due to a lack of OPV and the machine allows max pressure form the pump to be exerted on the portafilter. Additionally the materials used in the group are aluminum or aluminum alloy (to make it less expensive). BillC
Thanks you guys. I can testify! My old Salvatore had a 52mm massive brass filer holder and grouphead but there was no over-pressure valve in the system. The Ulka is still working solidly after 12-13 years (35-40,000 shots) even though I choked it often by overdosing and going too fine on the grind.
BTW, google, google images and yahoo turned up dozens of posts and threads about Ulka pumps but also the company's website, hundreds of photos, several exploded diagrams, performance curves and rebuild stories.
I play go. I use Macintosh,. Of course I ride a recumbent.
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