If you have the ability to measure the surface temperature of the group you are probably wondering what the optimum idle (i.e. before you start to pull the shot) group surface temperature is for the LONDINIUM I
We place the thermocouple on the surface of the body of the group at the height of the middle of the neck of the group (when viewed from the side of the machine) and about 25mm to the right of where the neck meets the body of the group (when viewed from above)
Whilst you will get a very good espresso with a group temperature above 80C, if you want to know the optimum i would say it is 86C, probably plus or minus 1C (the lower end of the range highlighting any acidity in the roast, the upper end of the range producing a sweeter caramelised taste, which i prefer)
Idle group surface temp above 90C produce poor shots, to my taste at least
Think of the group on a lever machine acting as a thermal shock absorber, quickly dragging down the peak inlet temperature and then at the end of the shot preventing the water from cooler too far – it protects against extreme brew temperatures, high or low, and ensures a progressive controlled decline throughout the extraction so the most is taken from the coffee at the start when the coffee has all the desirable solubles present, then a much lesser amount of the undesirable solubles as the extraction progresses as a result of the lower brew temperature. This is the magic of lever espresso