As far as I know, there are three espresso machines currently on the market (or soon to be) that offer adjustable brew pressure: Synesso Hydra, Slayer, and La Marzocco Strada (in 2010). There's a lot of info and a lot of hype surrounding all three and it's often difficult to cut through the excitement and get to brass tacks. Let's talk base comparison:
All three have PID, paddle groups, similar steam wand design, dual boilers, etc. and the main difference seems to be in the method by which one changes the pressure. Slayer uses pre-programmed "notches" in the paddle to determine pressure whereas the Hydra and Strada use a open, sliding grade making it more difficult to replicate shot per shot but giving the barista more overall control. Aside from that, the pros and cons seem to be about availability and aesthetic. For instance, I might love the wood groups and programmable pressure but unless I live in Seattle or have money and time to fly a tech across the country to fix my machine, I shouldn't go for Slayer.
I know that many of you have been able to play on these machines and can offer a perspective far more in depth than anything I can glean from internet articles. So waddaya say? Am I off base?
As far as technical differences I believe the Synesso and the La Marzocco actually have four boilers (correct me if I'm wrong), one for each group then a steam wand boiler. The Slayer has five, one for each group, one for the steam wands, and a boiler at the beginning of the chain that heats the water entering all the other boilers. From talking with the Slayer guys at SCAA, I was informed that the pressure allowed through on the first "notch" is changeable via a dial just behind the group, the second "notch" is full pressure from the pump. Also, as of now, I believe the prototype Strada has "knob" style steam wand valves, whereas Synesso and Slayer have opted for the lever type; seeing as Strada is still just a prototype this may (or may not) change. There are also the competing ideas of multiple pumps (Synesso) and single pumps with pressure regulation at the group (Slayer), not sure if the full implications of these two setups has been fully discussed here on BX or other forums. There are some little differences too, for example Slayer gives you a handy little built in "mirror" where the drip tray and body of the machine meet so that you can see you shots extracting without bending over, I'm sure everyday Synesso users can chime in on the little perks that the Hydra brings to the table, and those who were at "Out of the Box" can give us some examples from Strada.
I'm not clear on something: are you planning on buying a machine now? If so, the La Marzocco will not be for you since it's realistically about a year (or more) from production. Lots of great stuff going on with the Strada, and if you're looking for the greatest machine out there and can wait up to a year (or more) then the Strada will be worth waiting for.
But if you need one now and need a pressure profiling machine, then the Synesso and Slayer are your only machine choices. Greg Scace has developed a pressure profiling adapter that will retrofit to any machine and create variable pressure during the brew cycle, but there are certain limitation s to that device based on your current machine.
The difficult part of either Synesso or Slayer is their lack of national service. La Marzocco has the advantage of a large service network whereas Synesso service is spotty and Slayer hasn't had enough time to establish any sort of service network due to its' infancy. This means that you should be reasonably comfortable with doing machine work yourself.
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