I just finished building and installing a bolt-on, programmable pressure profiling pump system to my uber-stable LM Frankenlinea. I did this because I wanted to test prevailing ideas about preinfusion, the benefits of lever machine pressure profiles, etc. on a very stable platform on which selected brewing parameters may be quickly changed while holding others constant to values which are the current state of the art. At the same time, if the tests warranted, I wanted to make the system transparent enough that I could put together similar systems so that interested folks could just do a pump swap, with no extra effort involved. Depending on how this all goes over the next few weeks I may end up writing something more comprehensive, but for now I'll give progress reports as I learn stuff.
Acknowledgements up front for folks who who ought to be recognized: Andy Schecter implemented brew pressure profiling over a year ago, and should be credited as being a true pioneer on this. I'd like to thank Michael Teahan for tipping me off to a source of the required pump, and to Clyde Smith at Fluid-o-Tech, for his advice and encouragement. I should also mention here that the path I'm treading is not new. There is a body of work that has recently been performed by machine manufacturers, but I am not privy to details.
As I said earlier, the idea here isn't necessarily to pioneer any device, but to test the idea of pressure profiling in a reasonably systematic way, look at any perceived benefit from pressure profiling, publish the results in places where the results might have some impact, and in the end maybe make it possible for others to tailor brew parameter space for themselves if the results warrant.
Now, a little about the device: Unlike Andy's, the programmable pressure-profiling system (PPPS) is built from an array of commercially available parts. The pump is a rotary vane pump, which is magnetically coupled to 3-phase magnetic drive. Power is supplied to the pump drive by an accompanying 3-phase, variable-frequency converter that operates the drive over the range from 1100 to 3500RPM. The RPM of the pump is varied in response to an output signal provided by a fast-acting, industrial process controller. An electronic pressure transducer mounted in the water delivery line to the espresso machine provides a feedback signal to the process controller. The pump / drive combination is really quite interesting. Power consumption of the magnetic drive is 1/3 the power required by a traditional AC motor / pump combination. The system also has reliability advantages. Since the pump is magnetically coupled to the drive, there is no shaft seal to wear out. The pump plumbing is configured so that most of the water delivered by the pump impeller is recirculated to the pump inlet. The amount of recirculation is adjusted so that the pump runs at nearly maximum speed when the output pressure is approximately 140 psi. This sets the pressure range of the pump from almost no pressure at 1100 RPM to 140 psi at slightly less than 3500 RPM.
The test machinery: The test platform includes equipment considered to be state-of-the-art within the coffee industry. The pump is installed on a 2-group, LM FrankenLinea AV. Water supply to the pump is from a 5-gal. plastic carboy, filled with carbon-filtered water conditioned by an acid neutralizer and solar salt ion-exchange softener. Unlike a stock Linea, the Linea used here has no exposed plumbing. Flowmeters and group solenoids are mounted on top of the group caps, with water passageways integrated into the cap design. Temperature stability and reproducibility of this machine is better than 1 degree F under all duty cycles. Temperature control is achieved using a boiler mounted thermocouple, with industrial fuzzy-logic process controller. The machine has 0.6mm gicleurs installed in both groups. A Mazzer Robur is used for grinding the coffee. Coffee is roasted in-house (literally, since that's where Espresso Research Company HQ resides) on a Has Garanti drum roaster with 1 kg. capacity.
Progress: I've got the pump installed on the Linea and I've learned how to control the pump. The coupled system of pump and espresso machine produces an interesting control problem on startup. Cool makeup water entering the boiler expands, raising the boiler pressure to 12 bar. On pump startup, the brew solenoid opens, ejecting a small amount of water. At the same time the pump activates, with speed controlled by the process controller. The controller at first detects falling pressure, and speeds the pump to compensate, resulting in a pressure burp on startup. The problem is reasonably managed by selecting the fuzzy logic control parameter in the controller, and assigning an initial pressure setpoint of around 1.5 bar, rather than my initial setting of zero. The controller now detects a falling pressure, but the response is to allow the pressure to fall, ramping quickly to 1.5 bar for 1 second, then initiating the programmed pre-infusion cycle. There is no issue of pressure burping if the pump is configured to mimic a normal AC-motor-driven pump. In this case, at startup the pump speed rapidly increases to that which is required to support 9 bars pressure as measured in the group.
I have loaded a program into the controller that mimics an e-61 pre-infusion profile on startup, with straight-line (linear) increase in pressure over an 8-second period to 9 bars of pressure. The pressure profile remains constant at 9 bars for 5 seconds, then reduces linearly to approximately 7 bars at pump cutoff. The pre-infusion result is visually interesting to me in that liquid appears at the same time over the entire surface of the brew basket (bottomless portafilter), indicating that saturation of the cake is occurring more or less completely. It's intuitively satisfying, at the least in that the subsequent liquid flow buildup is uniform and very similar to what I have observed from e-61 machines. The effect of the pre-infusion ramp on required grind fineness is that my Robur needed to be set to produce finer grind by several divisions of the knurled outer surface of the adjusting collar. This is similar to what I experienced when reducing gicleur diameters in the Linea during comparison tests against a pre-production LM GS3 last winter ('05 - 06). I attribute this to reduced "slamming" of hot water into the cake, reduction of fines migration, or whatnot. Anyway it seemed reasonable to me based on past experience.
The pressure tail-off is an arbitrary exercise just because I could do it, and because Andy Schecter reports some benefit. It appears possible to control the flow rate of the extraction by controlling pressure tail-off, and by doing so somewhat delay the onset of blonding. I don't know much more than that at this time. Paul Pratt kindly sent me the pressure profile of his Faema lever machine, which I will try when I get a little more orderly in my methods. Essentially his machine starts at near zero pressure, builds very quickly to 8.5 bars (piston release), and then reduces to 3 bars at the end of the extraction. I'm presuming the pressure reduction is linear since the piston is spring-driven.
Taste - Obviously this is the important, bottom-line issue. Qualitative observation of extraction is only meaningful if it can be correctly correlated to optimum taste. It's REALLY early to draw any meaningful conclusions. But I didn't screw up by installing the pump system. The coffee is at least as good as I was producing before, and I was doing a pretty good job prior to the pump install. I've not yet had time to alternate back and forth between straight 9 bar extractions and profiled extractions, because my first efforts have been in sorting out the system. I hope to do testing soon that will be reasonably valid. I may bring another conical grinder home to pair with the Robur so that I can one grinder correctly for profiled shots and one for constant pressure shots. And I've got to find some test victims - maybe local folk that can really hold their coffee.
This is cross-posted at alt.coffee
. Both sites are not necessarily visited by HB folk, and both sites have different target audiences.