MAINTENANCE MANUAL
Mirage technical manual
page 19 of 65
Boiler temperature vs steam power
The factory settings of the steam boiler temperature is 124 or 125 °C (255, 257
°
F) and is maintained
by an electronic thermostat (see Figure M 7) connected to a Pt100 temperature sensor. The tip of the
sensor is located in the steam boiler above operational water level.
At idle, the temperature of the steam will be the same as that of the water and the heating element will
be switched on for a moment every once in a while to maintain the set temperature.
By placing the sensor in the steam (instead of the water), the machine reacts quickly to heat loss. As
soon as a steam valve is opened, steam pressure drops and steam temperature decreases instantly
by adiabatic expansion (while water temperature remains nearly constant). Any drop in steam
temperature is noticed by the controller which in turn activates the heating element. When cold water
flows into the boiler after the hot water tap has been used, the small decrease in temperature
immediately lowers the steam pressure and again, steam temperature drops.
Saturated steam pressure at 124
°
C is about 1.25 bar (overpressure), at 125
°
C it is about 1.33 bar
(see Graph M 1 and Graph M 2). This pressure is high enough for powerful and long-lasting frothing of
milk.
Figure M 7. Showing the temperature controller. The indicator dot in between the 1 and 2 of the
display is not lit which shows that the controller is not sending out a "heat-on" control signal.
Graph M 1. Saturated steam pressure versus temperature (data from www.engineeringtoolbox.com).
Note that most pressure gauges read overpressure (pressure as compared to atmospheric pressure).
90
100
110
120
130
140
0
1
2
3
4
°C
bar
1
.3
5
B
ar
o
ve
rp
re
ssu
re
water temperature 125 °C
