Espresso Machine (OPV) Over Pressure Valve is used to regulate the brewing pressure in an espresso machine and prevent overextraction of the coffee. OPV is an important component in the brewing system of an espresso machine and it needs to be set correctly to help you achieve the best quality espresso.
OPV does not set the brewing pressure | Contrary to popular belief the OPV does not set the brewing pressure or pump pressure of the espresso machine, it is impossible to set the machine to extract at a certain pressure. What the OPV does is to set an "upper limit" to the brewing pressure by limiting the pressure applied to the brewing system by the pump, with excess pump pressure (or flow) being either returned to the pump inlet or water tank depending on the type of pump.
OPV is typically set to 9-10 bar | it is generally excepted that the optimum espresso extraction is achieved at 9-10 bar of pressure. Above this pressure, undesirable flavours tend to be forced out of the ground coffee and into the drink, this is commonly known as over-extraction. If your espresso machine has an extraction pressure gauge then when brewing coffee the pressure should indicate 9-10 bar of pressure. When backflushing your machine, you would expect the extraction pressure to be about 1 bar higher than during a normal extraction.
Extraction pressure is still set by the barista | the extraction pressure is still determined by the grind, dose and tamp of the coffee. Too course a grind, too little ground coffee, or insufficient compression of the coffee will cause the
OPVs are are found in both commercial and high-end domestic espresso machines. In commercial espresso machines fitted with the most common Procon or Fluid-o-Tech pumps the OPV is typically incorporated into the pump between the inlet and the outlet. The OPV can be adjusted to set the pressure at the outlet with the excess water being distributed back to the inlet.
On espresso machines that use vibration pumps; tylically domestics espresso machines; the OPV is a separate component usually located just after the outlet of the pump. In this configuration the OPV can be adjusted to set the pressure at the outlet with the excess water being distributed out of a return. This return is typically connected to a silicon sube that runs back into the water reservoir (this is what the second water reservoir tube is for).