Vibiemme Domobar Junior - Manual (Page 6)

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The large rectangle represents the case of the Vibiemme espresso machine. It houses all the mainly
working elements of the machine as shown above: Brew boiler, Steam boiler , Boiler pressure gauge,
Brew gauge and Grouphead
3.1.1 Coffee Boiler: hydraulic principle
The coffee boiler works following the recessed surface principle ( same quantity of water in and out) so
there will always be only water without steam.
The water inlet is previously heated by passing through a heater placed inside the steam boiler: when it
is on, the heated water will arrive inside the coffee boiler at the constant temperature of approx. 88 °C,
so assuring a thermic stability during all working conditions.
When it is off ( only on Domobar super – double boiler version), thermic stability will be assured by a
boost of coffee boiler element driven by the electronic box in case the probe feels temperature decrease
of more than 2 °C.
Note: from our tests, even when steam boiler is off, the boost never lighted; in case this should happen,
an electronic check on the rated powers will always assure a power consumption below 15 AMP ( only on
Domobar super – double boiler version).
3.1.2. Coffee boiler: temperature checking (PID)
Coffee boiler temperature is electronically driven through the PID technology: When the desired
temperature value is inserted, the electronically driven probe will activate the heating element; once the
setted value is reached, the electronic box will disengage the heating element.
Temperature values can be setted by 1°C or 1 F step.
Only on the Domobar Junior 2B version, the coffee boiler temperature is mechanically adjusted by means
of a thermostat.
3.1.3. Coffee Boiler and offset technology
There is heat energy lost between the boiler through the copper thermosyphon pipes as the water is
pumped out of boiler, and again in the grouphead, so there is a difference (in terms of °C) in the temperature
read by the probe in the coffee boiler and the temperature out of the grouphead : this is the OFFSET.
This parameter is strictly linked to the manufacturing features of the machine and it must be considered
a default value ( factory parameter ). The idea is to have the correct temperature at the coffee, so to
compensate for the loss of thermal energy the designers compensate with a higher boiler temperature.
In the Vibiemme espresso machines the offset parameter is adjusted by means of the electronic box, so the
displayed temperature ( during setting as well as shown on the display ) is already taking this difference into
consideration. Besides, since the peculiarity of coffee boiler inlet water pre-heating ( see 3.1.1.), the OFFSET
values differ in the different configuration phases; Steam boiler On – Steam boiler OFF.
Don’t worry: the electronic box is preset to the automatic change of that value, relating to the selected
machine configuration. ( only on Domobar Super model).
3.2. Steam boiler
Differently from the coffee boiler, the steam boiler is not fill in with water, because it has to respect a
certain proportion ( approx. 2:1 or 3:2) between water and steam quantity; this ratio is determined by
the water sensor level. The water level sensor is a special wire that is passed through the outside of the
boiler. It sends a small electrical current through the water to the boiler’s inside surface. If the sensing
wire is not touching the water it cannot send the electricity through the water. An electrical wire is
connected to the end of the sensor to the control unit. If the water level is too low the control circuit turns
the pump on and at the same time a valve opens to direct the water into the boiler. How far this sensor is
pushed into the boiler determines the level of water in the boiler. Pushing it in further maintains a lower
water level. Pulling it out creates a higher water level in the boiler. Among other things, the water level
determines the amount of steam stored in the boiler.
Inside the case the boiler is the largest single component. In normal operation the boiler will be about one
half to two-thirds filled with water. Immersed near the bottom of the boiler is the heating element. When
the heating element is energized and the water begins to heat, some of the water turns to steam, and the
pressure inside this closed vessel begins to increase.
To control the temperature of that water there is a device called a pressurestat- a pressure-controlled switch.
When the pressure in the boiler reaches a certain level the pressurestat turns off the element. This cycling
can be seen through the activity of the Heating Element Indicator Lamp on the front panel.