Customers do ask why the brew pressure gauge on top of the grouphead shows a different value compared to the gauge for the pump pressure.
The difference between the 2 gauges is due to 2 things:
1) The position where they are mounted, on the e61, it is more accurate.
2) The tolerance of the gauges. Here is an example. The gauge installed in the machine may have a tolerance 0-16 while the one on the group has tolerance 0-12. The gauges on the group divides 12 bar in 270° while the one in the machine divides 16 bar in 180°.
So the customer has to look at the pressure gauge on the group while dispensing coffee.
The other use is only look at the pump pressure gauge to see if there may be an issue with a rotary vane pump.
If there is a vibration pump inside the espresso machine, then the pump pressure gauge is really a brew pressure gauge. The reason is that it only measures the full maximum pressure with a blind filter. With coffee grinds, it measures the resistance in the coffee puck.