Bopp & Reuther of Germany holds the earliest patent for the oval gear meter in 1932.This meter was later popularized by Oval Corporation of Japan , which introduced the oval gear meter in the early 1950s. Oval gear meters are used for liquid measurement. They are quite popular for oil applications, especially for downstream oil distribution involving custody transfer. Here they compete with Coriolis meters, which are gaining market share in downstream oil measurement... PD flowmeters operate by repeatedly filling and emptying compartments of known volume with the liquid or gas from the flowstream. Flowrate is calculated based on the number of times these compartments are filled and emptied. The main types of PD flowmeters used for gas applications are diaphragm and rotary. These types of meters differ according to the way they trap the liquid into compartments with different shapes... Positive displacement (PD) flowmeters are truly a member of the class of traditional technology..