Pressure differential flow meters and especially orifice plate flow meters continue to dominate industrial flow measurement despite the flood of ingenious and spohisticiated new instruments in the past three decades. This dominance is partly dependent on the essential simplicity of these devices and partly on the confidence that their performance can be predicted without the need for extensive calibration. The invention, Slotted Orifice Plate Flow meters, patented by Hall et al. is discussed in this paper. This new design helps minimize the difficulties caused by upstream flow disturbances and multi-phase fluids, thus making the orifice meter an even more desirable flow metering device.