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4 With the vacuum breaker valve closed and the heating element still energized, pressure in the boiler in-
creases which is indicated by the boiler pressure gauge on the front panel of the DD. As long as the heating
element is energized, pressure will continue to increase. There has to be some way to turn off the heating
element.
5 The Pressurestat is a switch that is operated by the pressure in the boiler. In this inset illustration, the pres-
sure in the boiler has dropped and the diaphragm relaxes (indicated by the red arrow) and moves the con-
tacts together (black arrow) closing the circuit. Electricity flows through the wires in the completed circuit
and the heating element is energized. The pressure in the boiler begins to increase.
6 When the pressure in the boiler reaches the set point of the pressurestat, the diaphragm is pushed by the
pressure (indicated by the brown arrow), and it separates the electrical contact (light blue arrow), opening
the circuit. The heating element no longer receives a flow of electricity. The steam boiler now begins to cool
until the pressure drops below a point designed into the pressurestat. The switch eventually closes, and the
cycle begins again. This is why you will see the needle of the boiler pressure gauge slowly rise, slowly fall,
and then rise again.
But how does this control temperature? Boyle’s Law, as applied here, states that for a given volume
of gas, when the temperature is increased, the pressure of the gas increases a predictable amount. So in our
example, as the pressure of the steam in the closed boiler increases, the temperature of the steam and the
water has also increased. The pressurestat is used to sense the pressure in the boiler, and so controls the
temperature like a thermostat - thus, we can call it a “pressure thermostat,” or pressurestat.
7 We need to control the water level in the boiler. Too high and water will come out of the steam wand making
a mess with your milk. Too low and the heating element can be exposed which can easily damage it. Addi-
tionally, the water, when heated, holds a lot of thermal energy which adds greatly to the power and volume
of available steam. The hot water itself actually works as a steam storage system.
To help maintain the optimal level of water in the steam boiler, there is a metal probe which passes
through the outer surface of the boiler. It is electrically isolated from the metal boiler. When the water level
drops below the tip (indicated by the yellow arrow) the electrical connection is broken and the control box in
the DD “senses” this to start the pump and simultaneously opens the fill valve to allow water to be pumped
into the boiler. When the water level touches the tip of the sensor, an electrical circuit is completed from the
probe, through the water, to ground. The control box responds and closes the fill valve and stops the pump.
8 A pipe draws steam off the top of the boiler and directs it to the steam valve.
9 A similar pipe, but this one connected to the bottom of the boiler, directs hot water to the hot water valve.
10 Finally, what if the system somehow malfunctions? If the pressure in the boiler gets excessive it can be
dangerous. All steam boilers are equipped with a safety valve to release dangerous pressure in emergency
situations. If the pressure rises above a predetermined level, the steam will push the valve open against its
internal spring pressure, releasing steam and lowering the pressure inside the steam boiler to a safe level.
WARNING: If you hear a violent release of steam from inside of the machine, immediately discon-
nect the power cord from the outlet and when the steam release subsides, carefully turn the power
switch to “Position 0.Call for service assistance from your reseller before using the machine
again.
3 When the machine is turned off, or the power switch is set to “Position I,” the heating element in the steam
boiler no longer receives electricity, and pressure in the boiler begins to drop. As the water and steam cool
and contract, the pressure in the boiler eventually drops to a little below atmospheric pressure and the
vacuum breaker valve (3) opens allowing air to relieve the partial vacuum in the boiler.
16


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