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Time to extract that caffeine

So your machine has done quite a bit of work for this next stage, now lets get that water through your grinds.
On very early espresso machines, extracting the espresso was was achieved using a hand operated lever that employed a series of pistons and springs

Thankfully, the baristas work has been made a lot easier with the use of electrically operated pump, electrical valves and cleverly designed group heads.
Whilst they are several designs on group heads, by far the most popular is the E61 type shown below and quite a few machine manufactures now use this format.

Depending on your machine, the layout of the group head may look different at first glance, but once you study the design, you will see that the format is the same as laid out here.
You will also notice that the modern group head does contain quite a lot of moving parts and tiny channels that could easily become blocked, either by dirt or the build of coffee oils from the brewing process.

For the above reasons, you can understand why we stress the use of a quality water filtration system and the need for a good cleaning regime. 

How it works

group head

Once your machine is up to the correct operating temperature, water circulates around the group head and the heat exchanger using thermal dynamics which maintains the temperature, see the animation on slide 3.

Upon pressing the dose select button the following sequence occurs:

Machine pump starts and takes the water pressure up to 9 bar.
Group head solenoid opens allowing water into the group handle.
Machine stays in this state until either you stop the group, or the flow meter informs the control circuit that the correct level has been reached in the cup.
Machine pump stops.
Group head solenoid is switched off which allows the pressure to be relieved through the 3 way valve.
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