Mechanical flowmeters are tested using a known volume in a tank. The water goes by gravity through the water meter and its accurate to with 1% of true flow.
They are only within 1% when they are installed correctly and when they are not old, fouled with sand or debris, or not working for some other reason. As I said, the one in my house was off by a factor of 5. That was a mechanical unit that I believe was installed in the 1980s or 1990s.
Of course any instrument can malfunction, but in my experience with cold fusion calorimetry, flowmeters are the most problematic.