Control and Sense the Pressure
There are two things regarding the pressure on a Gaggia Classic:
- Pressure Sensing
- Pressure Control
While pressure sensing is quite easy (just use a manometer), pressure control is not really possible.
That's because a vibration pump, like the Ulka EX5, has a constant pressure to flow rate curve.
On the other hand rotational pumps have a characteristic per rotation speed. That means for every speed you set the pump, it will give you a separate curve of pressure and flow rate.
That means a vibration pump will have the maximum flow at no pressure and no flow at maximum pressure. Between these two points, the curve is quite linear.
This is also the reason it is quite hard to get for example 25ml of espresso in 25s. That would mean you want the pump to have a flow rate of about 100ml/min (assume that 40ml of water are left in the basket and coffee). If you take a look to the datasheet, you will find out that the Ulka EX5 for example will produce a flow rate of 100ml/min at about 13 bar. But 13 bar are too high for some coffee, and the espresso will have a bitter flavour.
A solution to the problem is to use an overpressure valve, which is already built in to the Gaggia Classic but is set to about 14 bar by default. You can adjust the valve by screwing the adjustment plate out. Beware that you need a 17mm nut to unscrew the hose fitting and a 5mm Inbus to adjust the valve. If you can not move the screw, put the whole OPV into descaler first.
Here is a video after the OPV adjustment:
You can clearly see that the pressure drops down to 7 to 8 bar while the amount of water in the cup is after the 30s just about 45ml! In my subjective opinion, the coffee is now much smoother and not so "spiky" sour or bitter. To test this, I tried the same amount of coffee (21g) for the same time (30s) and got similar results of fluid in the cup (around 45 to 50ml).