Something strange has been going on with Silvia for a few days now. The shots are starting out faster than expected for the given grind setting/dose and then getting slow after approx. 10 seconds, yielding a very short volume shot compared to what I've always gotten using similar grind/dose.

Initially I thought it might be a grinder problem, but after more experiments today, it is doing this with both my grinders. It is unlikely both grinders somehow became damaged at the same time. Before anyone asks, the coffee is from top level roasters and is fresh...definitely not the coffee.

Are there any potential mechanical issues in the Silvia (4.5 yrs old) that could be causing this?

Thank you for any replies.

did you clean the shower screen?

Clean screen, routinely backflushed. Everything has been fine for 4.5 years and all of a sudden this change happens seemingly without anything else changing.

Try running the pump without coffee in the PF and measure the flow rate at various intervals. e.g., during the first 10 seconds, second 10 seconds, etc. If you can build or borrow a PF pressure gauge, check pressure at the group head during a 30 second pull.

Can't think of a cause offhand, but maybe a bad pump (rate varies as it heats up) or some air is seeping into the input hose somewhere along the line.

Thanks Peppersass - I came across the following method as an easy way to approximate pressure and flow rate:

"Place a blind filter into the group handle, lift the water return hose from the tank so it will empty into a calibrated container. Turn on the brew switch and time, say, for 20 seconds, noting how much water is pumped into the container in that time. If it works out at 250 mls per minute -- the machine is doing fine, set for ideal extractions."

I did two tests and wound up with very high rates in the 450ml - 525ml per minute range. So my pressure is way too high? How/why would this happen?

djmarc1200 wrote:I did two tests and wound up with very high rates in the 450ml - 525ml per minute range. So my pressure is way too high? How/why would this happen?

How long did you run the test? Your original post indicated that the shots start out way too fast, then slow way down. Did you run the tests at least the length of time it would take for a shot (20-30 seconds?) If you ran the tests for a shorter time, you may only be seeing the window in which the flow rate is very high and are not seeing when (or if) it slows down.

For some additional insight into pressure and flow rate, check out this post from Eric Scace regarding flow rate and pressure for the Ulka EP5 pump used in Silvia: The importance of brew pressure, purpose of adjusting OPV?

The chart indicates that your information is correct: 250ml/min is just about right for 9 BAR pressure. So it certainly doesn't seem right that the flow rate has increased to 450-525 ml/min.

One thing I don't understand about the chart is that flow rate is inversely proportional to pressure. Evidently, the pump can deliver more water at lower pressure than at higher pressure. I've always thought it was just the opposite: the higher the pressure, the more water you would get per time interval (sort of like electricity: the higher the voltage, the more current you get for a given resistance.) But I guess it means more pressure slows the pump rate down. I'm not sure whether this means your pressure is too high or too low.

I'm also a little confused about how flow rate relates to extraction. For example, my La Marzocco GS/3 runs at about 420 ml/min, and I've been told that's ideal. The spec from LM USA is 500ml/min +/- 10%. So why would the ideal flow rate be different for the two machines?

Perhaps the experts could comment on these apparent anomalies?

The high flow rate you're measuring correlates with the shots running unexpectedly fast in the beginning. Perhaps once the coffee cake compressed, the increased pressure is slowing the flow rate. It could be a problem with the OPV or the pump itself is worn out, but that's just a guess.

erics wrote:No, your pressure is way low - caused by a OPV that is stuck partially open or as a real long shot, the OPV spring has broken. Replace the OPV internals and the copper sealing washer and you should be in good shape.

I also think this is the most probable diagnosis. Since scale is a likely cause, Marc may be lucky enough to be able to fix it by forcing water through the OPV (blind filter in basket) while descaling. Disassembling and cleaning would be better, of course, but is also a lot more work

Jim

Thanks for your help so far guys.

I got the opv removed and cleaned, but there is still a small problem after re-installing. I am getting steam/water coming back through the overflow tube once the boiler nears temp. I think the problem is lack of a proper seal somewhere inside the opv?

One parts diagram I have seen lists a rubber gasket, while another does not.

Any insight into this? Or perhaps pictures of a disassembled opv with all of the parts?

Thanks.