The Warm-Up Process
For a number of good reasons, the temperature sensor (thermocouple) included in most PID installations
is located on the top of the boiler.
This spot is very quick to come to operating temperature, usually set by the PID to be between
225F and 230F (107C and 110C).
Typically, it requires only 6 minutes or less to get the boiler up to final temperature.
But other important parts of the machine require much longer to reach a stable,
The brew group, or grouphead, is a relatively massive brass casting that lies below, and in front of,
In order for the machine to be considered completely warmed-up, not only must the boiler have reached
a steady operating temperature, but the heavy brass
brew group must have also reached a steady temperature.
The "group" is responsible for directing hot water from the boiler into the portafilter,
through the coffee puck and into your cup.
As the heated boiler water flows through the brew group, it's temperature is influenced (in this case, cooled) by the
brass in the group.
The Brew Group
If the group is too cool, the brew water will lose too much heat, and the water that hits the coffee will not be
at the desired temperature.
So for consistent, repeatable shots, it is necessary that both the boiler
temperature (well controlled by the PID)
and the group temperature, be at the correct temperature.
OK - So How Long is Long Enough?
The most common recommendation seems to be that a 30 minute warm-up is the minimum.
But a quick glance at the graph above suggests that the group temperature does not completely
stabilize until after 50 minutes.
Our recommendation is that you give the machine at least 45 minutes,
preferably an hour,
for optimum results. Assuming that your PID is set within the range described previously, this will result
in the group reaching a temperature of around 195F (91C), give or take 3-4 degrees.
Our testing has shown that
very good intrashot temperature stability
can be achieved
when the group temperature
is in this range.
Our Test Setup
The temperature measurements on the graph above were made on our bench Silvia while under the control of
a Watlow series 96 controller set to 228F (109C). The machine was simply turned on from a cold start
and allowed to warm up without intervention.
To record boiler temperatures, a fine wire type T thermocouple was placed below the stock brew thermostat
and held in place by the clamping action of the screws and bracket.
Group temperatures were recorded using a 1/16" type K probe inserted through a custom
the 6mm threaded socket in Silvia's group (thanks to Eric Svendson for his very valuable assistance).
Datalogging was performed by a 2-channel Omega HH506RA digital thermometer with RS-232 interface to a
desktop Dell PC. Microsoft Excel was used to format and plot the resulting data.