Oene Zwittink schreef: vr 21 jan 2022, 12:52
Dank voor jullie reacties! Het is bij die ene keer doordrukken gebleven, ik denk dat valse lucht de oorzaak was. De maling iets minder fijn gezet, waardoor ik het water nu doordruk met 7-8 bar. Levert goede koffie op, wel blijft het wat wisselend...
De Strega ziet er erg goed uit. Vraag me wel af of daarmee ook het watervolume in de kop regelbaar is.... Ik zit ook te kijken naar de Lelit Bianca vs2, heeft oa een tijdens de extractie volledig regelbare zetdruk met een paddle... O, is dit vloeken in de hefboom kerk?
Over een sponzig gevoel en valse lucht gesproken.... Voor de technici hieronder een tekst daarover uit de FB groep La Pavoni van Coffee Sensor. Hij zegt er iets tegen te hebben, ik weet nog niet wat
Trapped Air & Spongy Pulls
This post is about manual lever espresso machines with boilers. Many of these have a cylinder in the group head to hold water for pulling a shot. A piston in the cylinder can be moved up and down by a lever.
When the piston is in the down position, it prevents water from exiting the group head (left image). To brew espresso, the portafilter is removed, filled with finely ground roasted coffee beans and re-attached to the group head.
The operator begins the brewing process by raising the piston (right image). This movement has multiple effects. One, is that the upward movement forces some water above the piston back into the hotter boiler. Another effect is that rising piston creates a vacuum underneath it which is readily filled with air drawn through the dry porous coffee grounds. Finally, when the piston reaches the top, a hole is exposed on the side of the cylinder, allowing water to enter the area below the piston.
Ideally what would happen next is that all the air under the piston is driven back through the dry coffee grounds, and the area under the piston is filled with just enough water at the perfect temperature for brewing espresso. Then, the forceful lowering of the piston drives that water through the ground coffee to extract a perfect espresso.
However, many users of direct lever machines will occasionally experience what is called a "spongy pull". That is, as they force the piston down with a lever, they notice the lever springs back up if they let go. This is due to some (compressible) air trapped in the area below the piston. This is not an ideal situation because not enough water will be available for pulling the shot. Also, when the portafilter is removed, any remaining compressed air may make a mess of things.
It is important to understand that it is steam, not liquid water, that effectively drives out the air underneath the piston. When the water enters this area, it undergoes a rapid depressurization. That is, the pressure of the water in the boiler is higher than the pressure of the air under the piston. The rapid depressurization results in a small percentage of that water turning to steam, known as "flash steam". While the percentage of water that turns to steam is small, the volume that steam occupies is large. To better understand this, we can calculate the volume occupied by water that turns to steam, and the volume occupied by the water that remains liquid. We can then compare these two volumes as a ratio.
By examining chart, we can see that at 105C, the water entering the area under the group head occupies about 15 parts of steam to 1 part liquid water. In other words. at 105C, enough steam is generated to fill the area under the group head 15 times over. At 100C or below, none of the water entering the area under the piston turns to steam. This then is the reason why introducing cooler water into the area under the piston results in spongy pulls. The remedy of course is to increase the water temperature by, for example, maintaining a higher group head temperature.
However, brewing espresso benefits from water at a temperature in the range of 91-96C to keep it from over-extracting the coffee, which yields a bitter taste. This intricate knife-edged trade-off of these types of lever machines makes them endearing to many.
With this understanding then, it should be easier to understand the thinking behind the "air buster".
