@Tedi
3.4 Simulation of the Coffee Cake:
Specification: The volume of the filter basket normally filled by the coffee cake shall be filled
with a solid material having a thermal conductivity of less than 0.5 Watts/meter*Kelvin
(W/m*K). The volume of the solid should be approximately the same volume as a coffee
cake, but may contain deviations from the actual shape of the cake in order to
accommodate the temperature probe, metering valves, etc. The distance between the group
dispersion screen and the top of the proxy cake (headspace) should approximate the
headspace in the presence of an actual coffee cake.
Technical rationale: A reasonable substitute for the coffee cake enhances practicality. Filling
the portafilter with coffee prior to each test run would be wasteful of coffee. Variations in
dosing would introduce varying water flow rate, and therefore varying boundary conditions.
While tamping the coffee cake is possible if bare thermocouple sensors are employed, use of
more robust, sheathed probes complicates tamping by normal means due to the probe
position above the coffee cake.
Replacing the coffee cake with a substitute introduces potential measurement errors.
However, the proxy cake is located downstream from the temperature probe, and is not
likely to affect the temperature of the brew water above the proxy cake, provided that it is
not constructed from material possessing high heat capacity. The specified thermal
conductivity range from .1 W/m*K to .5 W/m*K encompasses the thermal conductivity of
dry wood to wet foodstuffs such as bananas and apples, and is a reasonable approximation
for the thermal conductivity of coffee as it transitions from dry to saturated grinds. Many
easily machined plastics such as poly vinyl chloride (PVC) fall within the specified range of
thermal conductivity.
The proxy cake further deviates from a coffee cake in its water absorption characteristics.
This topic will be discussed further in the measurement procedure.
3.5 Water Flow-rate Adjustment:
Specification: A water flow regulator, positioned downstream of the thermometer probe,
shall simulate the flow resistance of the coffee cake and provide flow rate regulation as
specified in Section 4.3.
IT - Instytut Tedipresso
