Zevi Senior Member Joined: 28 Mar 2013 Posts: 124 Location: Ann Arbor Expertise: I love coffee
Espresso: Quick Mill Vetrano 2B Grinder: Baratza Vario
Posted Thu May 9, 2013, 10:54am Subject: Re: Reviews of Vibiemme DD V 3.5, esp. compared to Duetto 3
Have you any scientific evidence or done any testing to verify that a PID'd sensor cannot respond quickly enough (to pressure changes and thus temperature changes) in the steam boiler? While I was an art major and am just theorizing based on what little science I have in me, I believe that it is Boyle's Law (among others) which would indicate that pressure changes are directly proportionate to temperature, so the delay in switching of the heating element would be tied primarily to the response time of the thermocouple. The water holds a LOT of thermal energy and as the pressure decreases this energy is released as steam. While we could argue that the pressurestat may react sooner, the amount of time differential should be insignificant compared to the amount of time it takes for the heating element's temperature to rise above that of the surrounding water. The proper design of the thermocouple, again I am guessing, would make the contest between the pressurestat and the PID even closer to a tie in this application. With the added dependability of the thermocouple compared to the PID would seem to make the PID the winner in this contest, all other things being equal. One less pipe, one or two fewer pipe joints to leak, fewer wires to direct through the machine, etc.
The added benefit is that if there is already a PID for the brew boiler, the addition of one more thermocouple and a small amount of additional wiring is all that is needed - actually, less wiring than the pressurestat would require. pressurestats, being a mechanical device compared to the PID's electronics would make the pressurestat a "wear item," that would need replacing eventually (worn electrical contacts, leaking diaphragm, clogging from scale, etc.).
The Duetto is a fine machine, and if I had to choose between the two it would be difficult. I own the Vibiemme DD (Heck- I wrote the owners' manual!). In E-61 machines, the design of the Vibiemme group is unsurpassed, and dare I say, unequaled. The work done by the US importer, 1st-Line, has made the temperature stability excellent in this machine. On the other hand, there are some oddities in this machine I wish they would change. The case design is dated and noisy. The drip tray screen could be a better design. But as I stated earlier, I would not choose the Duetto over the Vibiemme because of the the PID vs. pressurestat issue alone.
In either case, I am a huge fan of the simplicity, dependability, and ease of maintenance of the exposed E-61 group manual group. I actually created an anniversary poster to commemorate the 50th anniversary of the group. But between these two machine it would be a tough choice. I took the DD in trade for the manual, so am still quite glad I had the decision made for me as even today, just a month short of two years of use, I am not sure which I would choose.
I do like the placement of the lights, PID control, and PID display on the Vibiemme. The lower profile of the Vibiemme drip tray area is more pleasing to my eye. The valves on teh Vibiemme are easy to "overhaul" without opening the machine. Don't know about the Duetto. The v.3.5 Vibiemme actually costs less... I cut a hole for the 3-way discharge in the drip tray cover yesterday and that is now a factory feature, and the new boiler insulation is very nice (which I think I will be doing to mine in the future. I wish I had the ability to test them side by side. it may come down to footprint differences with the Vibiemme being deeper, and the Duetto being wider, but really, I think it comes down to a coin toss.
Excellent assay, Randy! But more importantly - candid. Owning espresso machines of that class is often driven by passion, and passionate people often tend to defend the virtues of their "object of desire" while overlooking its shortcomings. You didn't fall into that trap in this matter of PID vs. pStat.
As an engineer, I think that both you and Keith are correct, although Keith is "more correct" given the context of this discussion. The physics law that govern the pressure/temperature relationship is Gay-Lussac's law (Boyle's_law is about presure and volume, although these are related laws). The point is, that Gay-Lussac's law concerning the relationship between pressure and temperature applies to a sealed system: increase the temp - the pressure will increase at a given ratio and vise versa. In the case of the boiler in espresso machine however, we open the system and take out steam. Yes, the pressure drop will cause a drop in temperature - but it will be delayed as the "heat mass" of the water and boiler will keep things hot. The good news (for the PID) is the existence of technology to put a much finer-resolution electronic temperature sensor than the mechanical pStat.
Having said that, I totally agree with you, Randy that the PID/pStat question alone should not the deciding factor. I just want to be in a position that once my decision is made -- I walk into it with open eyes (posts like this of a failed hightly-acclaimed Jaeger pStat after about a year would make anyone concerned). It's not so much the cost/effort of replacing the sensor -- it's the frustration of dealing with a non-functioning/degrading machine and the whole troubleshooting process. But I already said that "owning espresso machines of that class is often driven by passion", right? ... :-)
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