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small adjustment keeping in mind that it should be set to best match the coffee you drink most of the time. The
best test to judge the setting is your palate, but it will take time and practice to get consistent enough to sense the
difference in taste.
The pressure gauge on the front panel should only be used as a general indicator as to what is happening
during the brewing cycle. The only time when the gauge will display any useful information is when there is a
properly prepared portafilter, filled with properly ground and tamped coffee, locked in the grouphead, and the
brewing process is underway. At all other times, even when the pump is refilling the boiler this gauge is not dis-
playing any useful information and should be ignored. Even when the pull is underway the brew gauge will read
a little higher than the actual brew pressure at the coffee.
If the pressure gauge is reading a very low pressure during a pull, the first thing to suspect is the coffee. The
problem could be:
- not enough coffee in the portafilter
- coffee not properly distributed before tamping
- coffee not tamped properly or evenly
- coffee ground too coarse
If the pressure gauge is reading a high pressure during a pull it is almost always a matter of a poorly adjusted
over pressure relief valve. The indicated brew pressure during a pull should never indicate more than about 9.5
to 10 BAR.
If after attempts to resolve a pressure related problem by adjusting the above factors have failed it is time
to try adjustment of the pressure regulator. Do not attempt to adjust the valve by means other than described as
above.
Pressurestat Adjustment
The pressurestat controls the brewing temperature by sensing the pressure in the boiler. If the pressure is too
high the brewing temperature will be too high and the coffee can taste burnt or bit-
ter. It can also cause excessive steam in the heat exchanger which contributes to the
over-extraction and can create so much force that the steam can blast through the
puck and cause a fast flow. On the other hand, if the pressure is too low, the brew
temperature will be too low. The coffee will taste sour, under-extracted, and thin.
Of course, these can be experienced through a whole range of tastes, and incorrect
brewing temperature may not always be the cause of bad taste, bitter or sour.
Accessing the pressurestat adjustment is simple. Simply remove the cup warm-
ing tray and beneath that you will see a perforated panel with a black plug. Remove
the plug and the adjustment screw will be seen underneath as shown here. Turn the screw to adjust the boiler
temperature:
CLOCKWISE LOWERS the TEMPERATURE
COUNTERCLOCKWISE RAISES the TEMPERATURE
This adjustment is not very sensitive, so even a full turn of the screw is a fine adjustment, and that’s a good
thing because the generally accepted range is between .9 and 1.1 BAR. Still, it is important to make changes
slowly and in small increments. Temperature of the brew water is a critical component of good espresso, and a
change of one degree in the brewing temperature can make a big difference in the taste of the coffee.
In actual operation the function of pressurestat will create what is referred to as a “deadband.” That is the
range of temperature fluctuation caused the lag in time between when the heating element is energized to the time
it turns it off. The deadband of the Vibiemme Domobars is about .15 BAR. So if you set the pressurestat for about
1 BAR, the gauge indicating boiler pressure might range from about .9 to 1.1. This is normal operation for all
machines with pressurestats and is a function of the pressurestat design. To hold a smaller range of temperature
the pressurestat would be clicking on and off every two or three seconds and would rapidly burn its contacts.
40


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