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USER MANUAL - operation
Slim Jim User manual
page 9
Pre-infusion cylinder
The piston rod will rise up during the initial stage
of a brew. A double spring assembly inside the
pre-infusion cylinder assures a slow-starting and
gradual pressure build-up to about 3 bar when
the maximum displacement of the piston is
reached.
Figure 11. Piston-rod ends of the pre-infusion cylinders.
At the start of a brew water leaving the group-valve can go into the coffee-bed and into the pre-
infusion cylinder. The water will flow in the direction of the least back-pressure. At start the coffee-bed
will have no back-pressure but once the grinds are wetted, dominant water-flow changes towards the
pre-infusion cylinder. Pressure gradually increases until the cylinder is completely filled and all water
has to flow through the coffee-bed. From that moment brew-pressure quickly increases to pump-
pressure.
Caution!
The piston-rod of the pre-infusion cylinder can become very warm.
The advantage of the pre-infusion cylinder over a reduced flow-rate at start is that the entire coffee-
bed will be soaked relatively quick and then has time to swell and settle at reduced pressure before
high pressure is established. With reduced flow-rate during pre-infusion it will take a longer time to wet
all grinds and swelling and setting of the coffee-grinds starts where water reaches the grinds first such
that the upper part of the coffee-bed becomes more impermeable to the later inflow.