I'm going to eat some crow now, because for 2-1/2 years my eyes and ears told me it made no difference where you put the paddle during the line pressure preinfusion phase.

I did an experiment tonight. I put a Post-it on the top of the brewhead and marked the point where the line pressure flow started to be more than just a trickle and the point where the pump came on. Then I marked two points at about equal intervals in between. So I had three points marked within the line pressure/preinfusion range.

I let the water run for 20 seconds at each of the 3 marked points in the line pressure range and then repeated it. I "stopped" the flow by pulling my beaker away when the timer hit 20 seconds. So these are clearly not scientific measurements. Weights are in whole grams.

1st Run
81
128
117

2nd Run
90
129
117

Since the third (closest to pump-on position) was not the fastest flow, I assume that some element of flow interruption happens just before the valve reaches pump-on position. I'm not sure this will make the MP useful for flow profiling. On the other hand the earlier poster's idea of a manometer on top just might do the trick (if it doesn't destroy the machine).

Thanks, Marshall. This is curious, although in any mechanical valve there's likely to be some variation in flow, but I have no idea where the roughly 2:3 range you've measured would fall within those expectations.

But this flow was measured without any load, right? So how this might translate to true and usable pressure variations into resistance is still unknown; that crow in your oven may not be quite up to temperature yet.

From what I understand, this is just a flow experiment, not a pressure experiment, right? You aren't necessarily saying that the position matters because the pre-infusion pressure is variable, but more because the flow is quite variable, right?

Thanks Marshall for posting your results. I've only used the automatic (sans paddle) GS/3; when the paddle model came out, I assumed it was the same as the Strada except for the gauge. Evidently not...

Marshall wrote:Since the third (closest to pump-on position) was not the fastest flow, I assume that some element of flow interruption happens just before the valve reaches pump-on position. I'm not sure this will make the MP useful for flow profiling.

For what it's worth, the pump on the Strada comes on with the slightest movement of the paddle, but no water flows until the paddle nears the midpoint. As I've noted in other posts, I've used this "crossover point" between the valves to tweak the pressure at the end of the shot (it's fairly easy to hit 6 bar consistently; of course not with the consistency of the programmable EP model). From what you describe, it sounds like they moved the GS/3 MP pump-on switch to allow for line pressure preinfusion. I've thought of adding a pump cutoff switch to enable the same capability on the Strada, i.e., line pressure preinfusion for the midpoint.

Marshall wrote:On the other hand the earlier poster's idea of a manometer on top just might do the trick (if it doesn't destroy the machine).

I would need to compare parts diagrams, but I've always assumed the main difference between the Strada/Shotpuller grouphead and the GS/3 MP grouphead is the riser mechanism on the former has a hole drilled through it to allow for the grouphead pressure gauge. That is, no modifications are necessary, just expensive parts.

In retrospect, since only the preinfusion flow rate is variable (and even that only to a small degree), I would not expect any significant benefit from a manometer to gauge it.

The only time I pay attention to the pressure readout is when going from pre-infusion to get to 9 BAR. Once there, I listen to the pump and keep it running at the same RPM (which keeps my flow constant) until near the end then I fade down. Other than that, I don't see the need for the pressure gauge on a shot to shot basis. If the most your machine will produce is 9 BAR, then you don't even need the gauge at all unless you can't approximate the pre-infusion.

I don't know if you should be doing this on your valuable GS/3. It'd probably be best to leave it to a professional. Just sayin'.

You could probably talk to the LM guys about the feasibility of modifying the pump microswitch to get you pump power from the start on your GS3 MP, Marshall. This would let you narrow and widen the orifice to controll flow above the puck, giving you indirect pressure control. You would definitely need a few Strada MP parts to get the gauge above the puck, though... otherwise you would be flying totally blind.

Thanks, Nicholas. I don't really have any interest in going down that path. This was more to satisfy my curiosity and correct some misinformation I had posted in other threads. E.g. Possible to install group-mounted pressure gauge on La Marzocco GS/3 MP?.

I'm very happy running my machine the way it is.

haunce wrote:I don't know if you should be doing this on your valuable GS/3. It'd probably be best to leave it to a professional. Just sayin'.

When I want custom work done, I leave it to someone who knows what they are doing. This 2007 mod served me very well for several years: http://home.earthlink.net/~mrfuss/.