Recap and Glossary
Recap
- Darcy’s law describes the decrease in flow rate as the coffee bed depths increase,
- For this reason, we tend to grind coarser for larger brew volumes.
- E.E. Lockhart (the architect of the Coffee Brewing Control Chart) recommends using a coffee depth of between 1–2 inches (≈ 2.5 and 5 centimetres)
- There is a lot of potential to lose heat from the air-water interface at the top of the slurry. Evaporative cooling is accelerated by moving air so we recommend you avoid placing ventilators directly above your brewbar.
- Air velocity of up to 3 meters (about 10 feet) per second can double the rate of evaporative cooling.
- Even a three ml per second flow rate causes significant disruption to the coffee bed. Visibly the top 2.5 cm is significantly disrupted.
- Ted Lingle, the author of the SCA Coffee Brewing Handbook advises, ‘Feeding water to the ground coffee must be done uniformly and gently to ensure that the entire area of the coffee bed receives equal treatment.’
Glossary
Flat-bed A basket and filter paper design that is flat on the bottom, rather than conical
Funnels Filter holders on batch-brewing machines are called funnels rather than filter cones because they have a flat bottom
Inserts Baskets that can be inserted inside a funnel to allow a recipe to be scaled down to smaller volume whilst maintaining an adequately deep coffee bed
Side-channelling The situation in which brew water travels around the coffee bed and out the sides of the top of the filter paper
Skimmed The side-channelling effect of water exiting the slurry without travelling through the coffee bed
Thermal mass A material’s ability to absorb and store heat
Turbulence The situation in which a smooth flow in water or air is interrupted
