Last week, we discussed why pour-over coffee is filled with water in a circle, and we learned that it is to increase the extraction rate while not letting the water flow around, and help it flow in a way that achieves an even extraction. When you see that the powder pit is in the shape of a "bowl", the powder wall at the edge of the filter cup forms a smooth gentle slope with the bottom, which indicates that the whole circle water filling process has been ideally controlled. 
In the past two days, some novice friends felt that the coffee they brewed was not very good, and the powder pit after brewing did not seem to be consistent with the tutorial, so they asked what the reason would be for Front Street. The "pit" provided by this friend looks like this: the powder wall on the filter cup is obviously broken into two, the middle circle of powder wall is very thin with a little bubbles, there are almost no particles, and a part of the coffee grounds is washed to the bottom, probably like the picture below.
The main reason for the breaking of the powder wall in two is that the water injected at the tail is too strong. Let's imagine that when the steaming is over, the second target amount of water is injected into the filter cup according to the brewing plan, and the stirring of the water column allows the coffee powder to be free in the touch of the liquid surface, and as the liquid level gradually drops, the surface particles stick to the filter paper to form a thick and consistent powder wall.
When the liquid level drops to 2 3 of the height of the filter bowl, if you are anxious to inject the last section in a large circle with a large stream of water at the same time, the water column with too strong strength will break through the original piled wall, and at the same time, this coffee powder will be washed down, and it will form a form of breaking into two. If you continue to pour water into the broken section, the hot water will flow directly through the filter paper and away from the ribs around the cup, causing a "channel effect", i.e. uneven extraction. This explains why the coffee that my friend brewed didn't taste right.
In addition to faults, in fact, there are some cases that are also caused by excessive circle amplitude, and this kind of pit looks like this.
Such a pit is usually caused by the fact that the water level in the later part of the water injection completely overflows the original powder wall, and although it does not reach the filter paper, the overflowing water will slip directly from the edge of the ribs, resulting in insufficient extraction or reduced concentration. The fine powder that is washed to the bottom may clog the filter paper and cause water accumulation. The judgment is based on the fact that the powder walls at the top of the pit are covered with scattered coffee particles, the extraction time is usually more than 2 minutes and 10 seconds, and the coffee has an unpleasant bitter taste.
I believe that many brewers have encountered the above two situations when they are novices. If you want to improve the taste of coffee and improve your own extraction technique, Qianjie recommends using a safe method to practice the basic skills of flow control. The higher the water flow, the stronger the penetration, and the more obvious the strength of the coffee powder layer; Conversely, the lower the water column, the softer the force and the less tumbling the ground layer will be. After understanding this principle, we can adjust the water flow and the size of the circle appropriately according to the different filling stages.
Take the three-stage formula used in Qianjie as an example, after the simmering is over, a large flow of water will be taken smoothly and vertically, and the powder layer will be washed in a constant circle at a distance of 3-4cm from the liquid level, and the range will be continuously increased in a clockwise direction from the center to the periphery, until the weight of the chronograph scale shows 125 grams. As the liquid level rises, there will be less and less coffee powder that can adhere to the filter paper, and when the water level drops to half, you can gently inject it in concentric circles along the radius of the liquid surface at 1 2 points and gently inject it with a small stream of water 2cm high. In this way, the particles in the filter bowl can be "evenly wetted with rain and dew", fully release the flavor substances, and can also effectively avoid the original powder wall being washed down, resulting in uneven extraction.
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