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Vibiemme Domobar Super Manual

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But why sense pressure in the boiler when it is temperature in which we are interested? Pressure is an indi-
rect measurement, but one that is easy to control and yet accurate without using more complicated electronics.
Heated water at a given pressure is always at a predictable temperature. That is indicated by the left-hand gauge
on the front panel of the Domobar Super. As the temperature changes in the boiler you will see the pressure gauge
rise and fall.
Boiler Pressure Gauge
The boiler pressure gauge on the front of the machine is calibrated in “BAR.” That is barometric pressure.
The boiler works like a pressure cooker in that as the pressure in the boiler increases the boiling point of water in
the boiler increases:
Boiler Boiler Approximate
BAR temp brew temp
1.0 - - - - 253 F. (123 C.) - - - - 196 F. (91 C.)
1.1 - - - - 255 F. (124 C.) - - - - 199 F. (93 C.)
1.2 - - - - 257 F. (125 C.) - - - - 201 F. (94 C.)
1.3 - - - - 259 F. (126 C.) - - - - 203 F. (95 C.)
A Quick Science Lesson (no homework required)
Gauge readings can be confusing. Let us try to make a bit more sense of it. The boiler pressure gauge is read-
ing the pressure above the outside pressure, so a reading of 1 BAR is actually a pressure of 2 BAR in the boiler.
The laws of physics teaches us that at a constant volume (in this case we are speaking of the enclosed boiler in
our Vibiemme Domobar), as the pressure of a gas increases or decreases, by the same factor does its temperature
increase or decrease. In plain English, as the pressure in the boiler rises so does the temperature in the boiler.
Just like in a pressure cooker, the rising pressure of the steam in the boiler allows us to actually raise the
temperature of the water without having it boil. This temperature which is above the boiling point of water also
allows some temperature drop between the boiler and the grouphead so that when the water reaches the coffee it
is at the correct brew temperature.
Brew Pressure Gauge
The information that this gauge displays can actually be confusing. This is because it is reading the pres-
sure inside of the Heat Exchanger. When the machine is at idle (or actually at any time other than when brewing
espresso) the readings of this gauge should be ignored. Its only function is to report brew force and at any other
time the reading is not reporting anything of note at all.
During the brew cycle the reading should be in the range of about 9 to 9.5 bar if everything else is correct.
That statement is only accurate if all other factors are right. More on that later in this manual.
Back to the Machine
The temperature for brewing espresso is in the area of 200 F. So why are the boiler temperatures so much
higher? There is heat energy lost between the boiler and the heat exchanger, and again through the copper ther-
mosyphon pipes as the water is pumped out of the heat exchanger, and again in the grouphead. 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.
The Heat Exchanger is partially immersed in the boiler water. The HX is a tube through which the brewing
water passes. This water gets heated through an exchange of heat energy from the water and steam in the boiler
to the metal of the heat exchanger, and then from the metal parts to the water in the HX tube itself- thus its name,
because heat energy is exchanged from the boiler water to the brew water.
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