Field Testing Methods Section 2

B. Measurement of capacity a.) Magnetic Flow Meter
A calibrated magnetic flow meter is an accurate means of measuring flow in a pumping system. However, due to the ex-pense involved, magnetic flow meters are only practical in small factory test loops and in certain process pumping systems where flow is critical.
b.) Volumetric measurement
Pump capacity can be determined by weighing the liquid pumped or measuring its volume in a calibrated vessel. This is often practical when pumping into an accurately measured reservoir or tank, or when it is possible to use small containers which can be accurately weighed. These methods, however, are normally suited only to relatively small capacity systems.
c.) Venturi meter
A venturi meter consists of a converging section, a short con-stricting throat section and then a diverging section. The object is to accelerate the fluid and temporarily lower its static pressure. The flow is then a function of the pressure differential between the full diameter line and the throat. Fig. 4 shows the general shape and flow equation. The meter coefficient is determined by actual calibration by the manufacturer and when properly installed the Venturi meter is accurate to within plus or minus 1%.

sec b5 fig4Fig. 4 Venturi Meter

d.) Nozzle
A nozzle is simply the converging portion of a venturi tube with the liquid exiting to the atmosphere. Therefore, the same formula can be used with the differential head equal to the gauge read-ing ahead of the nozzle. Fig. 5 lists theoretical nozzle discharge flows.