V3-style volumetric mod for pre-V3 machines
One major issue with pre-V3 machines is that the water volume is measured before the overpressure valve (OPV). If the OPV diverts some water back to the tank, that water has already passed through the flowmeter but never reaches the cup.
It’s also important to keep in mind that the OPV isn’t digital. If it’s set to 11 bar, it doesn’t stay 100% closed below 11 bar and then suddenly open at exactly 11 bar.
As a result, the actual amount of espresso in the cup can vary quite significantly depending on the resistance of the coffee puck, despite the machine having volumetric dosing.
It’s a bit like unmetered air entering an internal combustion engine equipped with a mass airflow sensor.
What’s different with the V3?
On the V3, the OPV return line no longer sends excess water back to the tank. Instead, it feeds it back into the pump inlet after the flowmeter. This means that water which has already been measured by the flowmeter stays within the brewing circuit rather than being lost back to the tank. As a result, volumetric dosing is much more consistent, even when puck resistance varies.
I converted the water circuit of my Ascaso Steel Uno PID V1 to use this newer layout.
For the conversion, I used a Y-fitting and some silicone hose with a 5 mm inner diameter. I had already replaced all of the original hoses with higher-quality silicone hoses anyway.
The hose running from the flowmeter to the pump is cut, and the Y-fitting is inserted in between. The OPV return hose is then disconnected from the elbow fitting that leads back to the water tank and connected to the Y-fitting instead.
I also extended the hose between the flowmeter and the Y-fitting so that it can be routed naturally without any kinks or tension.
It works perfectly, and the variation from one espresso to another is now around ±1.5 g.
For the Y-fitting, I think POM is the most suitable material. There are plenty of generic fittings available, but I chose a high-quality Normaplast fitting made in Germany that is also officially approved for food-contact applications. It cost €1.15.
The hoses have an inner diameter of 5 mm. I still chose a 6 mm Y-fitting, which I would recommend.