Grind and Flow
When you layer more coffee up in a filter cone, the water percolates through more slowly . The most widely accepted explanation for this, in relation to percolation brew methods, is defined by Darcy’s Law. This equation describes the movement of liquid through a porous medium — in this case, water through a coffee bed. According to Darcy’s Law, the flow rate of water through the bed decreases with the distance it has to travel. A deeper bed will require more time for the water to flow through it.
Larger brews with deeper coffee beds take longer to finish, so the risk is that they will over-extract. To get around this problem, we need to grind coarser to slow down the rate of extraction. There are advantages and disadvantages to this. By grinding coarser, we create a bigger difference between the sizes of the largest particles and the smallest. In physics the speed at which water can move through a medium like coffee is called the hydraulic conductivity. A very coarse grind has a higher rate of hydraulic conductivity than a very fine grind. Imagine throwing a bucketful of water onto a sand castle, compared with throwing a bucketful of water onto a pile of bricks. It’s much more likely the water will penetrate the centre of the pile of bricks much more quickly.
Another major factor in what changes flow rate is the cross sectional area and the length of the flow column. If a coffee bed is wider and longer, then water takes longer to travel through it. This concept is easy to grasp. What is a bit harder is the idea of the concept of hydraulic gradient. In percolation brewing, the brew water at the top of the slurry applies more driving force to push the water through the coffee bed than water exiting the filter paper. Where there is more water above the bed, and the coffee bed is deeper, the water pressure is a little higher.