Any time you are called out to work on an espresso machine because “it’s not making coffee,” you are likely to enter into the world of fluid dynamics. Of course, there’s no need to get a degree in engineering to de-gunk a clogged group or replace a pump, but in both cases, you’re working to solve a problem that encompasses pressure and flow, the two bedrock concepts in fluid mechanics. Espresso extraction is highly susceptible to changes in either. The flow rate of water through the puck dictates your contact time and therefore is proportional to your extraction percentage. And pressure is what makes espresso espresso: a thick, quick-to-prepare coffee topped with a colloid of coffee and oils that we call crema. The fine grind and hard tamping creates a puck that’s hard for water to get through, which is whole reason pressure can build up to extract the oils out of espresso. So right away we see that flow restriction is used in espresso machines as a way of achieving a desired pressure. Consider the hunt for a soft pre-infusion. The idea is to subject a puck of espresso to a small amount of pressure for some period of time, during which the puck will swell up and the grounds will...