Have you ever wondered why the pressure on the brew gauge of your espresso machine changes, even when you are not extracting a shot? It’s normal for the brew gauge to indicate and move between three nominal pressures. The minimum likely to be observed is the mains water pressure at around 4 bar, depending on the local mains water pressure. If the espresso machine is not heating then the mains water pressure will usually be indicated.
When brewing, the pressure is typically around 9 bar as that is considered the optimum brew pressure for espresso.
The brewing pressure is set by adjusting the regulator on the brew pump. However, this may be different for a few reasons. Firstly, the brew pressure may have been deliberately set at a slightly higher or lower pressure. Some baristi deliberately adjust the brew pressure to control espresso extraction. A worn pump may be unable to regulate the pressure originally set and this needs to be serviced or replaced. If the mains pressure has changed since the brew pump regulator was adjusted then the brew pressure will also change as the pump regulator output pressure is dependent on the input or mains water pressure.
The maximum pressure you should see on the brew gauge is dependent on the adjustment of the thermal expansion valve.
If you have just run water from a group the pressure will climb above brew pressure as the cold water that has just entered the boiler increases in temperature, expands and therefore increases boiler pressure. The thermal expansion valve opens to relieve the expansion pressure and it is usually set at around 12 bar.
If you have ever witnessed a sudden and temporary drop in brew pressure while pulling a shot then it may be that the steam boiler has refilled at that time. If the steam boiler level probe senses a drop in water level, it sends a signal to the espresso machine controller asking the autofill valve to open and fill the steam boiler. Unfortunately, if this happens while you are extracting your sweet shot then the mains pressure to the pump regulator will be reduced and this will cause a drop in the output or brew pressure which ain’t so sweet!