LaSpaziale uses the same mechanical part used for the expansion valve in the VI and VII to regulate group pressure in the Mini. In the VI and VII this part has the group pressure on it and is set to dump to the drip tray if boiler pressure exceeds 12 bar. In the Mini this part is actual in-line with a tube that goes from the output of the pump to the input of the pump and is set to 9bar (or whatever group pressure you set it at).
Inside that valve is a heavy duty spring with a rubber disk on the end which pushes against the end of a metal tube. This keeps the valve closed until water pressure overcomes the spring tension forcing it open and allowing water to pass.
Consider how this acts when pulling a shot with the Mini. Pressure builds from the pump until it exceeds 9bar, then the valve opens. When it opens and water flows through it, the pressure is reduced and the valve shuts. Immediately the pressure exceeds 9bar and the valve opens again. This process repeats during the entire shot. You see this as the vibration of the needle in the group pressure gauge. To me this means that the minimum and maximum needle readings when it is vibrating are real pressure changes seen at the output of the pump. I would think you'd want the average pressure to be 9bar which means you should set the pressure so that the needle vibrates evenly around 9bar. The pressure gauge reads much closer to the pump than to the group head. With luck the additional plumbing has a dampening effect on this pressure vibration by the time it gets to the puck.
I think there are two take aways from this:
- - Set group pressure so that the reading you want is the mid-vibrate needle position when pulling at shot
- - The constant opening and closing of the valve is going to wear the rubber disk at the end of the spring MUCH faster than in the VI/VII. The next time you order a new group gasket or any other items from Chris Coffee, consider tacking a couple of those rubber disks onto your order(part #291 from the LaSpaz catalog)