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Pygmalion DavecUK: if it is due to some air somewhere in the system, do you think it would be possible to bleed it out somehow?
Probably not and with coffee in I am surprised the effect was as sustained as it was unless the coffee was quite a tight grind. Also when I say air and water hammer, I would expect the effect to be very transient. Air itself compresses to feck all at 9 or 10 bar….which would cause the hammer and can be fairly quickl;y absorbed at those pressures..
In your first video, what happens if you repeat the experiment, but fill the blank portafilter basked with cold water and carefully lock it into the group, spilling as little as possible.
Otherwise I’m mostly out of ideas.
- could it be a sticky expansion valve (I don’t think so), unless there is a manufacturing defect? But it still doesn’t explain why after a shot or two the problem always goes away.
- Some weird water heating expansion effect (but that doesn’t fit with the way the Hydraulics are connected.
- Some fault on the gauge but then why does it self correct after a few shots?
Nope I’m a little stumped. We can eliminate heat effects, if someone wants to disconnect the brew boiler heating elements, turn the steam boiler off then repeat the blind basket experiments.