One of the major factors which control the quality of your espresso is the pressure with which the water is delivered to the coffee. Too low and you need to grind more coarsely and the espresso will be under-extracted because the large particles do not expose enough surface area to the flow of water. Too high water pressure and a very fine grind is necessary and over extraction will result. All pumps, whether vibratory or rotary, are capable of delivering the water at far more force than the generally accepted standard of about 9 BAR (about 130 psi). Most espresso machines control that pressure by using a simple pressure safety valve, often referred to as a pressure relief valve or over-pressure valve (OPV). Call it what you like, its job is to keep the pressure from exceeding a predetermined level.

      Here is the valve in its at-rest state. At the top is a spring which presses against a piston. The piston (the green part) seals against its seat keeping the flow of water (blue) through its normal path. If the pressure become too great...

      ...the pressure of the water pushes the piston upwards against the force of the spring allowing some portion of the water to flow through the bypass port as indicated by the red arrows.

      Getting the pressure just right is done by the screw shown at the very top. Turning that screw inwards (tightening it) pushes against the spring, compressing it further. The more the spring is compressed the more force the water needs to push the piston upwards. Adjusting that screw outwards (loosening it) lowers the spring's tension and the piston can be moved by a lower pressure from the water.

      In actual operation the piston will be displaced slightly and a small flow of water will pass by the piston. You can observe this. In the water reservoir of your machine there might be two hoses. One is shorter and that is the hose connected to the pressure relief valve. Hold that above the water level during an extraction and you can see the amount of water being sent through the pressure relief valve and back to the reservoir.

      It is important to note that these valves do not maintain a set pressure; they only control the highest pressure which can be reached. So if you grind too coarse the resistance to the flow of water will be low and the pump will not develop sufficient force to create a good extraction. That s, no adjustment will make the pressure higher if you have problems in grind, dose, or tamping. That is why they are referred to as over-pressure valves or pressure relief valves and not pressure regulators. A regulator would maintain the output pressure regardless of the input pressure.

      Most machines equipped with an adjustable pressure relief valve are also equipped with a pressure gauge so that the actual set pressure can be seen during the extraction which makes it easy to verify its proper operation. It also is an indicator of your grind, distribution, and tamping. There are some machines that are adjustable but do not have a gauge so the user has to supply a gauge to properly adjust the valve. If you adjust your OPV using the machine's gauge on the front panel, be aware that the gauge is connected in such a way that it reads a pressure slightly higher than that which is delivered to the coffee. On the other hand, most gauges are only marginally accurate, so the best determiner as to the correct setting of the OPV is your palate.

      Like many other parts, the design of these regulators varies. For example, instead of a screw, some use a hollow bolt that have a hex fitting, and the bypass water goes through the valve and through the adjusting bolt to get to the bypass hose. But the function of these is the same— to set the maximum water pressure which can be delivered to the coffee.