Ok, so I have to confess -- I've repla....err, supplemented my beloved PID Silvia with a La Spaziale Vivaldi II (bought from Chris Coffee).

I am loving the machine to death, particularly for the ease with which it makes making milk-drinks (cappuccinos, lattes, etc.) compared to Silvia. It is also much easier to produce consistent shots with than Silvia although the best shots from Silvia are as good as the best shots from Vivaldi.

I am curious about one thing, however, and that is why the group pressure gauge always read 9BAR (or whatever I've adjusted the pump output to) when the pump is running. In Silvia, 9BAR represented the MAXIMUM pressure (i.e. the pressure at which the overpressure valve opened). If I run Silvia with no portafilter in the group, the pressure is less than 1BAR. This was actually a very nice diagnostic tool to tell me when I hadn't gotten the dose and/or tamp right.

But in the Vivaldi, I see 9 bar on the pressure gauge even with no portafilter in the group. From the diagram, the water flow is:

INLET->PUMP->PRESSURE VALVE->GAUGE->GROUP BOILER->GROUP->DRIP TRAY

I do not see how the gauge could show any pressure absent any back pressure or flow restrictor downstream of the gauge. I called Chris Coffee and they assured me this was normal for a rotary pump machine (as opposed to Silvia's vibratory pump) but I am at a loss to understand how. And I are un unguneeeeer.

The rotary pump is able to sustain a much higher flow rate than the vibe pump.

Ok, I get all that (that the rotary pump has a higher capacity). But explain this:

The Vivaldi does line-pressure pre infusion. I have my line pressure set at three bar (~40PSI (editorial aside: why do espresso machines insist on being calibrated in BAR, instead of PSI? That's stupid)). I preinfuse for 3 seconds.

If I take out the portafilter and hit the brew button, water starts coming out of the brew head per pre infusion and the machine's pressure gauge shows 3 bar. Then the pump comes on and the pressure gauge leaps to 9BAR.

Yet the flow out of the group remains unchanged, or changes so slightly that I cannot tell.

How is that possible if the pressure at the group head is going from 40PSI to 130PSI?

I guess I could see this if there is some kind of restriction between the group shower screen and the group boiler that lets only a very small volume of water through no matter what is in the portafilter, thus acts like a gicleur between the group boiler and portafilter. The VII Water Flow diagram does not show one, but it would make sense.

More I think of it, the more it makes sense. Rough calculation is that the 1/4" inner-diameter plastic tubing feeding the machine water at 40PSI can deliver 7 gallons per minute of water.

Filling a double shot glass with water at 25 seconds per two ounce of water is 0.0375 gallons/min. In other words, nearly 200 times less than the water line is capable of delivering.

nitpick wrote:I guess I could see this if there is some kind of restriction [...]

There is in fact a flow restrictor on the S1. It creates the whoosh sound you might hear as pressure rises during the beginning of a shot.

You can see it in the Vivaldi parts catalog, part 276 if I'm not mistaken.

Louis wrote:There is fact a flow restrictor on the S1. It creates the whoosh sound you might hear as pressure rises during the beginning of a shot.

This forum is incredible. "What the **** is that ****** noise?" was going to be posting #2 today.

All pressure is relative to where you are measuring. In most cases the pressure gauge on an espresso machine shows the pressure inside the boiler not in the portafilter.
Rotary pumps also have an OPV called a relief valve. Most of the rotary pumps have enough capacity so the relief valve is circulating water all of the time. This allows you to set a pressure and have it remain constant over different flow rates. When the flow demands exceed the rating of the pump (like when the steam boiler is filling and you let the machine free flow through the group) then you usually see a pressure drop on the gauge because the pump cannot keep up with the demand.
The water path from the boiler to the coffee usually has a valve with a small orifice (about 1-1.5mm diameter) and some tubing (tubing has resistance but is usually negligible compared to the valve orifice). Additionally allot of machines have a flow restrictor in the form of a small diameter orifice (some as small as 0.5mm) that restrict flow even more.
If you were to let your machine free flow through the group and the gauge is reading 9 bar, there is a pressure drop from the boiler to where the water is exiting of 9 bar (because the water is exiting to the environment at 0 pressure (Gauge). This just shows that pressure is relative to where you are measuring.

You see the pressure drop in a vibe pump machine immediately because the capacity of a vibe pump is very small and you exceed the capacity unless coffee is resent as a flow restrictor.

I will stop now because I am rambling and have to go coach my son's baseball team.
BillC