OPV/PRV's; what they do & how to adjust them.

An over pressurization valve/pressure relief valve is a safety feature found in better espresso machines. Whether of the adjustable type or not its intended to limit the pressure build up in brew groups by diverting excess pressure.

Conventional Wisdom holds that the following are true;

• Espresso is best brewed ~9 bar.
• Three things affect the flow of a shot;
  • pump pressure
  • OPV/PRV setting
  • grind/dosing/tamping of beans

If the pump passes a water debit test it's capable of 14 - 16 bar. If the OPV/PRV is properly adjusted the brew pressure will be limited to ~9 bar, with the excess being redirected to the water reservoir. If the beans are freshly ground to the proper degree of fineness and are tamped consistently, the resulting espresso shot will be 1½ - 2 ozs in 25 - 30 seconds.

It's best to establish that the pump & OPV/PRV are functioning properly before attempting to master dosing techniques. If the brew/group pressure is set too low the coffee must be ground coarser and/or tamped less, resulting in under extracted coffee. If the brew/group pressure is too high the coffee must be ground finer and/or tamped more, resulting in over extracted coffee.

Ideally, once the brew/group pressure is adjusted to ~9 bar it can then be ignored for a long while. CW holds that tamps should be consistent from shot to shot, light to firm is up to you. That leaves the grinding of the coffee beans as the only variable. If shots finish before 30 seconds, or begin to blond before that, adjust the grinder to a finer setting. If the shots don't finish by 30 seconds, adjust the grinder to a coarser setting.