Precision filter baskets

Holds the dose and decides, through the pattern of holes in its floor, how evenly water is allowed to leave the coffee bed.

A stainless cup with a perforated floor, and everything that distinguishes one from another is in that floor: hole diameter, hole count, how evenly the holes are distributed across the area, how consistent they are with one another, and how much of the floor is open in total. Water leaves the puck through those holes and it leaves fastest wherever the open area is greatest, so a floor whose perforations crowd towards the middle drains the middle first and pulls flow to the centre no matter what the barista did on top of it. A precision basket is one whose holes are formed to a tight enough tolerance and laid out evenly enough that the floor stops being a variable at all — the point is not that the holes are better, it is that they are the same as each other and the same as the next basket you buy. The wall does the other half of the work: its internal shape decides how the puck sits, and its depth decides how much headspace is left between the tamped bed and the shower screen once the portafilter is locked in. Too much headspace and the puck can lift and shift as the chamber fills; too little and the screen prints into the bed and disturbs it before the pump has even started. That is why a basket is specified against a dose range rather than against a volume, and why moving dose without moving basket eventually stops working.

• Specify the basket from the dose you actually run, and know that dose before you order anything. A basket has a working range, and a dose at either end of it changes the headspace and the bed depth together — which is why a bar that quietly crept its dose upward over a year is often a bar whose baskets stopped suiting it without anyone connecting the two.
• Rule out the grinder and the distribution before you buy precision baskets. The basket floor is a real variable and it is also a small one; spending on it while a clumping grinder or an untrained pour is still in play means paying to remove the least of three faults and then concluding the money did nothing.
• Buy basket, portafilter and tamper as a matched set from one decision. The tamper face must reach the basket wall, the basket must seat flat in the body, and a chain assembled from three separate purchases at three separate times is a chain with a gap in it somewhere.
• Hold a spare basket per group and retire on inspection, not on a date. A basket knocked out against a hard edge deforms at the rim, and a deformed rim seals badly and dents the puck. Look at the rim and the floor in daylight once a month; that is the whole procedure and nobody does it.
• Do not mix basket types across a bar unless every one of them has been calibrated. Two groups running two different floors will need two different grind settings for the same coffee, and the team will average them into a setting that suits neither.
• Check headspace physically, with the portafilter locked into the group and a real tamped dose in it. The screen imprint on the puck after a shot tells you the answer for free, and it is the only test that accounts for your machine, your basket and your dose together.
• Treat baskets as consumables with a service life, and put replacement in the budget rather than in the emergency. Coffee oil eventually polymerises in the perforations, and there is a point past which soaking restores nothing — a basket that no longer comes clean is finished, whatever it looks like.