This is because of
 Accuracy within +/- 1%
 Robust & Simple Design
 Proven Design 
 Suitable for Almost All Types of Fluids
 First Principle Device - Which never Fails
 Proven Solution for Flow Measurement
 Suitable for All Pipe Sizes from 1/2'' to 36'’
 Very nominal cost compared to other flow elements
 Zero Maintenance
 Because of proven data available, no need of frequent   calibration like other element