There are many areas where flow measurement can be inaccurate or invalid.Such common errors are discussed here.
Erosion:
Erosion causes the material to wear away. Suspended solids or debris in the piping will not only plug up the sensing lines, it will erode the sensing device. Erosion reduces the differential pressure between the HP and LP line. Orifice plate, flow nozzle and even venturi can be damaged.
Vapour formation in the throat:
Vapour formation in flowing liquid can occur at both LP and HP side. Differential pressure cell will show higher pressure difference as the gas expands at the LP side of the DP. This will result to show high flow rate than usually shows.
If the vapour formation is on the HP side due to cavitation, there will be a lower in differential pressure in such condition. This will eventually turn the flow rate low.
Over ranging Damage to the D/P Cell:
The system pressures are usually much greater than the differential pressure and three valve manifolds must be correctly used.
Clogging of Throat:
Particulate or suspended solid particles in flow stream hit the flow sensor in high velocity can damage the flow sensor. Also damages the material in the throat of the sensor which increases the pressure across the cell. Which increases the flow measurement too.
Plugged or Leaking Sensing Lines:
Sometimes DP sensing line can leaks. The effects of plugged or leaking D/P sensing lines is the same,
On the HP side, a plugged or leaking sensing line will cause a lower reading.
On the LP side, a plugged or leaking sensing line will cause a higher reading.