I've been doing a pressure study over the weekend to understand what is going on in the basket relative to what I see on my Mara brew pressure gauge and I thought I'd share the results in case you've been wondering too.

This first plot shows two extractions overlaid. One with a trickle preinfusion of 30 seconds and the other with no trickle. On the trickle pull I goose the pump momentarily every 10 seconds just to keep the juices flowing ... as seen on the Mara gauge (blue curve).


You can see where the preinfusion chamber is filling in between the early part of the yellow and gray lines, and it is also interesting to note that apparently somehow the preinfusion chamber is filling during the trickle on the other pull (I'm guessing ... since the two pressures track almost identically once the pump is full on). Still need to digest these results a bit. Please fell free to share your insight if so inclined.

This next plot shows that with the E61 group you can get a decent tailoff at the end of the shot by simply turning off the pump without opening the drain valves (going back to the just before midpoint spot on the brew lever when the pump just turns off). If you are setup to do the trickle preinfusion you are setup to do this too.


And finally, just for kicks, here's a blind basket. There must be something to be learned here since the only two things I can think of that are happening are air compression and preinfusion chamber filling.


Again, if you have thoughts or insights, do share. And thanks Eric, for continuing to make your pressure gauge enhancement!

Happy Pulls!

Jim,

Thanks for sharing this. I'm fascinated...

Are you using the "E61 bolt" where a thermometer would mount to get these group readings?

Cheers!

- Jake

Yes! Eric makes an add-on pressure gauge kit that uses the same adapter as his thermometer. Really convenient ... no need to touch the crush washer or remove the adapter.

Excellent.

Any idea what size gicleur you have in there? I am surprised by your results...

*Edit*
What's your water debit and what does the brew gauge (Lelit) read when flushing with the pump running?

I haven't come across the gicleur diameter in literature and I haven't removed the cap, nor do I think I have gauges to measure it even if I had, so I'm afraid I don't have that information.

As for the water debit, I'm trying to remember what that is, but haven't recovered it ... how about a hint?

I'll record (video) the brew and group gauge responses to flushing, but probably not until the weekend ... just too much procedure for a workday morning. If I do some after work pulls I'll have it a bit sooner.

Which part(s) are surprising, other than the trickle apparently filling the preinfusion chamber? I'm wondering if they have a super weak spring for that valve or maybe they've modified the E61 lower section all together.

Sorry.

Typical definition is to weigh a 30 second dose of water straight out of the group with no portafilter. This allows you to calculate the flow coefficient for the factory gicleur, which is useful in determining expected performance in a variety of situations. The typical water debit number is "such and such grams in 30 seconds".

Thanks!

- Jake

Is 'trickle preinfusion' basically using the boiler pressure to 'pre-wet' the coffee? Someone told me that's what Lelit means but I want to confirm. If so, Profitec and ECM E61 users can also do this.

I do the trickle preinfusion by raising the brew lever just past midpoint so the brew valve is just open but the pump has not started. I don't know if you would technically call it "boiler pressure" even though I have too, because it is an HX so it is the HX loop pressure and I don't know how that relates to the boiler pressure. I also do that pump goosing every 10 seconds to keep the loop fed.

Then we're on the same page -- awesome. That's exactly what you can do on Profitec / ECM machines too. I've not 'goosed' the pump to get water coming though so I stop after a few seconds. so it looks like you can prewet for 10 seconds and then get a 1 bar preinfusion for 20 seconds in your shot. I'm so about to try this on my Pro 500!

What does Eric's brew pressure gauge look like when installed?

I tried your method on my Pro 500 including tapering off at the end. Very smooth 60 second shots (including the long trickle PI) of Olympia Sweetheart and Big Truck. I was never able to keep it going due to not knowing to pulse pump every 10 seconds. It kept water flow going as you can tell via the thermometer if it is flowing or not.

Nice!