Unic Stella di Caffe, Maintenance Procedures, Page: 7 / 16

Pump Pressure:
By SCA definition, “ espresso is a 25-35ml (.85-1.2 ounce) beverage prepared from 7-9 grams (14-18 grams for a double) of coffee through
which clean water of 195 °-205 °F (92 °-95°C) has been forced at 9-10 ATMOSPHERES OF PRESSURE , and where the grind of the coffee is such
that the brew time is 20-30 seconds”.
Brew (pump) pressure is registered as the value ‘P: X.Xbar’ on
the display during coffee extraction. Adjustment can be made
by turning the by-pass screw on the output of the pump
clockwise for higher brew pressure, or anti-clockwise for
lower brew pressure, while coffee is extracting. Access pump
through right side panel.
Expansion Pressure:
The expansion (HP) valve allows by-pass of excess pressure exerted on the high-pressure hydraulic circuit. This occurs when cool (condensed)
incoming water expands as it is heated following an extraction. An expansion valve that is stuck, or set too high, may cause the 3-way brew
valve to leak shortly after an extraction.
To check expansion pressure, first locate the drainage tube that dispenses by-pass from the expansion valve into the drip pan. This drainage
tube is located beneath the backsplash panel to the right-hand side of the machine. Prompt a chosen group to flow without restriction for
5sec. Then, while the machine is in an idle state, observe the displayed (increasing) pump pressure and note when the expansion valve begins
to by-pass (drip from drainage tube). Bypass should not occur until the gauge registers approximately 12-13bar. If adjustment is necessary,
follow the drainage tube to the expansion valve, loosen the 24mm jam-nut and adjust the threaded (spanner cap).
ALWAYS RE-CHECK/VERIFY EXPANSION PRESSURE AFTER MAKING AN ADJUSTMENT.
Group Flow:
Each group head is equipped with a Ø0.7mm restrictor (gicleur). These small orifices are prone to obstruction by mineral deposits and debris,
however, they play a very important role in the thermodynamic and hydrodynamic properties of the coffee brewing system. Without a
portafilter installed, each group should displace 100mL (+/-25%) of water in 10sec. Of equal importance, is the relation in flow rates from
group to group, which should be +/-10% of one another.
Example:
On a 3-group machine, group 1 displaces 80ml of water in 10sec, while groups 2 and 3 displace 110mL and 105mL, respectively. While all
three groups are within the general 75-125mL target range, group 1 demonstrates sign of a partial obstruction, in that its flow rate is only
74.4% of groups 2 and 3. Intervention is required to correct the flow rate of group 1.