“15 bar” or “19 bar” on a box refers to the pump’s maximum output, not the pressure at the coffee.
What actually happens
A vibration pump can generate 15 to 19 bar unloaded. The over-pressure valve dumps the excess, capping brew pressure at whatever it is set to — usually 9 to 12 bar.
The coffee never sees 15 bar.
Why 9 bar
It became the standard with the pump machines of the 1960s and it has held up: enough to extract a fine bed in 25 to 30 seconds, without over-compacting it.
Modern practice often prefers 6 to 9 bar, finding lower pressure sweeter and less prone to channelling.
Why more is worse
Higher pressure compacts the puck harder and increases the chance of channelling — water blasting a hole rather than passing evenly.
The output is a faster, harsher, less even shot.
Why the number is advertised
Because it is a number that goes up, and bigger sounds better. It is one of the least meaningful specifications on a coffee machine.
What to look for instead
Temperature stability — a PID, a real boiler, or a well-implemented thermocoil. A standard 58 mm portafilter and single-wall baskets. Real steam power if you drink milk. Parts availability.
And on your machine
If it has a gauge, watch it during a shot. The needle sits where the OPV sets it, not at 15.
Last reviewed 8 August 2026