Just to confirm my understanding, or possibly display my ignorance... Supposing my pump was capable of delivering one litre a minute at nine bar, the operation of the OPV would be far more critical, would it not? I assume one reason OPV's are less of an issue with small vibe pumps is the flow rate reduces under pressure. Took a look at that PDF on OPVs, none of them look like the one in my gaggia, which seems quite well designed, albeit small. Same basic operation though.