Dialled in at 9am, sour by 2pm: what changes through a shift?

Weigh your dose before you touch the grind. On a single-hopper bar the hopper is full at 9am and nearly empty by 2pm, and the weight of beans sitting on the burrs changes what falls into the basket. A drop of half a gram reads exactly like your description: thin, sour, and it chokes the moment you go finer because you are now grinding finer for a lower dose.

Check in this order, because each one is cheaper than the next:

1. Dose on the scale. If 2pm shots are 17.4 g against 18.1 g in the morning, you have your answer. Fix: mark a fill line on the hopper and keep it above that line, or switch to single-dosing at changeover.
2. Yield and time on the scale, not by eye. "36 g" by eye drifts by 3–4 g through a busy service.
3. Burr temperature. After 200+ shots a flat-burr grinder can be 10–15 °C warmer, which grinds slightly finer and faster. That would make shots slower, not faster, so it is unlikely to be your cause, but log it.
4. Beans. Eight days is fine. But a bag opened Monday behaves differently by Thursday, and a bag left next to the machine warms up. Keep the working bag closed and away from the group heads.
5. Water. Filtered and tested is good. Check the softener is not regenerating mid-day.
6. You. Tamping pressure drifts when you are tired. A calibrated tamper or a leveller removes it from the list.

Two habits that end this conversation for most small bars:

• Re-dose and re-check at every changeover (roughly every 3–4 hours), on the scale, and write the numbers on the board. Thirty seconds.
• Keep a one-line log: time, dose, yield, seconds, taste word. After a week the pattern is on paper and you stop guessing.

Members: if you have a different first check, add it below. The order above is what the most experienced technicians in the room tend to agree on.